270 Offy

Discussion in 'A Work In Progress' started by mayhugh1, Aug 12, 2019.

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  1. Nov 14, 2019 #81

    awake

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    A continuing work of art!

    I'm an utter newbie with respect to building an engine, much less something of this amazing detail and quality, so I hesitate to offer any suggestions ... but though I have not yet built an engine, I have been an active home-shop machinist for around 14 years. So with trepidation and humility, hoping I am not guilty of going too far off topic, here goes:

    With respect to the problems you had with boring the StressProof ... I have gotten excellent results machining stress proof with HSS tooling. In fact, I have generally gotten excellent results machining the vast majority of materials using HSS tooling - I have been surprised that so many of the builders here seem to use carbide exclusively, even on machines that arguably would do better with HSS.

    A stout boring bar definitely helps, of course, and so does a stout way to hold it. For much of the boring that I do, I mount a tool in my quick-change tool post. But when I need the ultimate in rigidity, and especially a larger size boring bar, I turn to an "Armstrong" style boring bar holder. No, not the lantern tool post tooling; rather something like this:

    OnLathe3.jpg

    This is one of the very first projects that I made on my lathe, for my lathe - very easy to make, and works extremely well. Here are pictures of the components and assembled unit:

    Finished-Components.jpg Finished-Assembled.jpg

    Again, I hope that I am not going too far off topic - if needed, I will be glad to delete the post!
     
  2. Nov 15, 2019 #82

    John Antliff

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    I use my Quorn grinding spindle for similar jobs and it has a TIR of 2 tenths. I tried 3 times to better that but each time the spindle came out at 2 tenths again! I then found out that the lathe mandrel had a 2 tenths TIR which is why I could never better it. The Quorn spindle design uses angular contact bearings with a spring box and an oil bath and compares favourably with commercial TPGs. It's a good design and produces very acceptable results. I also have problems finding good quality internal stones so if anyone knows of a reliable source please post details.
     
  3. Nov 15, 2019 #83

    mayhugh1

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    The cylinder sleeves were sealed to the block with Loctite 680. The annular gaps between them were on the order of a thousandth and so, even though their fits were very close, the liners slipped into the block without distortion of their bores. Four oil return tubes were similarly installed. These will return waste oil to the crankcase from the engine's top-end so a scavenger pump can then draw it out to an exterior oil tank. The tubes were cut from hard thin-wall stainless steel tubing having a .120" o.d. and a .096" i.d. One of the photos shows a top view of the assembled block. The three pairs of holes between the cylinders are passages for coolant flow from the block to the head.

    Uncertainties in the engine's lubrication system have been holding up the completion of a number of nearly finished assemblies, and so I hope to start working on it next. - Terry

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  4. Nov 15, 2019 #84

    ShopShoe

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    Interruption, sorry.

    I just re-read post No. 81 and decided I might like to make your Boring-Bar Holder.

    Did you drill or bore the holes for the bar before cutting the cylinder-shaped piece in half? Or was there some other order of operations?

    Thank You,

    --ShopShoe

    PS: I always admire the totality of your skills in making all kinds of things.

    --SS
     
  5. Nov 15, 2019 #85

    awake

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    ShopShoe,

    I'm happy to share more details. As I mentioned, this was one of the very first projects I made, some 14 years ago, so my memory is a bit fuzzy ... but as best I recall, made the outside cylinder from a piece of heavy tubing, onto which I welded a top. I then drilled the hole for the 5/8-18 threaded portion to protrude through. Next I turned the inside cylinder, sized to a close sliding fit with the outer cylinder. The bottom of this inside cylinder was turned to act as the "t-nut," and the top was turned down and threaded to 5/8-18. Finally I made a nut, threaded 5/8-18, which will fasten everything together. Here is an important point: the size of the inside cylinder stops short of the top of the outside cylinder, so that when it is assembled and the nut is tightened, the nut pushes down on the outside cylinder while pulling up on the inside cylinder, and in the process locks the tool to the compound t-slot.

    Now with the tool assembled and locked together, I put a 1/2" drill bit in the lathe chuck, and moved the carriage over to drill the first (smallest) hole through both the outside and inside cylinders at once. I loosened the nut and turned the outside cylinder 120° while keeping the inside cylinder in its original orientation, re-tightened the nut, then used a 5/8" drill bit to drill through the outside cylinder, enlarging the hole in the inside cylinder in the process. Once again I loosened the nut, indexed the outer cylinder by 120°, re-tightened, and drilled through using a 3/4" drill bit.

