piston fit

[Jeff Lutz]

I had the privilege of spending a day with Mart Davis in the shop. he showed me a liner pincher that was made with a tapper cut into it.

He used it on one of my RS7 sleeve. he would turn the liner to preset marks on the jig using the centering pin notch as a reference. he used a torque wrench to tighten it at every mark.

He said this would give a .002 taper from the top of the ex to the top of the liner.

I still run this sleeve with new piston from CMB and it holds the pinch for a very long time.

The only reason I have changed pistons one time was the skirt broke at the bottom of the ex side.

For those of use that can't make are own liners we are stuck with what ever the factory gives us.

Now this specialized pincher dose give a chance to try different tappers.

But building one is probably just as involved as making the sleeve.

David

David, I have 2 stock 101 new p/l to check I'll get back to you and tell you were they fall, stock with no lapping and honing. You are doing good building custom pistons for the 101's. The piston walls are thin and not much material under the wrist pin boss. I have also broke one. Engine had just hit the pipe, and it did not stick, it just broke. I have not stuck or burned a piston because of some mods that I do to the motor.The old purple head 1 inches had more meat under the wrist pin, and I never broke anything on one of those motors, we just wore them out. Maybe with time cmb will modify the piston mold to make it right.

 

[Marty Davis]

I had the privilege of spending a day with Mart Davis in the shop. he showed me a liner pincher that was made with a tapper cut into it.

He used it on one of my RS7 sleeve. he would turn the liner to preset marks on the jig using the centering pin notch as a reference. he used a torque wrench to tighten it at every mark.

He said this would give a .002 taper from the top of the ex to the top of the liner.

I still run this sleeve with new piston from CMB and it holds the pinch for a very long time.

The only reason I have changed pistons one time was the skirt broke at the bottom of the ex side.

For those of use that can't make are own liners we are stuck with what ever the factory gives us.

Now this specialized pincher dose give a chance to try different tappers.

But building one is probably just as involved as making the sleeve.

David

David:

I also don't have the ability to machine my own pistons and liners and must rely on other methods to get a good fit. As you know, I love the .21 engines and in order to get good performance, I must have good pinch at the top. The very best liners are ones that the piston just stops at the point of pinch. No mushy feeling, it just stops. I started using Brad Christy's pistons in all my MAC & CMB .21's and they lasted for a long time (only thing that usually resulted in having to replace them was detonation).

With the Nova Rossi's and the new Go Engines, the material in the pistons is great and they hold a seal a long time. I did find out the hard way that with a head button fit into the liner that was too tight, I lost any pinch that I had VERY QUICKLY. I found that I needed at least .001" clearance on each side of the head button (360 brass) to keep the pinch from going away. Hard Lesson Learned....

I did hang out with Mike Wisniewski many years ago and he taught me how to get a great fit between the liner and piston. He made a cast iron lap that he could put the piston into that had been bored with the same taper that the liner had. The piston would go into the lap maybe 3/4 of the height of the piston so that it didn't rock much. He was able to make a lapped area right at the top of the piston that exactly matched the taper of the liner. This was a PERFECT fit in that area. He carefully measured the liner taper from the top of the exhaust to tdc and put that taper into the lap. He used non-imbedding garnet lapping compound to do the lapping. If you draw this in large scale, you will see that the top of the piston has a band right at the top that has the same taper as the liner and when you push the clean and dry piston into the liner it just STOPPED immediately. These were some of the best engines that I ever ran. Now, I use some of the Helical Lap Companies lapping compound that Ackerman uses for years on pistons. I forgot to say that I usually first made the piston really round with a helical lap before using the Wisniewski lap. I would like to find a source for some chunks of cast iron to make some more of these laps - anyone???

I guess this is half way between Jim Allen and Jeff Lutz systems since I am limited in what I can achieve with my tooling. BUT, this is a system that works very well too.

 

[dwilfong]

I had the privilege of spending a day with Mart Davis in the shop. he showed me a liner pincher that was made with a tapper cut into it.

