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dwilfong

dwilfong

Well-Known Member
Joined
Jul 1, 2008
Messages
5,968
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
 
dwilfong said:
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
Click to expand...
I could be wrong but I believe how much it grows depends on how much U feed it!!!!!!!!!!!!

dick
 
David,

Maybe I can give you an idea based on numbers taken from my .90 cu in engines & the Nelson engines. Piston top taper amounts from .001"/.100" to .003"/.100", per side, were tested. This means that the sealing area & the wear band would be .100" down from the top of the piston. Higher amounts of silicon did not change the final number, but did require a much tighter cold fit because high silicon (30%+) pistons expand much less. We found no difference in the HP amounts when comparing high silicon (30%+) to low silicon (16%+) amounts. However, high silicon pistons wear much longer than low silicon pistons.

I don't think there is a difference in expansion between a billet & a cast piston if they are made of the same alloy. However, some of the silicon aluminums that can be used for pistons require heat treating before final fitting to prevent growth problems during engine break in. Billet pistons will usually be much stronger than cast pistons because of the additional amounts of metal above the piston bosses & adjacent to the piston bosses that cannot be removed. This also allows billet pistons to be able to conduct more heat. Does this cause billet pistons to grow more in the piston boss area? We could find no evidence of this, but did see expansion problems in these areas due to wrist pin flexing in larger size engines.

Machine the parts, know the exact numbers you have machined & test, test & test.

Jim Allen
 
richard jones said:
dwilfong said:
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
Click to expand...
I could be wrong but I believe how much it grows depends on how much U feed it!!!!!!!!!!!!

dick
Click to expand...
I try to feed it a rich diet.

But you know how that goes when your a dumb a$$ and want more speed.
 
dwilfong said:
richard jones said:
dwilfong said:
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
Click to expand...
I could be wrong but I believe how much it grows depends on how much U feed it!!!!!!!!!!!!

dick
Click to expand...
I try to feed it a rich diet.

But you know how that goes when your a dumb a$$ and want more speed.
Click to expand...
Squeeze the water off till you can COOK BACON ON IT. No Need to needle down.......
 
Last edited by a moderator:
Interesting re-actions to the very last thing that is done in engine building. Prospective engine modifiers should have realized that fitting amounts as small as .000050" on the straight or tapered side of a piston can easily make the piston liner fit to tight, or to lose. The "crunch" number can not be anything for optimum engine performance! Only testing of different fits for different materials used can tell you what is best!

https://www.intlwaters.com/gallery/index.php?cat=10141; page 3; Laping tool, K-67 liners & pistons; page 2

Jim Allen
 
Last edited by a moderator:
Jack ODonnell said:
As for piston fit,where does the sleeve fit in the crankcase come into play.
Click to expand...
I always made the sleeve fit tightly in the crankcase to encourage heat transfer between the cylinder wall & the crankcase. I also kept the cylinder wall very thick. At the top of the liner a .200" thick flange, which was as large as the OD of the crankcase, kept this upper part of the liner very round, even at high operating temperatures. To further insure that this critical part of the motor was round during engine operation, the 8-8/32 head bolts went through the head button, the flange & then into the crankcase. Bolting the three pieces together in this fashion gave the maxium heat transfer possible & eliminated bending of the upper part of the liner that takes place when the bolts are outside the lip of the liner. The piston at TDC went .100" into the .200" area. Wear patterns on the piston's OD & ID of the cylinder proved how stable the upper end of the cylinder was. Henry Nelson & John Erik Faulk have applied this feature on their engines.

Jim Allen

http://gallery.intlw...x.php?cat=10141; page 2; completely custom built 7+ HP engine
 
Im always amazed by the things you do Jim .

Hugh
 
Last edited:
dwilfong said:
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
Click to expand...
David, Piston growth at the crown can vary greatly. The way 'I' have ran my 101's the piston taper has expanded out and contacted at the top, and shows 2 sealing bands, 1 towards the top, 1 as you know above the wrist pin.. Not hard contact at all, indicating it is a pretty good fit. Cmb 101's have about .002" TOTAL taper, .001" to the side, and they are ending the taper roughly .315" from the top. Based off some custom pistons I ve done in the past, I will say, if I walked in cmb's factory I would have them make some changes. I like the current sleeve taper just fine, it could be a little less, but this would require a different piston geometry that I will not get into. With the current 101 brass liner, if you want to do some custom pistons, I would put .0015" taper to the side, .003" total and would stop the taper higher up the piston than cmb, cmb is terminating the taper about .315" from the top. I would probably terminate mine about .250" from the top, with .003" total taper. As you change the length of taper, it will require the piston diameter to change. Moving the taper up, the diameter will be slightly smaller.To get you started, I would cut the piston with the taper indicated above, until the piston would come .280" from the top of the sleeve. This is a little less "Pitch" than cmb, and then slightly lap it and give it a try. That distance of .280" from the top of the sleeve, will get you started and I mean with your end finish on the piston, clean, room temperature, with a slight push with the thumb.(Real high tech huh).Push a piston in a few times and get something consistent working for you. Higher silicon content will have less expansion,but I would still fit the piston like I mentioned above to get you in the ball park. See ya,Jeff Lutz
 
