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Hi guys: I appreciate the application of the correct /best material for the job is required.
But (from what I remember being taught in the Engine Design office in Japan in the late 80s...) the big-end bolts are among the most highly stressed bolts in the engine, at Max engine speed, when the throttle is closed rapidly. They are designed to withstand the maximum stresses incurred, for a fatigue life of many millions of cycles. Therefore, in "production" engines, they always tend to be the highest grade steels, and tempered appropriately, to achieve the required lifetime and NEVER fail. Although, incorrect servicing, re-use of bolts, engine tuning (even caused accidentally by poor fuel and pinking, crashing tha car, etc.), can cause failures. But these are very rare and almost are all well outside the Warranty period. Warranty data can show show "no failures" year on year. (I read it for a couple of engines).
To have a "lower strength bolt" than required by the design, that "stretches before it fails", has actually failed by stretching, before it breaks. - And the higher stresses of a slack big-end from a stretched bolt will very rapidly cause it to fatigue and fail. (In engine testing we recorded every big-end bolt length before and after the longest durability tests on the dynamometer. Both off-the engine and as installed in the big-end assembly.).
But that is my "experience-based knowledge" and opinion.
So to re-iterate the "make it to the design" comment, if Brian R made his engine with 12.9 bolts, then those are the basis for anyone else making their own "copy". (Until proven that "lower grade" bolts are adequate..). Or course, most of this is immaterial to the model maker who's models are simply shiny ornaments, (Yes, I have a "display" model too.). But if my advice was that Brian has over-engineered the bolts, I would have justified that to you all by calculations at least, if I hadn't made the engine and tested it. (Which I have not). And yes, I assumed his bolts would be a "high grade" of steel.
What I have been presenting in the calculations, is an example of how the Designer thinks, before any metal is even ordered, never mind machined, built or tested. MANY problems are never seen down-stream of the design office, because the calculations say they won't work.
As a contributor to this website-of-"knowledge and best practice", I present my best knowledge, and recognise an expert will either confirm or correct me. - Hence I try to justify my opinions - sometimes with simple calculations. Therefore I always try and be considerate of every proposal, or counter-idea, because that way we can discuss it and all learn from what we agree is the right solution.
Minh Thanh has suggested he shouldn't chuck-in his ideas because they may be from insufficient experience (posts 226 & 235). But I think he should question us/me, as I am certainly not expert in many things that are discussed here. (And he makes things that work (His diesel engine) - and is not ashamed when they don't (his broken crankshaft), as he asks us for help.). And the learning from others is a great joy to me (so Thankyou ALL).
On the specific application of the "best" grade of steel bolts for these big-ends, I have put my ideas forward, but do remind you that I have not done the calculations of the loads on the big-ends, but have compared how different bolts can withstand stresses generally, so have only considered that "usual practice" (in industry) is for the high grade of steel bolts that Brian has selected.
I hope this makes sense?
K2
 

CFLBob

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This is hard to explain easily, but I've spent the last two weeks getting piston rings for this. Now I have a question for anyone bothering to keep up with this.

Brian suggests somewhere that this is a Vyton piston ring. I searched for these at McMaster-Carr and a couple of sources I can't remember (it has been about month) and while I could find Vyton I couldn't answer the question, "which blend of Vyton do you want?" I eventually went to the reliable source of cast iron rings, Otto Gas Engine Works (Dave Reed) and ordered three rings, assuming I needed one ring. I ordered the rings to the drawing width, 1.000" dia. by .093 wide.

My question is: should change over to two rings because of using cast iron instead of Vyton?

This is what the drawing looks like:

PistonRing.jpg


As you can see, there's one ring groove 0.170 edge to edge from the end of the piston. If I add a second ring, how much space should there be between them?

There's not enough room to put a second ring farther down (right) on the piston if it's spaced the width of the slot (.093") away from the existing ring. I did a quick sketch in CAD to check the room.

ClarenceTest.jpg


That construction line dropping down to the wrist pin hole shows that it cuts .020" off the wrist pin hole. I suppose I could just make the pin shorter so that it doesn't hit the piston rings.