He used it on one of my RS sleeve. he would turn the liner to preset marks on the jig using the centering pin notch as a reference. he used a torque wrench to tighten it at every mark.

He said this would give a .002 taper from the top of the ex to the top of the liner.

I still run this sleeve with new piston from COMB and it holds the pinch for a very long time.

The only reason I have changed pistons one time was the skirt broke at the bottom of the ex side.

For those of use that can't make are own liners we are stuck with what ever the factory gives us.

Now this specialized pincher dose give a chance to try different tappers.

But building one is probably just as involved as making the sleeve.

David

David, I have 2 stock 101 new p/l to check I'll get back to you and tell you were they fall, stock with no lapping and honing. You are doing good building custom pistons for the W's. The piston walls are thin and not much material under the wrist pin boss. I have also broke one. Engine had just hit the pipe, and it did not stick, it just broke. I have not stuck or burned a piston because of some mods that I do to the motor.The old purple head 1 inches had more meat under the wrist pin, and I never broke anything on one of those motors, we just wore them out. Maybe with time comb will modify the piston mold to make it right.

Well I won't comment on that. h34r:

 

[Marty Davis]

I had the privilege of spending a day with Mart Davis in the shop. he showed me a liner pincher that was made with a tapper cut into it.

He used it on one of my RS7 sleeve. he would turn the liner to preset marks on the jig using the centering pin notch as a reference. he used a torque wrench to tighten it at every mark.

He said this would give a .002 taper from the top of the ex to the top of the liner.

I still run this sleeve with new piston from CMB and it holds the pinch for a very long time.

The only reason I have changed pistons one time was the skirt broke at the bottom of the ex side.

For those of use that can't make are own liners we are stuck with what ever the factory gives us.

Now this specialized pincher dose give a chance to try different tappers.

But building one is probably just as involved as making the sleeve.

David

harder to make than a straight one.just have to mount the block in a lathe and cut the bore with the compound

Steve:

EXACTLY using steel for the lap and a 4 jaw chuck. I make a piece of aluminum that I turn using tyhe compound and moving until I get the correct taper. Then I bore the squeezer. with that setting.

I am not sure that my tapered squeezer is better than the one that Steve sells. I have used both and I am not sure I can tell the difference. For less than $50, I think that I will use Steve's for my other engines. After running the new Go Engines for a little bit, and using Steve's squeezer, I am happy with the results.

What a great thread..... People that are curious about the really technical side of the hobby can gain a LOT from this exchange!!

 

[dwilfong]

I had the privilege of spending a day with Mart Davis in the shop. he showed me a liner pincher that was made with a tapper cut into it.

He used it on one of my RS7 sleeve. he would turn the liner to preset marks on the jig using the centering pin notch as a reference. he used a torque wrench to tighten it at every mark.

He said this would give a .002 taper from the top of the ex to the top of the liner.

I still run this sleeve with new piston from CMB and it holds the pinch for a very long time.

The only reason I have changed pistons one time was the skirt broke at the bottom of the ex side.

For those of use that can't make are own liners we are stuck with what ever the factory gives us.

Now this specialized pincher dose give a chance to try different tappers.

But building one is probably just as involved as making the sleeve.

David

David:

I also don't have the ability to machine my own pistons and liners and must rely on other methods to get a good fit. As you know, I love the .21 engines and in order to get good performance, I must have good pinch at the top. The very best liners are ones that the piston just stops at the point of pinch. No mushy feeling, it just stops. I started using Brad Christy's pistons in all my MAC & CMB .21's and they lasted for a long time (only thing that usually resulted in having to replace them was detonation).

With the Nova Rossi's and the new Go Engines, the material in the pistons is great and they hold a seal a long time. I did find out the hard way that with a head button fit into the liner that was too tight, I lost any pinch that I had VERY QUICKLY. I found that I needed at least .001" clearance on each side of the head button (360 brass) to keep the pinch from going away. Hard Lesson Learned....