    Once the drilling was completed, I dissassembled everything and used round files to elongate the hole through the inside cylinder to provide clearance - this allows assembling the unit with the largest size (3/4") boring bar while it is loose, and makes sure it doesn't bottom out on the inside hole when it is tightened. (Alternately, I could have drilled through just the inner cylinder with a 13/16" or 7/8" drill.) Finally I cut the outer cylinder in half on the bandsaw at the center line of the holes, and used a file to ease the edges.

    In use, the outside cylinder (now in two pieces) is turned so that the desired size of boring bar is lined up with the hole through the inside cylinder. The boring bar is inserted through and lined up on the lathe axis, and the nut is tightened down, squeezing the boring bar securely between the lower and upper halves of the outer cylinder while simultaneously squeezing the inside cylinder's "t-nut" up to lock the whole unit in place. The boring bar is automatically on-center, but of course you have to cut the holder(s) for the tooling that the boring bar will take so that the tool will be on center - that, or plan to grind the tooling as needed. I made my boring bars to take square HSS tool bits that I can grind to the needed shape.

    Obviously, you can make this design to fit whatever size(s) of boring bars you want. If I were to do it again, I would skip the 1/2" and 5/8" sizes (since I can do those fine using my QCTP), and instead make this unit to fit a 3/4" and a 1" boring bar. (I would also produce a better finish, especially on the deburring - looking at this, I can tell it was one of my first projects!) With larger stock, you could make the design fit as large a boring bar as you wish.

    I hope the explanation makes sense - the explanation is probably longer / more complicated than the actual making of the project!
     
  6. Nov 16, 2019 #86

    ShopShoe

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    Thank You awake.

    The holder looks so simple that I imagined fewer steps in the process. It clearly is a clearly-thought-out design and very elegantly made.

    Thank you for your complete explanation. I think I can speak for everyone as well as myself to say how much we appreciate detailed explanations and complete sharing of methods.

    I can also say that in reading your explanation I could see some other situations where that particular set of techniques could be used to make other bits and pieces.

    Thanks again,

    --ShopShoe
     
  7. Nov 25, 2019 #87

    mayhugh1

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    The Offy's lubrication system isn't trivial. Although I browsed its design while working on other parts of the engine, the last week was spent totally focused on it. Its complexity is difficult to convey in a few 2D parts drawings, and so I created a 3D model based upon information in the manual that I modified for the split crankcase.

    The two pumps (pressure and scavenger) are machined into a stacked pair of 3/16" aluminum plates and topped off with a 1/16" thick cover. The pumps are mechanically coupled and sit between the crankshaft which drives them, and the water pump which is driven from their common shaft.

    The pump assembly is located inside a recess in the front of the crankcase, immediately behind the front cover. Drilled passages in the floor of the recess behind the pumps carry oil from the bottom (pressure) pump to the main bearings. Waste oil that collects in the crankcase is drawn out by the upper (scavenger) pump and returned to an exterior oil tank. Compared with the original one-piece crankcase, the redesigned main bearings in the split crankcase greatly simplify oil distribution in the bottom end.

    An interesting feature of Ron's crankcase oil returns is their different diameters. They're designed to equalize oil accumulation between the bearings and reduce chances of the scavenger drawing air. I carried this detail over to the split crankcase as well.

    Getting oil to the engine's top end is considerably more complicated, and care will be needed to prevent leaks and pressure loss along its tortuous path. The top end will actually receive oil from the scavenger pump. A portion of the oil that would otherwise be returned to the tank is diverted to the top end through a needle valve located on the side of the engine. This oil, though, must be pumped across any clearance that exists between the pump assembly and the side wall of the crankcase recess that it's mounted in. So, this fit must be snug.

    After crossing this boundary, the oil flows through a vertically drilled passage in the side of the crankcase and then into a horizontally milled trough underneath the gear tower. It then continues up between the gear tower and engine block inside a milled channel in the rear of the tower. Before crossing another boundary to reach the head, a portion is turned back into the tower in order to drip lubricate its gears and bearings. The remainder enters the head on its way through the cam blocks for distribution in the top end via the hollow camshafts. Top end waste oil will be returned to the crankcase through the four vertical tubes already installed in the block.

