piston question -urgent
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,164
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From: Beringen
piston question -urgent
If you buy a piston say for a 4.030" bore, for which a manufacturer recommends a .004" piston clearance, is the piston 4.026" wide at the skirt or is the bore 4.034". I'm talking about an off the shelf piston here.
Thanks
Thanks
Melting Slicks
Joined: Aug 2003
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From: Dayton Oh
If the video "How to hot rod a small block chevy" is accurate, then what you've said is correct. The video suggests inserting the piston into the bore and then measure the skirt to wall clearance using a feeler guage. Machine shops don't finish the bore completely to 4.030 over and do the final honing on each unique bore/piston to get the exact clearance. IE, each piston will be matched to only one bore.
Le Mans Master
Joined: Mar 2004
Posts: 7,493
Likes: 1,502
From: Little Rock AR
What people say and what actualy happens might be different. .
I will bet you that your shop will bore the block a hair under size and then run the hone to get it to 4.030". The pistons are designed to run in that size bore "off the shelf". You can verify that the clearance is right using common feeler gauges. That is only "close enough" to let you know if something is realy wrong, but not enough to tell you it is dead nuts right.
Not saying all shops do it that way but to say that shops routinely size a particular piston to a particular bore for every rebuild is unrealistic at best.
Pistons are pretty difficult to measure with common tools so that is why they tell you to drop it in the finished bore and measure in one specific area. The bore sides of a piston are not actualy round!!! They are more barrel shaped. Measure with the feelers on the side of the skirt, and down at the wrist pin height to get the clearance.
-Mark.
I will bet you that your shop will bore the block a hair under size and then run the hone to get it to 4.030". The pistons are designed to run in that size bore "off the shelf". You can verify that the clearance is right using common feeler gauges. That is only "close enough" to let you know if something is realy wrong, but not enough to tell you it is dead nuts right.
Not saying all shops do it that way but to say that shops routinely size a particular piston to a particular bore for every rebuild is unrealistic at best.
Pistons are pretty difficult to measure with common tools so that is why they tell you to drop it in the finished bore and measure in one specific area. The bore sides of a piston are not actualy round!!! They are more barrel shaped. Measure with the feelers on the side of the skirt, and down at the wrist pin height to get the clearance.
-Mark.
Thread Starter
Melting Slicks
Joined: Mar 2002
Posts: 2,164
Likes: 7
From: Beringen
I read something on piston clearance the other day and started thinking about the clearances of my own engine... do not have all the specific details anymore, but I remember that I had something of about 0.006-0.007" of clearance. The bore was slightly larger than the recommended size. I do not exactly remember the values anymore, but it was something in the order of 0.002" bigger. I refered then to a work of D. Vizard and found values of clearance for forged pistons between 0.004 and 0.007 and deemed it to be good to go. Didn't have exactly much choice since the block was already machine and since i live on the other side of the ocean of where my machineshop was. Anyway. As the pistons where in the box, weighed etc, I'm pretty sure they were not matched to a specific bore.
Would 0.006-0.007 be a problem with a forged piston and a combo that can run around 7000-7500 rpm ?
Would 0.006-0.007 be a problem with a forged piston and a combo that can run around 7000-7500 rpm ?
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The clearance is built into the piston. I.E.,,,,,4.030 bore will use a piston with say 4.026 diameter.
Cast pistons might only be .001-.0015 smaller..forged race type pistons (2618 material) might be .004-.006 smaller. Forged ones made from 4032 material run a little tighter clearance.....maybe .002-.003.
Most machine shops in a production environment bore straight to 4.030 and count on pistons to be correct. REAL meticulous machinists or race type shops will bore to say 4.028-4.029 and then measure each piston and hone the cylinder with TQ plates until they get whatever clearance they are shooting for.
.006-.007 won't hurt anything really. It will be a noisy engine when cold with the average skirt design, and might use a little more oil....but it will definitely run strong under hard use and never worry about sticking a piston. Just run it!
JIM
Cast pistons might only be .001-.0015 smaller..forged race type pistons (2618 material) might be .004-.006 smaller. Forged ones made from 4032 material run a little tighter clearance.....maybe .002-.003.
Most machine shops in a production environment bore straight to 4.030 and count on pistons to be correct. REAL meticulous machinists or race type shops will bore to say 4.028-4.029 and then measure each piston and hone the cylinder with TQ plates until they get whatever clearance they are shooting for.
.006-.007 won't hurt anything really. It will be a noisy engine when cold with the average skirt design, and might use a little more oil....but it will definitely run strong under hard use and never worry about sticking a piston. Just run it!
JIM
