A new attempt at making piston rings

[Brian Rupnow]

I have just received and read thru the Trimble method of making piston rings. It is very well done, and a bit "long haired" so it will require a bit of concentration to re-read and fully understand exactly what Trimble is saying. He treats the subject "in depth" and does provide very good guidelines for making compression and oil rings for small engines. I would recommend it to anyone who has had trouble making their own rings.---Brian

 

[Brian Rupnow]

WOO-HOOOO---I just got my second covid shot this morning. Wife and I drove to a nearby town and after they confirmed that we had our first shot in early march and that we were "essential caregivers" to my 100 year old mother they went ahead and injected both of us. Arm is not sore, no hives, no sick feeling (at least so far).

 

[petertha]

Since this post seems to be a current collection point for ring making methods & we seem to be on a roll, I’m going to throw out some cartoon visuals of things I’ve wondered about regarding the heat setting aspect. Brian if you feel this is more appropriate in another post, I’m happy to relocate. And what follows is above my pay grade so I may be barking up the wrong tree. I’m trying to be careful here because others have constructed rings using alternative heat set methods that obviously work. Now do they work to 70% of what a Trimble ring could have yielded or 105% we will likely never know. I’m just trying to provide common starting point visuals for discussion purposes.

But if one assumes the Trimble heat method set as being the correct starting point basis for the underlying theoretical reasons he mentions, it might be interesting to visualize deviations to that. This cartoon sketch shows my takeaways from the SIC article. The ring is cleaved, not slit sawed. It is supported on a dowel pin of calculated diameter. The dowel tangent contact point occurs on the ring’s neutral axis. This arrangement is by intent & comes about by the resultant mandrel diameter, dowel diameter & center distance using his formulas. So, the reaction force aligned to neutral axis. Even heat is then applied with ring supported in this manner. The intended result is once the ring is heat set, it exerts consistent pressure radially to the bore wall along its circumference once installed in the bore.

(My visualization) if you unfolded the ring, applied heat & the reaction force to the ends it would become some slightly altered dimension once heat set. But the takeaway is that the force acts through the neutral axis while ring is hot & relaxing, so the ring strip would stay in a straight line. So theoretically this ‘matches’ a similarly unfolded bore. For reference this unfolded ring strip would be 3.14” long with rectangular section 0.043” x 0.023”. A pretty skinny, delicate stick of cast iron.

 

[petertha]

Here I’m showing another heat set mode. Identical ring OD & thickness, but this time supported on a plate of same thickness as dowel and again evenly heated. Whether it drops off or just heated is yet another variable but let’s just say perfectly heated. It looks very similar but one could visualize the reaction forces applied at an angle, not consistent with neutral axis. The ring is allowed to relax under heat & freely adopts its new shape because its only supported on the ring corners. But visualizing as a strip, it is no longer straight, it becomes curved. Using the analogy of fitting to a similarly unfolded straight bore, it would result in unequal bore pressure. Or likely worse if it bridges across bore resulting in non-contact annulus. That’s exactly what the Trimble Fig-12 graph & description was all about as I mentioned in #271 post. His graph shows an open gap between ring & bore of ~0.003” on a 1” OD.

 

[petertha]

Here is a circular dowel support with the ‘heat until it drops off’ method. I’m having trouble visualizing this comparatively. Maybe it was supported near the neutral axis for majority of heating? Or maybe the last point of contact was the outer ring corner which might be a variation of the plate gap example? What is probably evident is that the equivalent gap is going to be influenced by the (top view ring) thickness & dowel diameter combination. The Trimble dowel diameter is not equal to the maximum ring gap opening, its whatever it took to position the contact tangent at the neutral axis at the appropriate opening. So that’s what his fixture was about – controlling these variations.

 

[petertha]

Lastly heat application. One can visualize if the heat s not evenly distributed across the ring while setting, the ring could take on non-uniform shape on that basis alone. I could see this particularly happening with a high heat torch in ring suspended mode where say the mid-section gets hotter than the open ends. Or maybe the gap support / plate acts as a heat sink so is colder there. Eventually the ring drops off whenever the gap has sufficiently enlarged, but that could occur prematurely (and incorrectly) with a hotter mid section.

 

[petertha]

This is what I was visualizing regarding the unfolded heat set ring to the unfolded bore. Straight to straight is good. Curve to straight is bad. Hope this makes sense.

 

[Brian Rupnow]

Perertha--I think your posts are right at home here. I have read thru the Trimble method about three times now, and I probably can get my head around about 80% of it. Only four things really jump out at me. #1---He uses a calculated diameter of round pin on the neutral axis of the ring as his "gag" when spreading the rings for heat treat. #2--He clamps the rings flat in a fixture for heat treating, which makes sense, because that keeps the two ends of the ring in perfect alignment whereas spreading the rings on a "gag" but not clamped flat does allow the rings to "squirm" a little bit and become misaligned. #3--He uses an expanding collet style fixture to mount the rings on one at a time to deburr the inner and outer diameter of the rings on the "blind side" which couldn't be deburred when parting off from the original parent stock. and #4--He is quite adamant about not trying to do any further work on the o.d nor on the i.d. of the ring before mounting it on the piston. To me, his rings look awful damned thin at 0.022" on a 1" piston. I could not make the 1/16" wide rings as purchased from Debolt work for me. That is probably my fault, not Debolt's. I am buying a 1 mm (0.039") wide grooving tool to put the ring grooves into my piston, I'm going to make my new piston from aluminum, not cast iron, and my compression rings will be made 0.038" wide x 0.038" radial thickness.----Brian

 

[Gordon]

Trimble says that the width is critical only because of the friction against the cylinder so a few.001 is not going to make a lot of difference within reason. If you make the rings.038 thick you are into the. T/B area on his diagram which makes wall pressure under 30 psi which he says is too low.

