270 Offy

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Terry excellent explanation on timing as are your thoughts on machining gears for small engines
The cam gear covers were gasket'd with Oracal vinyl before being installed, but the cam box covers were only temporarily set in place for now. There's an oil flow adjustment screw on the front starboard side of the engine which will be finally set while watching the top-end oil delivery with the engine running.

The block and crankcase side covers were installed next with each being sealed with a Teflon gasket. Since it would have been easy to drop one of the 0-80 block cover fasteners inside the crankcase, the crankcase covers were installed first.

The magneto (which is actually a distributor) was then mounted on the front cover bracket. A pair of timing adjustments involving the phasing's of the rotor and trigger magnets completed its installation. Since the ECM was needed for these adjustments, it was time to move the engine to its display/running stand.

With the crankshaft set to TDC in the number one cylinder's compression stroke, the rotor was aligned with the number one high voltage tower. Since the shaft driving the magneto spins at the same speed as the crankshaft, a 2:1 gear reduction inside the magneto drops the rotor speed down to match that of the camshaft. An alignment mark, visible on the top inside surface of the magneto, will be inline with the grub screw located on the rear of the rotor when it's properly positioned. This grub screw locks the rotor in place.

A collar containing the pressed-in trigger magnets was slipped onto the distributor driveshaft during the magneto's installation. Three grub screws lock it to the driveshaft so it spins at the same speed as the crankshaft. The pair of diametrically opposite magnets generate the four trigger pulses required for every two crankshaft revolutions. Before tightening the grub screws, the collar was aligned with the magneto so the CDI is triggered at TDC in the number one cylinder's compression stroke. A lever on the side of the magneto can manually advance the ignition timing up to thirty degrees.

Finally, the carburetor and exhaust subassemblies were added. Each was sealed to the head using one of the nineteen custom Teflon gaskets made earlier for this engine.

A set of plug wires was made up using repurposed automotive vacuum fittings (Dorman 47411) for plug boots that nicely fit the VR2L spark plugs. Before installing the plugs, they were temporarily fixture'd outside the engine to verify the ignition one last time. After they were installed, the engine had plenty of compression.

A little oil and gas were added and the engine cranked for a few seconds to verify it at least wanted to run before adding the radiator and (messy) coolant. With the four carb screws opened 3/4 turn, the little engine started right up. I shut it down after a few seconds since I really didn't want to run it without coolant.

There's just a few loose ends including the addition of the already completed radiator to finish this build. With no experience with multi-carb set-ups, nor carb design for that matter, tuning promises to be a learning experience. It seems I've elected to put four unknown and untested carburetors on the same engine. - Terry

It looks like you are getting dangerously close to the point where you won't be needing to make the Vroom-Vroom noises anymore. You'll be able to let the Offy speak for itself. As usual, this has been an interesting and educational ride.

Fascinating seeing how you have made all of the parts. Stunning build (as always). I know it takes ages to document builds, especially with this much detail, but I can assure you it is greatly appreciated! I am a lover of Alfa twin cam motors and you've got me thinking if I could make a model....
The radiator was installed along with the engine's rather delicate water outlet pipe. The pipe was bolted down to three tiny flanges on the head through .010" thick Teflon gaskets before filling the system with 1200 mL of coolant. Clear silicone tubing was used for the top radiator hose so coolant flow could be monitored while running. Earlier pump testing showed at least 500 rpm being needed to get any appreciable flow and, with the spark plugs installed, my starter drill can manage only a few hundred cranking rpm.

Texas is currently in the middle of an historic arctic front with near 0F temperatures and some six inches of ice and snow. The trees and vegetation in our area weren't designed to deal with this, and large over-stressed limbs have been falling and creating a lot of havoc for property owners. Almost half our population has been without electricity, and many of those also have no water. My own power was off for some 30 hours while the outside temperatures hovered around 20F.

Although trivial in comparison with all the real problems around me, the weather has limited progress on the Offy. The engine must be run indoors, and I'm currently dealing with a lot of oil smoke. Only brief (less than one minute) runs are possible between necessary half-day fume clearing pauses. On the positive side, the engine is remarkably easy to start and responds well to its throttle while still using my original 'all carb screws open 3/4 turn' guesstimate.

Oil smoke is a problem though and watching and listening to the exhaust seems to indicate all four cylinders are smoking. My guess is that the issue is a combination of too much oil in the engine and not enough runtime on the rings. As it turns out, the Offy is a tough neighborhood in which piston rings must live.

Ideally, the Offy's 2x scavenging pump should stay ahead of its 1x pressure pump and evacuate the crankcase so there's only a light oil mist remaining for the cylinders. I've learned from experience, however, this is seldom the case. For example, the Hodgson radials are dry sump engines, but their 2x-1x pumps require drip feeding the pressure pump to limit the amount of oil in the engine and prevent flooding the cylinders.

The Offy really doesn't have an actual oil sump but has instead four floor drains situated between the main bearings. The huge mains create enclosed compartments around the crank web pairs that straddle each rod. Unlike the Merlin with its deep open sump and 2x-1x pumps, the Offy's connecting rods whip up much of the oil that would otherwise collect on the floor drains for removal. The resulting oil storms inside those four compartments also fill the bottoms of the cylinders directly above them and challenge the oil rings. The only oil that can be removed by the scavenger pump is that which falls out of suspension on the floors of the drains.

I originally began with 50 mL of 10W-30 in the oil tank with an expectation that the scavenging pump would always leave about 5 mL inside the oil pump, crankshaft, and scavenger return passages. At the end of the run there was some 27 mL still inside the engine. Most of it had likely been in suspension until the engine was shut down. Upon removing one of the crankcase side covers I found the oil levels in all four drain areas well above the connecting rods. This was the oil that was feeding the smoky exhaust.