I did hang out with Mike Wisniewski many years ago and he taught me how to get a great fit between the liner and piston. He made a cast iron lap that he could put the piston into that had been bored with the same taper that the liner had. The piston would go into the lap maybe 3/4 of the height of the piston so that it didn't rock much. He carefully measured the liner taper from the top of the exhaust to tdc and put that taper into the lap. He used non-imbedding garnet lapping compound to do the lapping. If you draw this in large scale, you will see that the top of the piston has a band right at the top that has the same taper as the liner and when you push the clean and dry piston into the liner it just STOPPED immediately. These were some of the best engines that I ever ran. Now, I use some of the Helical Lap Companies lapping compound that Ackerman uses for years on pistons. I forgot to say that I usually first made the piston really round with a helical lap before using the Wisniewski lap.

I guess this is half way between Jim Allen and Jeff Lutz systems since I am limited in what I can achieve with my tooling. BUT, this is a system that works very well too.

Thank for the great info Marty.

especially the info of the fit of the brass head buttons.

David

 

[Marty Davis]

I would be interested in hearing from Jim, Jeff and Steve about what they consider to be the most appropriate brass to use as head buttons on our production engines. I realize that liners may be manufactured using different brass, but they must be similar. I would like to find a brass that is easy to get and has a similar expansion rate as the production liners (or maybe they need to expand less). Thoughts..........

The different brass that is easily available from McMaster is shown here:

http://www.mcmaster....-metals/=ipcj2v

Your choices......?

 

[Jim Allen]

I would be interested in hearing from Jim, Jeff and Steve about what they consider to be the most appropriate brass to use as head buttons on our production engines. I realize that liners may be manufactured using different brass, but they must be similar. I would like to find a brass that is easy to get and has a similar expansion rate as the production liners (or maybe they need to expand less). Thoughts..........

The different brass that is easily available from McMaster is shown here:

http://www.mcmaster....-metals/=ipcj2v

Your choices......?

I tested #360 brass for head buttoms when I made liners of the same alloy. Then I realized that the brass cylinder with the chrome plating, had a slightly larger expansion rate when compared to the silicon aluminum pistion's expansion rate. From then until now I have made the things out of 2024-T3 aluminum. Didn't you say you had to give the brass .001" clearance to keep from blowing the taper out at the top? Why do you think that happened? The aluminum buttons I make fit tightly in the liner.

 

[Marty Davis]

I would be interested in hearing from Jim, Jeff and Steve about what they consider to be the most appropriate brass to use as head buttons on our production engines. I realize that liners may be manufactured using different brass, but they must be similar. I would like to find a brass that is easy to get and has a similar expansion rate as the production liners (or maybe they need to expand less). Thoughts..........

The different brass that is easily available from McMaster is shown here:

http://www.mcmaster....-metals/=ipcj2v

Your choices......?

I tested #360 brass for head buttoms when I made liners of the same alloy. Then I realized that the brass cylinder with the chrome plating, had a slightly larger expansion rate when compared to the silicon aluminum pistion's expansion rate. From then until now I have made the things out of 2024-T3 aluminum. Didn't you say you had to give the brass .001" clearance to keep from blowing the taper out at the top? Why do you think that happened? The aluminum buttons I make fit tightly in the liner.

Jim:

I realize that the expansion is more than I want. I used to make all my buttons from 2024 but switched to brass because of the detonation problem. I am looking for a brass that is closer to what I want. Maybe I should look at 2024 and see the expansion of that and then look for the closest brass!!! Sometimes it is more simple than we realize. :wacko:

I always thought that a button that didn't fit tight would allow charge to blow up the sides, thus the reason that I always made them a snap fit.

 

[Jim Allen]

Marty,

It's very hard to nail this down because the brass liner is chrome plated, two alloys, & the button is made of one alloy. I cannot find any information in my notes as to why I stopped using the brass. The thermal expansion of aluminum is 13.2 in/in/deg F & brass is 11.2 in/in/ deg F.