    The 'snug-fit' requirement (among others) concerns me, and so I took a break from modeling to find out how much of an issue it might become since an o-ring seal in this location doesn't seem practical. I wanted to experimentally determine how close of a fit I might expect for the pump assembly inside my already machined crankcase. A thousandth clearance should allow the assembly to come in and out of the recess without damage to either and maybe keep leaking oil and pressure loss at manageable levels. Another issue, however, is that the rear bearing for the pump assembly will sit in a pocket in the bottom of this recess, and it too has already (and maybe prematurely) been machined.

    The recess was part of the lower half crankcase machining done much earlier on my Tormach using CAM tool paths that I can reuse. However, effective cutter diameter and machine backlash can unpredictably affect the fit I'm hoping to achieve. In order to get the best possible result, I machined three trial pump blanks using the same tool paths and end mill that was used to machine the recess. I modified the CAM of two of the blanks by adding a thousandth to the outer perimeter of one and subtracting it from the other.

    To verify the alignment, a bearing pocket was added to the rear of each trial blank using the same CAM and cutter that was used to machine the pocket in the crankcase. A dummy bearing was used between them during the fit checks. The CAM parameters for the trial blank that gave the best fit were saved for use later when the pumps are actually machined. Even though there was a perceptible gap between the two, it was small enough to prevent a .001" feeler from passing between them.

    I've provided some renderings from the modeling. I made use of a SolidWorks feature that I recently discovered that permits an x-ray view behind an individual surface. The head and cam box models aren't yet finished, and so they aren't included. I also have some o-ring and gasket details to work out.

    After a full week in front of the computer, I desperately needed to return to the shop and make an actual part. I was fairly certain that its design won't change, and so I machined the oil manifold that will eventually connect the engine to its oil tank. A hose on its top barb will supply oil to the pressure pump, and the bottom hose will return scavenged oil to the tank. - Terry


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  8. Nov 26, 2019 #88

    awake

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    The amount of work in just the oil distribution is staggering. When all of this engine is done ... it will be a testament both to craftsmanship and to perseverance!
     
  9. Nov 26, 2019 #89

    oldCB

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    For sure...........considering building a 2.4L Ford OHC engine , kinda cheating would use my assembled model as a pattern?
     
  10. Nov 26, 2019 #90

    RonC9876

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  11. Nov 26, 2019 #91

    mayhugh1

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    RonC9876 wrote:
    [Terry: My engine has been running for twenty years now and I have never had a problem with the lubricating system I designed. I pondered a long time on just how to do it. I admit it is a bit difficult to employ, but it has worked flawlessly. I am happy to say that you were able to understand exactly how it works. Using the scavenged oil to lube the upper end and making the flow adjustable with the needle valve on the side is probably a little unorthodox, but it all works perfectly. I came up with this design at the very end of construction. I left it till the very end because I knew it would be difficult to get it all to fit and actually work. I don’t know how it is done on the full sized engine. I had very little info from which to work with. It is what it is!]
    ---------------------
    Thanks Ron. That's what I hoping to hear! - Terry
     
  12. Dec 6, 2019 #92

    mayhugh1

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    The two pump bodies and cover were machined from 6061 aluminum. There's a lot going on inside them with limited space for screws to hold them together. Both sides of each were lapped on a glass plate with Time Saver to minimize leaks. Since the thin top cover was likely to be troublesome, I initially tried machining it from a piece of 14 gage steel that I planned to grind perfectly flat on my surface grinder. I wasn't able to control its warpage during grinding, however, and so I instead machined three covers from sheet aluminum and then selected the flattest one to work with.

    It was encouraging to hear from Ron that the Offy's lubrication system has been performing well for him since I had been concerned about the pressure pump's seemingly small size. My only change to Ron's original design was some rerouting to accommodate the modified crankcase, but this required relocating some of the oil channels inside the pump bodies. Before irreversibly drilling the crankcase, I thought I'd better assemble and test the pumps outside the engine just to make sure there won't be any surprises after final assembly.

    I made up a simple fixture to which I could mount the pump assembly and connect some hoses so I could operate the pumps outside the crankcase. This fixture was also used to ream the final diameters of the shaft holes through the stacked pump assembly while it was doweled together and bolted down to it.