The solution, of course, is to break-in the rings for the best possible piston oil control. If an unacceptable amount of smoke persists, the amount of oil in the tank can be reduced while carefully watching the oil flows in the oil tank hoses during running. If necessary, oil can be removed from the tank up to the point where the flow in the tank's return line is affected.

Before continuing, I decided to open up the crankcase breathers on the exhaust side to also make them functional. In addition, I milled a groove on the inside faces of the covers to connect the breather ports and make them more available to the two compartments in the middle of the engine. Although crankcase pressure pulses are prominent in the breather hoses while running, there's little to no oil inside them. In order to reduce the indoor smoke during the rings' break-in period, the amount of oil in the tank was reduced to 25 mL. After a number of short runs and nearly four ounces of gasoline, the smoke has been steadily decreasing.

Remarkably, the only engine leak I've found so far has been an exhaust leak. A slight warp in the silver-soldered stainless flange was discovered and ground away. The ports underneath the flange had their own story to tell. All four were equally wet with oil, but the rear three were also sooty. Those three cylinders had obviously been running rich compared with the front cylinder.

A magnetic dipstick made up from couple brass tubes and cylindrical magnets was Loctited to the oil tank lid.

The current plan is to continue with the brief indoor break-in runs until the exhaust is running clean before I begin playing with the carb adjustments. Hopefully my next post will include a video of a well running engine.

In the meantime, ... brrrr ... - Terry

I'm not sure if a four carb setup having 8 individual adjustments on a quarter scale engine having a marginal cooling system inside a shop with a 20F fluctuating temperature can ever be fully optimized, but I've reached a point of diminishing returns while trying. Attempting to methodically tune four carburetors by measuring their individual manifold vacuums proved to be inconsistent and inconclusive. I finally obtained results I can live with using the 'seat of my pants'.

The engine starts easily, hot or cold, and idles reliably at 1100 rpm. Although I've not been interested in finding out how fast it will go, I've routinely rev'd it to 5000 rpm. The exhaust is relatively clean except for a puff of oil smoke when the throttle is blipped. At the time this video was made, the engine had accumulated a couple dozen short runs with only a few lasting several minutes. Most wouldn't consider it broken in yet.

The Offy's coolant system could be further improved with a constant displacement pump as suggested by Ron. (Frankly, I'm just glad to have wound up with a coolant system that doesn't leak.) The pump that I designed has kept the surface temperature of the head below 155F during the longest (five minute) run that I've made. The coolant warms up quickly even at idle, but with the huge volume involved, the radiator fans weren't necessary after all.

The only oil leak that's shown up has been at the crankshaft's exit through the front cover. An o-ring used as a shaft seal was supposed to have been installed there, but after several tries, I couldn't keep it in place while the cover was being installed over the crankshaft. I eventually decided to omit it.

My real contribution to the project has been the split crankcase which I highly recommend to anyone who decides to build one of their own. Unfortunately, producing drawings wasn't part of my process since I created and worked solely with SolidWorks 2010 models.

I'd like to thank Ron for sharing his drawings for the Offy. Without actually building one, it's difficult to appreciate the effort that must have gone into producing such a faithful model of the original engine. The amount of error-free complexity is humbling especially considering the primitive 2D CAD tool that was used 20 years ago to produce it.

And so, this will likely be my last post associated with what has turned out to be an eighteen month build. I'm not yet sure what my next project might be or if there will even be a next one. The Offy has not only been very challenging but a lot of fun. - Terry

Amazing work as always, I have read this thread from start to finish several times now in readiness to starting my own offy but scaled x2 size. The end product is a beautiful piece of work and this has been the best thread I've read so far. Thank you for the insane detail and lengths you went to and documenting them for us all to read.
Yup. That's the Offy !!. Ive heard Ron's many times at shows. Such a wonderful sound.
Fantastic work Terry. I'll miss all the detailed writings about your work on this one and looking forward to how all the pieces go together. I've learned a lot of things since the beginning of this build. I'm sure the next one will just as educational.
I'm not sure if it was the Offy but I remember Ron describing how he tuned an engine by removing the exhaust pipes and looking in each exhaust port where I believe he tuned by the color of the flame (or whatever is to be seen there) and tuning them to all be the same. Rich / lean blue / orange. I'm not sure.

Great stuff (as usual).
Sorry to see the white stuff blanket your area & create so much distress, Terry. Hope you stay safe.

I sourced some clear silicone tubing to coupling sections of my radial induction pipes & subsequently discovered the tubing can also be had in colors including black just in case you cared for scale-ish appearance. No quarter scale hose clamps though LOL

I'd be interested to hear how you went about multi-carb tuning. I have a feeling my next project will require that knowledge. I've seen a few videos of people tweaking the carbs back & forth at low & high speed, occasionally holding holding a finger over exhaust (oilyness? temp?). These were glow mind you but the process looked mysterious.
What can I say? Just beautiful! Must be satisfyingly after so much work seeing run so well.

Terry, are you running the engine in a close shop? Is 20F out. Last time I did that I I felt nauseous and headachy.
Thank you Terry. It has been a wonderful and humbling experience following along with your build. I have learn so much from your detailed explanations, photos and CAD models. I can't tell you how much I appreciate the time and effort you put into the build log in addition to the time actually modeling and building the Offy. I wish you the best, you are a true artist.
Thank you for the great write ups and pictures of your progress! Some of the best work on the net!
I understand why my sprutcam post lists you as one of the authors!

Thank you for another great and inspirational build of another legendary engine.

You have so many skills that I always feel like it's Christmas when I see you have put up another post.

I also thank you for the generous way you respond to the questions you get asked.

Thank You for posting.


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