I would think the brass button with the brass liner would be okay, but maybe it doesn't work because of the chrome on the liner.

Think about what happens when the piston is made of high silicon aluminum. The thermal expansion of the piston is 8.3 in/in/deg F. The brass liner or aluminum liner with the chrome plate doesn't expand so much that the engine loses power or stops running.

Jim Allen

 

[Jeff Lutz]

I would be interested in hearing from Jim, Jeff and Steve about what they consider to be the most appropriate brass to use as head buttons on our production engines. I realize that liners may be manufactured using different brass, but they must be similar. I would like to find a brass that is easy to get and has a similar expansion rate as the production liners (or maybe they need to expand less). Thoughts..........

The different brass that is easily available from McMaster is shown here:

http://www.mcmaster....-metals/=ipcj2v

Your choices......?

I tested #360 brass for head buttoms when I made liners of the same alloy. Then I realized that the brass cylinder with the chrome plating, had a slightly larger expansion rate when compared to the silicon aluminum pistion's expansion rate. From then until now I have made the things out of 2024-T3 aluminum. Didn't you say you had to give the brass .001" clearance to keep from blowing the taper out at the top? Why do you think that happened? The aluminum buttons I make fit tightly in the liner.

Jim:

I realize that the expansion is more than I want. I used to make all my buttons from 2024 but switched to brass because of the detonation problem. I am looking for a brass that is closer to what I want. Maybe I should look at 2024 and see the expansion of that and then look for the closest brass!!! Sometimes it is more simple than we realize. :wacko:

I always thought that a button that didn't fit tight would allow charge to blow up the sides, thus the reason that I always made them a snap fit.

Years ago, I made a head out of 1045 carbon steel, just for the heck of it. It ran good in a cmb 45 evolution. All Ive got to say is I would stay with aluminum, and mod the engine to get rid of the detonation. I know this takes the fun out of running something different, but it is a band-aid for another problem. If I was going to use brass, I would use 360 brass, not bronze.

 

[dwilfong]

I have bin tossing around making a button from ductile iron. the only thing that concerns me is the corrosion with the water. would be a very high maintenance item.

Thinking this would give a more stable temp to the chamber.

Would the expansion rate be more than the sleeve?

 

[Terry Keeley]

Henry & I extensively tested the materials listed as well as A-390 & RSA-431, running the engines at WOT for 3 to 4 minutes. Different materials required different cold fits because of the different expansion amounts developed during engine dyno tests. The higher the silicon amounts, the tighter the cold fit must be. The Dispal-250, A-390 & RSA-431 we used had to be heat treated before final machining to prevent piston growth. The best alloy tested thus far is RSA-444 (30% Si). However, when using the RSA-444, the #360 brass previously used for liners had to be changed to #544 phosphor bronze. The engines with RSA-444 pistons & #360 brass liners sometimes could not be turned over by hand, but once started they ran fine. Changing to #544 bronze made cold fitting much easier.

RSA-444 works extremely well with #4032 aluminum liners. These two alloys are used in those very tight fitting .45 Nelson long stroke engines. When the liners have the correct taper amounts & the pistons have the correct top taper amouints, the engines would last forever.

Jim allen

Again, great info Jim, thanks.

Some of the pistons I made when I did this 15 years ago were from CMB blanks which when analyzed were close to A-390, they worked great. Also made a few from a 30% Si "mystery material" I got from a free flight engine builder in Australia, think it mighta been one of the Mahle variants, I had to be very patient breaking them in or they'd gall on the cold (intake) side easily. Here's a bad picture but you get the idea:

Looking forward to getting back to this, I'm convinced a very round, low taper liner with a mirror finish and a 30% Si piston is worth the effort and might even help me get outta the 1.9's, contrary to what someone here thinks... <_<

 

[Jim Allen]

Terry,

Henry & I spent many hours trying to determine exactly what caused piston galling or scuffing in engines that had plenty of oil (20%). Many engine builders thought the solution was in the use of higher silicon (30%+ Si) alloys for pistons. This turned out to not be true.