I have no idea if the spacing should be the same as the ring width or if something smaller, like 1/16 or .050 is acceptable or even better. Or if I should shrink the 0.170 from the top down and move both rings and spacer to the left.
 

minh-thanh

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Hi !
If you use a cast iron ring , one ring is not a problem .
Some of my engines have pistons without rings and they still run, so don't worry too much about a ring or two.
If you want two rings, a distance of about 1.5 - 1.8 is fine
 
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Brian Rupnow

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Bob--the piston you made is only for Viton o-rings. If you plan on using cast iron rings, then your piston should look like this. Cast iron rings can be a real bear to get them to seal. Viton o-rings are a much easier method of sealing the piston. Since you have already ordered your cast iron rings, then you will have to make a new piston that looks like this.----Brian
nolIPB.jpg
 
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CFLBob

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Hi !
If you use a cast iron ring , one ring is not a problem .
Some of my engines have pistons without rings and they still run, so don't worry too much about a ring or two.
If you want two rings, a distance of about 1.5 - 1.8 is fine

Thanks. I have one piston with no rings (compressed air wobbler), one with two rings (Webster) and now this.

Brian,
the piston you made is only for Viton o-rings.

I haven't actually done this part of it, yet. I wasn't going to cut the groove until I had the ring in my hands. Every other feature is machined already, the oval and circular counterbores, and all.

I had sent you an email three weeks ago asking for more details on the Viton and your source. I figured you were on vacation somewhere else.


Bob
 

Brian Rupnow

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Sorry Bob--I never seen that email. I just talked to Hercules O-rings here in Barrie, and the o-rings are brown in color and have a 75 durometer hardness. They sell for 60 cents each, and you only need one o-ring on the cylinder.
 

CFLBob

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Sorry Bob--I never seen that email. I just talked to Hercules O-rings here in Barrie, and the o-rings are brown in color and have a 75 durometer hardness. They sell for 60 cents each, and you only need one o-ring on the cylinder.

Brian,


A little searching shows these at Grainger. Hercules OEM only listed Viton ETP in their options that I found. Does this look right?


Grainger-Viton-ETP.png


The only advantage to Grainger is that they're less than six miles from me. OTOH, it's a BIG difference from 60 cents each to two for $14.


Bob
 
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I like Brian's drawing of the piston with 2 cast iron rings.
For anyone interested - my understanding from working on "Industrial" jobs with pistons, iron, steel, PTFE, Labyrynth, O-ring, etc. :-
Viton is an incredible material - copes with high temperature (see manufacturers' specs), low friction, oil resistant, etc. The key to Brian's success (IMHO) is the radial clearance and precision sizing of the bore and ring groove. The generates adequate contact pressure to commence sealing, so gas pressure increases the seal- pressure. But not too much pressure to wipe-off lubrication from the bore. Follow Brian's design exactly within tolerance. But at higher ring speeds Viton will fail due to heat - From friction, and piston, and cylinder wall. But in these engines Brian has success, so should be OK.
Cast iron rings. Advantages - higher temperature resistance, more durable (no long term material degradation), "tangential load" tolerant, I.E. variability in the "spring" pressure of the ring in the bore affects friction, sealing, etc, but engines are very tolerant of this up to a max when the oil-film breaks down. An advantage is the conduction of heat from the hot piston to the cooler cylinder wall. Necessary on full sized engines at moderate to high loads.
Steel rings: Generally, a lower tan load is set on these than for cast iron. 2 zones: the continuous ring curve, and the ends of the rings. Tan load changes in the last 30 degrees towards the ring ends, so cast iron rings must wear in the bore (running-in, lapping, etc.) to develop the lower tan load towards the point so the ring Shap is a more perfect circle when in the bore. The steel rings are made with a deformed circular shspe, so when installed they exactly fit the bore with a uniform tan load. So no running- in is necessary from new. Steels are selected and nitrided for a very hard and durable surface, so last much longer than cast iron. They are usually much narrower, with a curved outer diameter face for tangential contact on the bore. = lower friction overall, better lubrication, etc.
Enough for now, (flat battery!) unless someone wants to understand Labyrynth seals, PTFE, etc.?
K2
 

CFLBob

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Good start on the crank, This what i use

That's what I used. Well, I just bought a couple of the parts of the set since I've had the lathe eight years and haven't needed one yet.
 

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