Pistons dimensions
Thread Starter
Drifting
Joined: Jun 2001
Posts: 1,625
Likes: 1
From: Zagreb, Croatia
Pistons dimensions
I've bought some time ago a set of used forged SRP pistons.
Today i was cleaning them and I also did some measuring.
Pistons are SRP part nr. 138084 - 0.020" overbore:
Here is what I've got:
1. 4.0153
2. 4.0152
3. 4.0152
4. 4.0148
5. 4.0151
6. 4.0164
7. 4.0156
8. 4.0153
I've measured at .5" from the bottom of the piston, as suggested by JE/SRP.
So I have a few questions:
1. Doesn't difference of .0016" look too big (piston 6-4)?
2. My machinist will not do clearancing for each piston individually.
What would be the best way to tell what bore size do I need
(+ the clearance)?
Today i was cleaning them and I also did some measuring.
Pistons are SRP part nr. 138084 - 0.020" overbore:
Here is what I've got:
1. 4.0153
2. 4.0152
3. 4.0152
4. 4.0148
5. 4.0151
6. 4.0164
7. 4.0156
8. 4.0153
I've measured at .5" from the bottom of the piston, as suggested by JE/SRP.
So I have a few questions:
1. Doesn't difference of .0016" look too big (piston 6-4)?
2. My machinist will not do clearancing for each piston individually.
What would be the best way to tell what bore size do I need
(+ the clearance)?
Safety Car
Joined: Sep 2001
Posts: 4,161
Likes: 554
From: Glenbeulah Wi
What does SRP suggest for clearance? Typically you bore the engine to a set number like 4.02 and the pistons are designed with the necessary clearance to make it work. To me it looks like you anywhere from .0035 and .0052 for clearance is that what SRP recommends for your pistons?
Melting Slicks
Joined: Dec 2008
Posts: 2,772
Likes: 64
From: Mystic CT
If your machinist does not have a dial bore gauage and a CK10 Sunnen hone you need a new machinist. saying that measure the piston 90 degrees to the pin bore then at the skirts that will tell you how much the skirts have collapsed from knock... Given enough money anyone can hone to 8 different bore sizes but cheaping out on the pistons is the real issue. If the skirts are collapsed they will rattle burn oil and be a big issue in the end...
Dave
Dave
Thread Starter
Drifting
Joined: Jun 2001
Posts: 1,625
Likes: 1
From: Zagreb, Croatia
My machinist have everything needed, but usually he makes all bores the same size. I could ask him to make different sizes, but price could be an issue then.
How can you tell that piston skirt is collapsed'
The pistons that I've bought are from CF member and he said that they don't have too much mileage. There are some marks on 2 or 3 pistons that suggests that there was some mechanical failure in the engine but those marks are really minor and on the top of piston. Also, some of the pistons do have some scuffs.
Here is a pic of the worst one.
How can you tell that piston skirt is collapsed'
The pistons that I've bought are from CF member and he said that they don't have too much mileage. There are some marks on 2 or 3 pistons that suggests that there was some mechanical failure in the engine but those marks are really minor and on the top of piston. Also, some of the pistons do have some scuffs.
Here is a pic of the worst one.
Thread Starter
Drifting
Joined: Jun 2001
Posts: 1,625
Likes: 1
From: Zagreb, Croatia
What does SRP suggest for clearance? Typically you bore the engine to a set number like 4.02 and the pistons are designed with the necessary clearance to make it work. To me it looks like you anywhere from .0035 and .0052 for clearance is that what SRP recommends for your pistons?
But everybody says that forged pistons should use more clearance,
like .0035-.0045 even .0050".
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
The piston in the pic looks pretty good..except for the nicks at the bottom of the skirt. The numbers you posted seem close enough - the ring will make up the rest @ .02 over on all 8. Inspect them closely, weigh them individually (after you clean them up) and make your final decision. Doesn't look like they had much time on them....did you ask the seller "why" they were pulled before you bought them?
I'd run'em if they were close and undamaged.
I'd run'em if they were close and undamaged.
Thread Starter
Drifting
Joined: Jun 2001
Posts: 1,625
Likes: 1
From: Zagreb, Croatia
I didn't ask for selling reason, but it was stated in the add:
http://forums.corvetteforum.com/c4-p...-for-sale.html
I've bought them from respected member and I believe that those are
low mileage pistons. I'm really not too worried about those scuffs. It's not very unusual.