Last edited by a moderator:
Jeff Lutz said:
dwilfong said:
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
Click to expand...
David, Piston growth at the crown can vary greatly. The way 'I' have ran my 101's the piston taper has expanded out and contacted at the top, and shows 2 sealing bands, 1 towards the top, 1 as you know above the wrist pin.. Not hard contact at all, indicating it is a pretty good fit. Cmb 101's have about .002" TOTAL taper, .001" to the side, and they are ending the taper roughly .315" from the top. Based off some custom pistons I ve done in the past, I will say, if I walked in cmb's factory I would have them make some changes. I like the current sleeve taper just fine, it could be a little less, but this would require a different piston geometry that I will not get into. With the current 101 brass liner, if you want to do some custom pistons, I would put .0015" taper to the side, .003" total and would stop the taper higher up the piston than cmb, cmb is terminating the taper about .315" from the top. I would probably terminate mine about .250" from the top, with .003" total taper. As you change the length of taper, it will require the piston diameter to change. Moving the taper up, the diameter will be slightly smaller.To get you started, I would cut the piston with the taper indicated above, until the piston would come .280" from the top of the sleeve. This is a little less "Pitch" than cmb, and then slightly lap it and give it a try. That distance of .280" from the top of the sleeve, will get you started and I mean with your end finish on the piston, clean, room temperature, with a slight push with the thumb.(Real high tech huh).Push a piston in a few times and get something consistent working for you. Higher silicon content will have less expansion,but I would still fit the piston like I mentioned above to get you in the ball park. See ya,Jeff Lutz
Click to expand...
Thanks Jeff thats the info I was looking for as far as the eng and set up I am working with.

I have 1.140 at the top and 1.143 about .250 down now and it dose feel loser than the stock piston fit.

I have ordered a custom made Flex hone with Levigated Alumina extra fine compound to polish the bore with. will See how it feels after this.

I did notice on the last stock piston it did have a seal at the very top and one at the section just above the pin. Although on the last 30% piston their was no seal at the top with a .003 taper.

will see how the next one looks.

Will See you at Charleston and we can check the piston in between heats and see whats what.

David
 
<_< Jeff,David How are you guys cutting these tapers in your pistons? Just curious as to what you cut with and how you hold the piston when youre cutting the taper.

Hugh
 
OH,Thats how you hold the piston and cut the taper . Nice!!!! who ever would have thought that.
 
Last edited:
hugh hargett said:
OH,Thats how you hold the piston and cut the taper . Nice!!!! who ever would have thought that.
Click to expand...
Throttle back turbo! I had not seen your question.LOL. You can hold the piston on a mandrel fitting the bottom counterbore of a whole piston, keep in mind if you have interference this will give a taper at the bottom of the piston in the amount of the interference that you have. This can be good to relieve the bottom of the piston slightly. Really needs to be cut in a cnc, so you can cut the taper and o.d. all at once and adjust in multiple passes to get to the diameter you are looking for.
 
Here is how us cave men do it.

and a little 600 to 2000 grit and lot of Patience. It's not down to the .0001 but it will do.

David
 
dwilfong said:
Jeff Lutz said:
dwilfong said:
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
Click to expand...
David, Piston growth at the crown can vary greatly. The way 'I' have ran my 101's the piston taper has expanded out and contacted at the top, and shows 2 sealing bands, 1 towards the top, 1 as you know above the wrist pin.. Not hard contact at all, indicating it is a pretty good fit. Cmb 101's have about .002" TOTAL taper, .001" to the side, and they are ending the taper roughly .315" from the top. Based off some custom pistons I ve done in the past, I will say, if I walked in cmb's factory I would have them make some changes. I like the current sleeve taper just fine, it could be a little less, but this would require a different piston geometry that I will not get into. With the current 101 brass liner, if you want to do some custom pistons, I would put .0015" taper to the side, .003" total and would stop the taper higher up the piston than cmb, cmb is terminating the taper about .315" from the top. I would probably terminate mine about .250" from the top, with .003" total taper. As you change the length of taper, it will require the piston diameter to change. Moving the taper up, the diameter will be slightly smaller.To get you started, I would cut the piston with the taper indicated above, until the piston would come .280" from the top of the sleeve. This is a little less "Pitch" than cmb, and then slightly lap it and give it a try. That distance of .280" from the top of the sleeve, will get you started and I mean with your end finish on the piston, clean, room temperature, with a slight push with the thumb.(Real high tech huh).Push a piston in a few times and get something consistent working for you. Higher silicon content will have less expansion,but I would still fit the piston like I mentioned above to get you in the ball park. See ya,Jeff Lutz
Click to expand...
Thanks Jeff thats the info I was looking for as far as the eng and set up I am working with.

I have 1.140 at the top and 1.143 about .250 down now and it dose feel loser than the stock piston fit.

I have ordered a custom made Flex hone with Levigated Alumina extra fine compound to polish the bore with. will See how it feels after this.