 

[petertha]

I've wondered about this too.

Article says '4-cycle engine to develop 10 psi suction during intake strokes under low throttle, high rpm operating conditions. Using a safety factor of three to allow for our ignorance with respect to uniformity of pressure exerted by a less than perfect ring in a less than perfect cylinder'.

Multiplying 10 psi by 3 = 30 psi is what sets the graph constraint curve. Which btw I eyeballed off his graph to mimic. I have no idea if lower SF would look like this sketch, parallel-ish lines at 20 & 10 psi which would open up the target window. But I'm not clear if this is solely regarding induction effect - mitigating I guess reverse blow-by where you are sucking charge past the rings? Or if this is bundled with overall ring seal force. For example 30 psi resting ring pressure + 50 psi behind ring pressure = 80 psi total sealing pressure under combustion stroke. Vs say 10 + 50 = 60 psi & now maybe entering range of blowby if the combustion pressure was say 75 psi. I have no idea if these values are reasonable, just pulling numbers from my rear end. Maybe a low CR hit-n-miss this is OK but not for his V8 & similar engines which might explain variable results?

 

[Brian Rupnow]

I think Mr. Debolt probably knows what he's doing, and he sells a ring that is 1/16" wide for a 1" piston. I have never seen a subject that brings in such widely varying answers which all seem to work---or not!!!

 

[Gordon]

I am also just amazed. One person cuts them with a hack saw and heat treats them with a BIC lighter and they work just fine. The next person holds everything within.0005 heats them to+/- 5* and they will not work. Moon phase or black magic?

 

[Nerd1000]

I am also just amazed. One person cuts them with a hack saw and heat treats them with a BIC lighter and they work just fine. The next person holds everything within.0005 heats them to+/- 5* and they will not work. Moon phase or black magic?

Perhaps you need to make an appropriate sacrifice to the piston ring gods? I think some blood (from accidentally cutting yourself on a burr) and a brief prayer ("Ouch, darn it!") might appease them.

 

[DavidLloyd2]

The first thing that struck me on the video is that he made the tube he was going to part the rings off of "a nice slip fit" inside the cylinder. Even after doing measurements to try to make it the SAME diameter as the cylinder
The rings are therefore smaller than the cylinder. A problem before he even gets going. This and many other "close enough" errors are what leads to problems.
He also did the light test without a mask to cover the central hole. The mask is a must because staring into a bright light you're never going to see tiny pin pricks or slivers of light leaking around the rings which is what you're looking for.
And yes he never mentioned the ring gap.

BTW Who the heck is Earl (the Guy this Guy is apparently following) and why should we believe Earl or this Guy.

Over IMHO I would rate this video as just another one to add to the pile of sketchy ring making methods not to be followed.

I would rate this video as another rabbit hole

 

[dsage]

**** Caution ****
I believe it is a well documented error in the Trimble article that he got the temperature wrong for the heat treating.
He had it too high.
Do some further research and others can comment here. I won't quote a temperature here for fear of getting it wrong.
Terry:
Maybe you can quote the proper figure. Or link back to where you have already quoted it.
I know that the proper (lower) temperature amounts to a very dull red glow of the fixture in a dimly lit room. Very evenly applied, preferably using a furnace over a period of time.
Brian:
Please get this corrected temperature figure before you proceed. And post it here to make this complete.

 

[Nerd1000]

Random thought: two compression rings is standard nowadays, but prior to the 1950s it was pretty common for engines to have three due to the inferior precision of manufacturing in those days causing poorer sealing.

Would it be worth adding a third ring if you're having compression problems?

 

[mu38&Bg#]

Why not go with 7 or 10 rings? Production model aircraft engines have had one ring for a very long time. Cylinders and rings have to be made correctly to seal. Adding more parts that don't seal won't help.

 

[Gordon]

If I remember correctly, not a given, only one of the three rings was a compression ring. One was to lubricate the cylinder wall, one was to seal the oil in the crankcase from getting into the combustion chamber and one was for compression. The only justification for more compression rings is the chance that one of them would work properly.

 

[petertha]

Trimble used 2 compression rings and 1 oil control ring on his SIC article V8 but I don't recall reading his justification for 2 vs 1. He discusses his testing results & requirements for oil control ring, but likely had a lot to do with his pumped/distributed oil lubrication system.

 

[minh-thanh]

Brian!
Maybe you'll hate me - Again, Cylinder
You already have a cylinder (not just one)
Make the engine run with ringless pistons (make aluminum pistons because aluminum is easy to work with)
If the engine has no compression, lap that cylinder and make a new aluminum piston
With a little of my experience , the cylinder is the most important and the hardest part - which of course everyone knows .
So first you have to make sure your cylinders are good enough, otherwise you buy rings or you make rings they are also hard to compress.