# Piston rings Trimble

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#### Gordon

##### Well-Known Member
HMEM Supporting Member
I am sure that some of you have a better understanding of the math involved in making rings using the Trimble method so I have some questions. The pin diameter is determined by .150 x Bore. This obviously gives some pretty strange numbers which may or may not be close to a standard rod size and trying to turn a rod to something like .1125 diameter is pretty difficult. In that instance that is pretty close to .125 which probably is close enough for a gap while heat treating. However that change will probably also change the mandrel diameter and perhaps the offset. Trimble has some pretty specific formulas for determining these sizes but I am not sure where the rather strange numbers come from. I am sure that it relates to the inside circumference and ring thickness but I am not following the math here. Can someone who is a better mathematician than I am enlighten me or is pin diameter something which must remain per formula?

Gordon

#### stevehuckss396

##### Model Engineer
Project of the Month Winner
HMEM Supporting Member
You can purchase hardened drill blanks. They are the same size as all the standard drill bits. That should get you within a thou or 2.

Do not do this with the close enough mentality. Try to stay within .001 with all your diameters if possible. The tighter you keep to the numbers the happier you will be with the quality of the rings.

I usually make my fixture a little longer so I can put a few spare rings on the mandrel in case I break one or one is deformed somehow. Make a few spares.

#### Gordon

##### Well-Known Member
HMEM Supporting Member
You can purchase hardened drill blanks. They are the same size as all the standard drill bits. That should get you within a thou or 2.

Do not do this with the close enough mentality. Try to stay within .001 with all your diameters if possible. The tighter you keep to the numbers the happier you will be with the quality of the rings.

I usually make my fixture a little longer so I can put a few spare rings on the mandrel in case I break one or one is deformed somehow. Make a few spares.
I realize that accuracy is the key to making rings. I am just speculating that by making the pin slightly bigger and adjusting the diameter of the mandrel and the offset would give the same result. The goal is to have the pin tangent to the center of the ring flat. I do not understand the math involved and wonder how he came up with things like 1.0072+.1569 etc. I am sure that these numbers are related to circumference and pi etc. Just increasing the gap a few thousands during heat treating should not make any difference as long as the associated mandrel size and offset were compensated. I have read his entire article a few times and do not understand where he came up with these numbers.

I am not trying to be difficult but I would like to understand the reasoning behind these numbers. So far my results on using this method has not been great in spite of trying to be as accurate as I can be with my less than super accurate equipment and my less than super accurate talent.

#### petertha

##### Well-Known Member
HMEM Supporting Member
This link, post #63, shows a screen grab of spreadsheet I wrote. It takes the Trimble formulas/methodology & shows the resultant optimal size on a graph. You can vary the input parameters in the cells & see how the sizing dot moves around relative to the various constraints discussed in the SIC article. If you alter the pin diameter by a bit, the dot moves one way. If you vary thickness it moves a different way. If you have a different material strength assumption, the constraint curve itself moves. To answer your question, my guess is he came up with those sizing factors to put it somewhere in the middle.

Its been a while since I looked at this but you can see my attempt at back-calculating a commercial RC engine ring which obviously 'works' in its intended applications. But there is no way of knowing exactly what their material is & other manufacturing factors so should be viewed as a guesstimate.

There has been a lot of discussion on forum about the Trimble method including some further insight/opinions about the underlying assumptions. A search should provide many happy hours of reading LOL

#### Gordon

##### Well-Known Member
HMEM Supporting Member
This link, post #63, shows a screen grab of spreadsheet I wrote. It takes the Trimble formulas/methodology & shows the resultant optimal size on a graph. You can vary the input parameters in the cells & see how the sizing dot moves around relative to the various constraints discussed in the SIC article. If you alter the pin diameter by a bit, the dot moves one way. If you vary thickness it moves a different way. If you have a different material strength assumption, the constraint curve itself moves. To answer your question, my guess is he came up with those sizing factors to put it somewhere in the middle.