In the case of A-390 (12% Si), before final machining to size was done, the following heat treatment was required to prevent unwanted extra growth. The heat treatment used raised the A-390 to a T-7 strength.

1) 935 deg F for 8 hours-quench in boiling H2O with a glycol base- cool to room temperature.

2) 450 deg F for 8 hours- cool to room temperature.

Without this heat treatment A-390 could easily create the galling problem.

Engines that showed no signs of this problem on 3 to 4 minute dyno pulls at WOT, sometimes failed during actual lake testing. Eventually we realized three things contribute to piston scuffing or galling problems. 1) insufficient liner taper for the length of the stroke, especially from the top of the exhaust to the top of the stroke; 2) incorrect piston top tapers for whatever liner taper was used, evidenced by more than one wear band or no wear band at all; 3) to much cooling.

After finding the correct combination of these three things & with the use of a correctly designed steel roller rod, oil contents dropped from 15% to 6-8%. Nitro contents went from 65 to 80% & the HP went above 7. There have been no failures of any kind since we figured this out.

Jim Allen

 

[Andy Brown]

By what mechanism does brass reduce/eliminate detonation? If in fact it does?

Terry, the 35% dispal would be more stable, but it is more brittle.

 

[dwilfong]

By what mechanism does brass reduce/eliminate detonation? If in fact it does?

Terry, the 35% dispal would be more stable, but it is more brittle.

The brass dose not show the peppering like the aluminum. you have to read the piston top.

Dick made a statement about how when detonation starts in the chamber the aluminum turns to aluminum oxide adding oxygen to the combustion and making a lean burn even leaner. kind like it will feed on its self.

David

 

[Andy Brown]

By what mechanism does brass reduce/eliminate detonation? If in fact it does?

Terry, the 35% dispal would be more stable, but it is more brittle.

The brass dose not show the peppering like the aluminum. you have to read the piston top.

Dick made a statement about how when detonation starts in the chamber the aluminum turns to aluminum oxide adding oxygen to the combustion and making a lean burn even leaner. kind like it will feed on its self.

David

Wow! That small amount of consumed aluminum must produce a huge amount of oxygen. Sounds like we should make use of this "free" fuel. Just richen the needle to go FASTER! LOL

Now I can buy that the brass does not show the peppering. The melting temp of brass is twice that of aluminum plus it is generally harder, so it will not show the evidence of detonation so easily. That does not mean the detontion is not there.

 

[Jim Allen]

By what mechanism does brass reduce/eliminate detonation? If in fact it does?

Terry, the 35% dispal would be more stable, but it is more brittle.

The brass dose not show the peppering like the aluminum. you have to read the piston top.

Dick made a statement about how when detonation starts in the chamber the aluminum turns to aluminum oxide adding oxygen to the combustion and making a lean burn even leaner. kind like it will feed on its self.

David

Wow! That small amount of consumed aluminum must produce a huge amount of oxygen. Sounds like we should make use of this "free" fuel. Just richen the needle to go FASTER! LOL

Now I can buy that the brass does not show the peppering. The melting temp of brass is twice that of aluminum plus it is generally harder, so it will not show the evidence of detonation so easily. That does not mean the detontion is not there.

Andy, I guess where the peppering can be seen should be expected since the brass has a thermal conductivity 1/2 of aluminum.

 

[hugh hargett]

why dont you inject water? A perfect project for your lil aurduino.

 

[RodneyPierce]

Now I can buy that the brass does not show the peppering. The melting temp of brass is twice that of aluminum plus it is generally harder, so it will not show the evidence of detonation so easily. That does not mean the detontion is not there.

Andy, I always wondered about this myself. How JUST changing the material of the button, and making no other changes would magically rid the detonation that was taking place. I was always a bit skeptical about it. I think its still detonating, just not showing on the brass, there for not eating up the head like the aluminum ones do.

 

[dwilfong]

My thinking is the brass keeps the chamber temp more stable.