Sliding

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)?

bjankuski

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?

ddahlgren

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

Sliding

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.

Sliding

Actually they recomend .0025-.0035 for small block applications.
But everybody says that forged pistons should use more clearance,
like .0035-.0045 even .0050".

Churchkey

Used slugs are ok if your building a grenade for Saturday night racing.

Suggest a new set of pistons, bore .030 over & sell the used set.

engle1147

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.

Sliding

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.

JackDidley

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.

Sliding

I just spoke to my machinist and he agreed to make individual
bores for each piston/cylinder! I can sleep easier now!

coupeguy2001

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.

mtwoolford

are your pistons for pressed in wrist pins or for floating wrist pins ?

Sliding

Floating.

Pete K

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.

Pete K

Some forged pistons do require a ton of clearance, but it will be due to piston construction. If SRP wants .0025-.0035, I personally would like
.0028 or so on pistons 1-6 and .0032 on #7 and #8.

Sliding

Thanks for your input Pete. Allways appreciated!

Pete K

Always happy to help.

Don't let the naysayer's get you down. SRP makes a nice piston.
Run it hard, and enjoy.

ddahlgren

Wash with hot soapy water not mineral spirits or anything petroleum. It is a solvent and will send the grit deeper. Soap is a suffocant and will get the grit to the surface where you can get it off. When done a quick squirt of wd 40 and clean wraping until ready to use. Newspaper works way better than a paper towel that is a ball of lint..LOL..

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