Its been a while since I looked at this but you can see my attempt at back-calculating a commercial RC engine ring which obviously 'works' in its intended applications. But there is no way of knowing exactly what their material is & other manufacturing factors so should be viewed as a guesstimate.

There has been a lot of discussion on forum about the Trimble method including some further insight/opinions about the underlying assumptions. A search should provide many happy hours of reading LOL
I also have a similar spreadsheet but there is no formula which changes M or O when D is changed. T and W obviously can be changed within reason but I am trying to find the relationship of M and O to a change in D.

#### petertha

##### Well-Known Member
HMEM Supporting Member
Again, I haven't looked at this in quite a while, but here I've exposed my formulas
If you spot any errors let me know

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• SNAG-2-8-2021 0000.jpg
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#### petertha

##### Well-Known Member
HMEM Supporting Member
And there was another spreadsheet floating around, had some of the background assumptions laid out a bit different.
I cant say I understood it all. Some of the posts I collected over time provided some insight

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• SNAG-2-8-2021 0001.jpg
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#### tornitore45

##### Well-Known Member
HMEM Supporting Member
There is nothing magic about the the Gap Setting Pin dimension. The gap is established to provide springiness = radial force. So a 1% error on the diameter translates to 1% difference in the radial pressure the ring exert initially onto the cylinder. Once ignition take place the ring is pushed from behind by the expanding gas fit a force much greater than the elastic force of the ring. Reserve your attention for accuracy to the ring OD and a close but free fitting of the ring in the piston groove. Also deburr both side of the ring lightly without too much rounding. Deburr the left side when you are half way to parting it loose.

#### Gordon

##### Well-Known Member
HMEM Supporting Member
Again, I haven't looked at this in quite a while, but here I've exposed my formulas
If you spot any errors let me know
Now I feel foolish. I have generated the same spreadsheet and I have looked at the formulas several times and kept looking at the D in the formula and interpreting it as diameter instead of dowel in my mind. Using the same formula and using a standard value for dowel diameter it will recalculate the value of M and O. I still do not understand where the formulas came from but I can blindly follow them. Actually I do that a lot because there are so many things I do not understand and do not necessarily have to in order to use them.

Thanks

#### Gordon

##### Well-Known Member
HMEM Supporting Member
I just reworked my spreadsheet so that it calculated the value of M and O based on a different value of D.

On my .750 dia piston the calculated value of D = .1125. Changing that to .125 made M .002 bigger and O .002 bigger. I am not sure if that is enough to change the actual ring since there is a small gap between the ring ID and the mandrel.

I have been breaking the ring using a heavy duty nipper instead of the cleaver shown in the article. The cutting edges are directly across from each other so it would seem to accomplish the same thing. I will have to think about that. Trimble said that the cleaver was important but he was comparing it to breaking the ring in a vise or over an edge.

#### mrehmus

##### Well-Known Member
HMEM Supporting Member
George Trimble, as you may know, was a mathematician working for one of the aerospace companies and who was working on anti-gravity when he died. In the 1960's he was loaned to NASA because their moon landing program was in serious trouble. He straightened them out and we, as you know, we got to the moon.
Dwight Giles, the resident ring-maker of the Bay Area Engine Modelers uses the Trimble method with a few changes. He heat treats the rings at 1150 degrees F for 1 hour in a SS bag. When he is machining the rings, he makes them 0.001" larger in diameter than the Trimble spec and then slits them with a 0.006" saw.
His rings almost always work well from the first startup and do not smoke.
He does insist that the ring grooves in the pistons are very important to the success of the rings in sealing the bore.
Most of his engines have oil control rings which are a standard ring with oil grooves cut through the ring. The 0.006 saw is used to make the cuts.

#### stevehuckss396

##### Model Engineer
Project of the Month Winner
HMEM Supporting Member
I am not trying to be difficult but I would like to understand the reasoning behind these numbers

Not saying anything negative. I was simply trying to express the importance of staying to the numbers.

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