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Compression height and hemi pistons

L

Louie

Member
My first predator build, I have a new non hemi with the piston in the hole 0.051". I am planning on 11.1:1 to 12.0:1 CR with a 0.045 gasket, a 22 cc head milled .065" for use with pump gas and 12:1 to 13.0:1 with a thinner gasket for use at some point with E-85. It looks like I'll need a hemi flat top piston. The trouble is that the hemi piston has a 0.020 shorter compression height, so even with a 0.040" longer rod I'm only gaining 0.020", or 0.031" in the hole from 0.051". Now I see listed a "208" piston, flattop, with .020" to .025" more compression height, giving me 0.011" to 0.006" in the hole. Now we are talking, 'cept for one thing, this has a footnote stating that pin clearance may be excessive. 0.003" instead of 0.001", actually I'm not too afraid of 0.002" but the extra thousandth may be a bit much. The catalogue doesn't say if the pin is included or not nor whether the pin is sloppy in the piston or the rod. I'm thinking that the original predator pin must be bigger, if so I could hone the piston to fit. I would rather avoid cutting the deck if I can find parts to fit. I'm sure some of you wizards have ran into this and know what to do or buy. Or the possibility exists that I have misread or misunderstood something. Measure twice cut once.
"Louie"
 
Bear in mind, whatever combination you use, you need to keep about .025" between the top of the piston and head. I have played with a thought in my head from time to time but haven't acted upon it with these motors.(did it once with a flathead) Has anybody set a head up on a rotary table and milled everything except the area over the piston.(could do it with a scribed line and some careful two handle work). It seems like it would work fine?? If you have the room to spare(.025"+ gap), you could raise compression to the same level without cuting as much off of the head.(saving some valve to piston clearance). I'm crazy sometimes but sometimes that's what it takes.
Take Care
 
Its gonna be real close to 0.025, I figure with the cam I want there will be an interference of about 0.011". As long as the cam is not retarded it should be OK. I would just use a 14cc head that would not need cut so much, but the danged thing has an RTC-8 head that I hate to throw away. Just looking at options and thanks for the tip. If I had my own mill I'd try it. I have access to one but I'm too rusty to try much two handle work.
Thanks again.
 
One build that I have run and like...

I use a Honda crank in the NoN Hemi block,with the Hemi flat top, and a Honda 3.403 Rod, gives about .010 pop up, that with a .045 gasket... Is a nice clearance...

I have also ran a 3.707 Rod and Wiseco set up and circle milled the head to allow for pop up, and used a thick gasket...

The Non Hemi block and a .040 Rod and the flat top I always ended up .020 in the hole and used a .009 gasket... Good combo too!!!
 
I just tore the rest of the engine down, checked the stock pin clearance, wouldn't you know; its 0.003"! The compression height is the same as the "208" flat top piston. Guess I'll be using the 0.040" rod with that piston and see how the ARC rod likes a loose pin. Maybe there is a Honda pin that works.
Just as happy as if I were in my right mind.
 
M.D.racing said:
Just think of it as a negative thickness gasket.:)
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Not that cutting the head is a bad idea, but I think you'll find that cutting a round circle with a manual mill is very very near to impossible. Been there, done that. I'm thinking the circle you cut would have to be within .001" of the bore. Or less. I think you could do it on a CNC mill, but the cost might be prohibitive.
 
Louie said:
I just tore the rest of the engine down, checked the stock pin clearance, wouldn't you know; its 0.003"! The compression height is the same as the "208" flat top piston. Guess I'll be using the 0.040" rod with that piston and see how the ARC rod likes a loose pin. Maybe there is a Honda pin that works.
Just as happy as if I were in my right mind.
Click to expand...
How are you measuring that clearance? .003" is a mile when it comes to piston pin clearance.
 
alvin l nunley said:
Not that cutting the head is a bad idea, but I think you'll find that cutting a round circle with a manual mill is very very near to impossible. Been there, done that. I'm thinking the circle you cut would have to be within .001" of the bore. Or less. I think you could do it on a CNC mill, but the cost might be prohibitive.
Click to expand...

It wouldn't have to be perfect or a perfectly smooth dia., as long as it was small enough to drop into the bore or probably just the chamfer on top of the bore. Your probably only talking about leaving .025"-.030" standing. Time and patience.
Cnc equipment has pushed alot of manual things to the back shelf.
You could use a rotary table or a Volstro milling head for a Bridgeport.
Sharpen a tool that would just cause a small groove if you missed the depth a little and "back" bore it......
 
Al, I'm using a micrometer and telescoping gauge, this is a brand new motor with a stock rod. I would expect a stock rod to be a bit loose, but am not sure how well the ARC rod will tolerate it,
the bad thing is that the big end was clearanced at 0.001", my experience has proven this to be too tight for a stock rod.
 
Yak, the flat top hemi piston is now out of stock, this would have been my second choice. I should end up with 0.056" with a 0.045" gasket, with a 0.015" gasket it would clear 0.026". I've been running a yellow clone at 0.017" and getting away with it.
Louie
 
alvin l nunley said:
Not that cutting the head is a bad idea, but I think you'll find that cutting a round circle with a manual mill is very very near to impossible. Been there, done that. I'm thinking the circle you cut would have to be within .001" of the bore. Or less. I think you could do it on a CNC mill, but the cost might be prohibitive.
Click to expand...
Easy on the lathe.
 
The compression height is right, if the pin is snug. Even the stock predator dish piston, pin and rod combo showed 0.003" clearance. I'm going to check my stock of clone pins and see if the length is right, maybe the run a bit bigger. I can always hone a couple thou. from the piston or rod.
Thanks.
 
Louie said:
The compression height is right, if the pin is snug. Even the stock predator dish piston, pin and rod combo showed 0.003" clearance. I'm going to check my stock of clone pins and see if the length is right, maybe the run a bit bigger. I can always hone a couple thou. from the piston or rod.
Thanks.
Click to expand...

I hope the piston works out for you. I ordered one to ensure it is the same height as the dome piston in my non hemi. Than I'll get the +.040 or +.020 rod. We shall see.
 
KMC The stock compression height is 0.570 to 0.575, so this piston should be in the ball park, as well as the "208" from NR. Its the pin fit on the NR "208" that is in question. Let me know what you find. I was too busy celebrating Halloween last nite to check pins. I'm going to order the .040 over rod and one or the other pistons Monday or Tuesday.
 
Louie said:
KMC The stock compression height is 0.570 to 0.575, so this piston should be in the ball park, as well as the "208" from NR. Its the pin fit on the NR "208" that is in question. Let me know what you find. I was too busy celebrating Halloween last nite to check pins. I'm going to order the .040 over rod and one or the other pistons Monday or Tuesday.
Click to expand...

Great info to know, I will let you know what I find.
 
I got my "208" piston yesterday, pin fit is good in both piston and rod. The rings are the wide pattern or old style. Ring groove clearance good. I will mock up case, rod piston and cam tonite, check the "in hole" and skirt clearance. If OK the rotating stuff and head are going to the machine shop for a balance and a shave.
Make sure you get the right rings.
Louie
 
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