    The pressure pump's small size make it susceptible to even small accuracies in its machining. A gear pump's transfer volume can be easily calculated and used as a sanity check on its machining. The volume moved through a gear pump per revolution is given by:

    Vol = pi/4 * (D^2 - d^2) * T

    where D is the o.d. of the pump gears, d is their i.d. measured across the roots, and T is their thickness. Once this volume is determined, the flow rate at any rpm can be found by simply multiplying it by the rpm.

    There's a 3:1 gear reduction between the Offy's crankshaft and its oil pumps. At 5000 crankshaft rpm, the theoretical flow rate of the Offy's pressure pump works out to be 5.8 cubic inches per minute and 13.1 cubic inches per minute for its scavenger pump. The expected pressures aren't easily calculated, though. For the simple pumps we typically make, their machining typically limits the maximum pressures they can develop. In the past, I've measured as much as 100 psi.

    I tested the Offy's pump assembly while mounted in my fixture and being spun by a drill at roughly 1700 rpm in order to simulate their operation in the engine at 5000 rpm. The pressure pump's open-end flow rate was 3 cubic inches/min, and the scavenger pump's flow rate was 9 cubic inches/min. The scavenger pump was 30% lower than theoretical, but the pressure pump was nearly half of what I expected. I also measured the blocked-output pressures to be 15 psi and 50 psi for the pressure and scavenger pumps, respectively.

    Although I was satisfied with the scavenger pump, the pressure pump results were disappointing. Under a microscope I could see no visible clearance between the pump body and the teeth on its gears. But, while removing the assembly from the test fixture, I noticed it and the rear surface of the bottom (pressure) pump were wet with oil. Suspecting leaks at the rear of the assembly, I made a .005" thick Teflon gasket to seal these two surface together. I remeasured the pumps' performances and found the blocked-output pressure of the pressure pump had risen to 25 psi, and its flow rate had increased to 5.2 cubic inches/min. The scavenger pump's flow rate also increased to 12 cubic inches/min.

    The addition of this rear gasket brought the flow rates up to within 10% of their theoretical values and their maximum output pressures to levels that I'm comfortable with. I've included a CAD rendering of an x-ray view looking through the bodies of both pumps. Keeping in mind that the pumps are on different levels, there should be no internal leakage between them except possibly through the central driveshaft. If the bottom pump isn't sealed to its mounting surface, however, there's a potential for its output to leak to the outside world or even into the scavenger pump's low pressure input. Similarly, the scavenger pump could find its own path to the outside world.

    More importantly, if there is a leak on either of the pumps' inputs, priming during starting and/or idling can become a problem. All my testing was done worst-case with the pumps located several inches above the oil reservoir. During testing but before adding the gasket, the pressure pump had to be manually primed with a syringe and again after standing overnight. The gasket seemed cure this. The pumps were disassembled/reassembled several times over as many days to make sure their operation remained consistent after adding the gasket.

    The common drive shaft running through the pump assembly combined with the pumps' .001" gear face clearances probably account for the current limitation on the pressure pump's maximum output pressure. Based on my earlier experiences with the Knucklehead, though, 25 psi should be more than adequate especially since there is no pressure relief valve. The pumps were disassembled one final time in order to drill the transfer hole through the side of the bottom pump body that will be used to supply oil to the engine's top-end. - Terry

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  13. Dec 6, 2019 #93

    ddmckee54

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    Where did you find that HUGE safety match that you were using in you photos? Make it yourself? Gonna' do a build thread on it too?

    Your reputation for excellence is well deserved.

    Don
     
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  14. Dec 6, 2019 #94

    petertha

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    Very nice work indeed. Some basic questions if you don't mind

    - is the higher capacity of the scavenger pump relative to the pressure pump related to some basic lubrication pumping rule of thumb. Or maybe particular to this engine implementation? You mentioned the top end will actually receive oil from the scavenger pump. I don't quite get why the pressure pump couldn't feed another separate area of the engine on a separate branch. How does the needle valve factor in this?

    - how do you select tooth form for when it comes to pumps? I assume bigger or smaller teeth influences rate & pressure but any words of wisdom in that regard? Are they conventional spur gear profiles?