Do you think that cleaning/polishing skirts (that would also remove some scuffs) with scotch brite would be good?
I'm only a little bit worried about that dimensions difference. I'll try to talk to my machinist tomorrow and ask his opinion and also ask him again if he would make individual bores.
http://forums.corvetteforum.com/c4-p...-for-sale.html
I've bought them from respected member and I believe that those are
low mileage pistons. I'm really not too worried about those scuffs. It's not very unusual.
Do you think that cleaning/polishing skirts (that would also remove some scuffs) with scotch brite would be good?
I'm only a little bit worried about that dimensions difference. I'll try to talk to my machinist tomorrow and ask his opinion and also ask him again if he would make individual bores.
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Race Director
Joined: Sep 2003
Posts: 16,839
Likes: 341
From: Database Error Indiana
Scuffs dont look like a problem to me. May be from being a little tight in the bore. Forged pistons do require more clearance than cast. An experienced machinist will know what they need. My 2 cents. 
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
weigh them and use a drill bit of 3/8 inch to even up the piston weights. They should weigh within 1-2 grams.
Use a jeweler's file to smooth out any imperfections on the skirt, but not on the scuff marks.
Have the pistons coated with a friction resistant coating, and use a wet sanding process to restore the piston diameters to your satisfaction. Use 320 grit and water, then wash the pistons with solvent or mineral spirits, blow dry, and use a twist drill by hand to clean out the oil return holes, then a broken piston ring to clean the ring lands.
But if this is a thow-away engine, just install them dry, and lubricate the cylinders prior to installing the heads.
Use a jeweler's file to smooth out any imperfections on the skirt, but not on the scuff marks.
Have the pistons coated with a friction resistant coating, and use a wet sanding process to restore the piston diameters to your satisfaction. Use 320 grit and water, then wash the pistons with solvent or mineral spirits, blow dry, and use a twist drill by hand to clean out the oil return holes, then a broken piston ring to clean the ring lands.
But if this is a thow-away engine, just install them dry, and lubricate the cylinders prior to installing the heads.
Race Director
Joined: Feb 2004
Posts: 12,518
Likes: 19
The instant a motor fires, new pistons become used pistons, and the skirts scuff.
Knock the burrs off the skirt scuffs, let the machinest custom fit each piston as planned, and enjoy the motor.
This forum cracks me up sometimes.
Knock the burrs off the skirt scuffs, let the machinest custom fit each piston as planned, and enjoy the motor.
This forum cracks me up sometimes.
Last edited by Pete K; Nov 9, 2009 at 07:34 AM.
Race Director
Joined: Feb 2004
Posts: 12,518
Likes: 19
.0028 or so on pistons 1-6 and .0032 on #7 and #8.
Melting Slicks
Joined: Dec 2008
Posts: 2,772
Likes: 64
From: Mystic CT
weigh them and use a drill bit of 3/8 inch to even up the piston weights. They should weigh within 1-2 grams.
Use a jeweler's file to smooth out any imperfections on the skirt, but not on the scuff marks.
Have the pistons coated with a friction resistant coating, and use a wet sanding process to restore the piston diameters to your satisfaction. Use 320 grit and water, then wash the pistons with solvent or mineral spirits, blow dry, and use a twist drill by hand to clean out the oil return holes, then a broken piston ring to clean the ring lands.
But if this is a thow-away engine, just install them dry, and lubricate the cylinders prior to installing the heads.
Use a jeweler's file to smooth out any imperfections on the skirt, but not on the scuff marks.
Have the pistons coated with a friction resistant coating, and use a wet sanding process to restore the piston diameters to your satisfaction. Use 320 grit and water, then wash the pistons with solvent or mineral spirits, blow dry, and use a twist drill by hand to clean out the oil return holes, then a broken piston ring to clean the ring lands.
But if this is a thow-away engine, just install them dry, and lubricate the cylinders prior to installing the heads.
To the person that suggested you attck them with a drill to balance. Let the guy balance the engine match heavy rods with light pistons to get the rod top end weights even with very minimal metal and structural removal. I have balanced thousands of engines and it is easier and better to do that for a one off every day of the week. If you change pistons every pass or month then yes all rods and pistons need to be interchangeable.
Dave