I did notice on the last stock piston it did have a seal at the very top and one at the section just above the pin. Although on the last 30% piston their was no seal at the top with a .003 taper.

will see how the next one looks.

Will See you at Charleston and we can check the piston in between heats and see whats what.

David
Click to expand...
David, With the 30%, if you are not seeing contact with .003", back it down a little, try different pitches, and try different taper lengths. I do not like seeing a void in the middle of 2 fits when looking for max h.p. Ideally you would want a sealing band, with mild contact up to the crown.Sounds like your on the right track, but it is going to be difficult with emery cloth and a steady hand! See ya, Jeff
 
Jeff Lutz said:
dwilfong said:
Jeff Lutz said:
dwilfong said:
I have a question on fitting a piston in a large bore eng.

Have noticed that on the CMB pistons in the big bore eng the tight spot is below the top gust above the pin hole.

My question is how much do you think the top of the piston grows as the eng is at running temp?

Will higher % of silicon in the piston make it grow more?

Will a billet piston grow more than a cast piston?

I know that in large V8 engs That I ran I would double the clearance with a forged piston.

so how much will a 1 1/8" billet 30% silicon piston grow at temp?

David
Click to expand...
David, Piston growth at the crown can vary greatly. The way 'I' have ran my 101's the piston taper has expanded out and contacted at the top, and shows 2 sealing bands, 1 towards the top, 1 as you know above the wrist pin.. Not hard contact at all, indicating it is a pretty good fit. Cmb 101's have about .002" TOTAL taper, .001" to the side, and they are ending the taper roughly .315" from the top. Based off some custom pistons I ve done in the past, I will say, if I walked in cmb's factory I would have them make some changes. I like the current sleeve taper just fine, it could be a little less, but this would require a different piston geometry that I will not get into. With the current 101 brass liner, if you want to do some custom pistons, I would put .0015" taper to the side, .003" total and would stop the taper higher up the piston than cmb, cmb is terminating the taper about .315" from the top. I would probably terminate mine about .250" from the top, with .003" total taper. As you change the length of taper, it will require the piston diameter to change. Moving the taper up, the diameter will be slightly smaller.To get you started, I would cut the piston with the taper indicated above, until the piston would come .280" from the top of the sleeve. This is a little less "Pitch" than cmb, and then slightly lap it and give it a try. That distance of .280" from the top of the sleeve, will get you started and I mean with your end finish on the piston, clean, room temperature, with a slight push with the thumb.(Real high tech huh).Push a piston in a few times and get something consistent working for you. Higher silicon content will have less expansion,but I would still fit the piston like I mentioned above to get you in the ball park. See ya,Jeff Lutz
Click to expand...
Thanks Jeff thats the info I was looking for as far as the eng and set up I am working with.

I have 1.140 at the top and 1.143 about .250 down now and it dose feel loser than the stock piston fit.

I have ordered a custom made Flex hone with Levigated Alumina extra fine compound to polish the bore with. will See how it feels after this.

I did notice on the last stock piston it did have a seal at the very top and one at the section just above the pin. Although on the last 30% piston their was no seal at the top with a .003 taper.

will see how the next one looks.

Will See you at Charleston and we can check the piston in between heats and see whats what.

David
Click to expand...
David, With the 30%, if you are not seeing contact with .003", back it down a little, try different pitches, and try different taper lengths. I do not like seeing a void in the middle of 2 fits when looking for max h.p. Ideally you would want a sealing band, with mild contact up to the crown.Sounds like your on the right track, but it is going to be difficult with emery cloth and a steady hand! See ya, Jeff
Click to expand...
Ya it takes a lot of slugs to get it right.
 
dwilfong said:
Here is how us cave men do it.

and a little 600 to 2000 grit and lot of Patience. It's not down to the .0001 but it will do.

David
Click to expand...
This is not a good method to use to hold the piston for final machining. A snug fitting 3.000" long pin placed in the wrist pin hole while it is being held with a Hardinge expanding collet, can be easily zeroed. This insures that all the final cuts made on the OD & the piston crown are square to the axis of the wrist pin hole. The wrist pin hole itself should be in the middle, unless offset is desired. Using the expanding collet will give the piston a semi barrel shape. Since the piston is rocking in the bore, this helps reduce side wall friction. It has no effect on what will happen if the wear band taper is correct & the liners taper is correct. Piston top taper amounts are tied to liner taper amounts. Liner taper amounts are tied to the stroke of the engine. Short strokes allow steeper liner tapers & vice versa.

Only poylcrystaline coated tool bits that have been lapped can cut the very small amounts, .00005"/side, that are necessary for fitting pistons to liners. The final machined finish should be very smooth, between 25 & 50 micro inch.

Jim Allen

Note: Both the final side cuts & the taper side cuts should be made at the same time. This can be easily done in manual lathe. The compound will have to be set to the desired top taper amount. A .00005" indicator can set the compound using a taper master. Accuracy of the finished taper cut will be .00005"/2.000". Anybody think this type of precision can be done with a piece of 2,000 grit paper held in your hand????
 
Last edited by a moderator:

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