    - what will you be using for oil (specifically I'm interested in viscosity)

    - on the scavenger pump side, what happens if it drains the crankcase sump or otherwise sees intermittent oil & air? Especially since its capacity is higher as mentioned. Does the pump care as long as its lubricated or slowly heats up spinning like this? I've never understood this aspect. Can the crankcase ever be drawn down below atmospheric?

    - anything to know about the oil supply/return tank from your previous engines? Does return oil foam at all (allowance for extra volume). Does it have an ideal position relative to the pumps (like gravity feeds pump elevation).
     
  15. Dec 6, 2019 #95

    mayhugh1

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    Peter,

    Lots of questions...

    The scavenger pump is usually larger than the pressure pump in order to insure the oil level doesn't have a chance to build up in a sump-less engine and cause problems. The scavenger may at times draw air, but it should be capable of re-priming. Builders of the Hodgson radial found that the size of its scavenger wasn't great enough to prevent the engine from filling with oil and locking up, and so a gravity drip fed had to be added to the engine to prevent the crankcase from filling with oil and locking up the engine.


    The oil will foam, but in a well design system, the air will have a chance to settle out in the return side of the tank. I even saw some evidence of foaming during the bench tests described above. Unlike my radials, I'm hoping to be able to set the Offy's oil tank below the engine.


    I'm speaking for Ron here, but yes, the top-end could receive its oil from the pressure pump along with the bottom end. In fact this is what was done on the Quarter Scale Merlin. But, a pressure relief valve was required to properly divide the flow between the critical bottom end and the not so critical top-end. What Ron did was actually pretty clever. He dedicated the entire pressure pump to the critical bottom end and diverted a fraction of the scavenger's return to the less critical top end. A simple needle valve can be used to control this fraction while observing the two flows without having to deal with difficult to set pressure settings.


    Spur gears are typically used in constant displacement pumps, but if you look back on my Knucklehead thread you'll see I tried a different profile, just for fun, that I felt would be more efficient. Then, I spent several weeks dealing with its monstrous flow that I didn't want or need.


    I don't know what you mean by 'bigger or smaller teeth.' The equation for transfer volume clearly depends on the size of the gears used, and the size of their teeth depend upon their diameter. Remember, the space between the teeth are the buckets that carry the oil around the outsides of the gears to the pump's output.


    I've tried oil viscosities between 5W20 and straight 80 weight, and frankly I'm not sure I see much difference between any of them in my model engines. I now typically use whatever happens to be left over from my last auto oil change.


    Good Luck. Can't wait to see your radial ... Terry
     
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  16. Dec 7, 2019 #96

    kvom

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    Tongue in cheek comment on Ron's engine: I've seen it at several shows over the years and heard it more often. The noise in running is such that it rarely if ever runs more than a minute at a time. Makes me wonder if the oiling system is super-critical for short runs like this. Disclaimer: I know nothing about IC engine lubrication.
     
  17. Dec 7, 2019 #97

    mayhugh1

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    I've often wondered myself if splash lubrication alone isn't sufficient for these model engines. With a roller (ball bearing) cam, leakage past the valve stems might be enough to mist lube the top end. The pressurized oiling systems that some of these more challenging models have could very well be unneeded although interesting complications. - Terry
     
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  18. Dec 7, 2019 #98

    RonC9876

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    The reason for short runs at the shows is two fold. First,If I allow the engine to run for any extended period, I get flak from people saying it is too loud and their conversations are being interrupted. Secondly the water pump and cooling system is totally insufficient for the heat this thing generates. Lubrication has never been a problem. The oil tank sits at the bottom of the wooden base used to support the engine. It has it’s own oil filter and the pumps prime without incident every time, even after sitting for months. I already discussed the cooling problem with Terry. The set up needs a positive displacement pump.
     
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  19. Dec 8, 2019 #99

    KennyMcCormick315

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    commercial model aviation 4-cycles are lubricated solely by blowby and they last for decades working far far far harder than any of the models posted on this site ever do.


    It isn't that critical on this scale. It's quite impressive when someone goes to the trouble of giving one full pressure lubrication but, for the majority of the models on this site that never see a day of honest work in their entire existences, they'd be lubed just fine by splash alone.
     
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  20. Dec 9, 2019 #100

    Ironmanaz

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    Blowby is identical to the fuel/air mix coming thru the intake manifold (reason for the PCV valve). How does this lube anything?
    Maybe with premix? Considerations on my BR2 build....
     

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