70 LS5 forged vs cast pistons

[Steve G.]

I am freshening up motor on my '70 LS5, and trying to determine if it needs to be bored out or not. Machine shop measured .0045". Forged pistons are supposed to have more piston to bore clearance than cast pistons. I believe mine are factory originals, and that they are forged. But I am not 100% certain. Does anyone know for sure whether the LS5 came with forged or factory pistons? Is there a casting number or part number that would say for sure? Steve

[Steve G.]

Re: 70 LS5 forged vs cast pistons

Note the part number on pistons is 396355 3/4 - I believe that means both 3963553 and 3963554. The also have an 'F' on the wrist pin bosses. Steve

[Joe L.]

Re: 70 LS5 forged vs cast pistons

Note the part number on pistons is 396355 3/4 - I believe that means both 3963553 and 3963554. The also have an 'F' on the wrist pin bosses. Steve

Steve-----l

All LS-5 pistons were conventional cast aluminum.

[Duke W.]

Re: 70 LS5 forged vs cast pistons

I am freshening up motor on my '70 LS5, and trying to determine if it needs to be bored out or not. Machine shop measured .0045". Forged pistons are supposed to have more piston to bore clearance than cast pistons. I believe mine are factory originals, and that they are forged. But I am not 100% certain. Does anyone know for sure whether the LS5 came with forged or factory pistons? Is there a casting number or part number that would say for sure? Steve

If you are saying that wear at the top of the bore is .0045", you need to go to the next oversize.

Duke

[D S.]

Re: 70 LS5 forged vs cast pistons

Steve-----l

All LS-5 pistons were conventional cast aluminum.

I didn't know that they were originally cast aluminum. When I had my 512 engine rebuilt I had it bored .30 over an had forged hype-eutectic flat top pistons installed. That was the recommendation I got here on the TDB about five years ago.

[Steve G.]

Re: 70 LS5 forged vs cast pistons

Interesting - the taper was measured at .001" to .0015". And yes, the clearance between piston and the wall (using direct feeler gage method) was .0045". Machine shop suggested I buy new pistons to see if they would tighten clearance up a bit. I have an old 'How to Rebuild Your Big Block Chevy' book. It says largest acceptable clearance for cast pistons is .0025". For forged pistons, the limit goes up to .008" as they expand and contract more. The book described characteristics of a cast piston (expansion slot between oil ring and the skirt as well as ribs on the piston pin boss inside the piston). My original pistons (with GM numbers on them) don't have either of these features. That's why I had thought they were forged.

Any thoughts on forged vs cast, whether I could try new pistons (I don't really want to bore the block), or what type of pistons I should be buying? Steve

[Duke W.]

Re: 70 LS5 forged vs cast pistons

One question often leads to another. As built by GM cast piston engines had nominally .001 clearance, so either your engine has already been rebored or something is wrong with the measurements.

What is now needed is the nominal bore dimensions in the unworn area below the taper to determine if it has been previously bored and the piston diameter, which is measured about halfway down the skirt 90 degrees to the pin axis. Also what is the thickness/part number of the removed head gasket.

What information is on the pistons - like a casting number?

You can google Keith Black and look at what they have for replacement pistons. If the engine did not detonate before, then don't lower the CR.

Your machine shop's recommendation to "buy new pistons to see if they fit tighter" is a backyard bubba approach. What is needed if FACTS - like the measurments I requested.

Duke

[Steve G.]

Re: 70 LS5 forged vs cast pistons

Thanks Duke. I am just getting this message as I have been away on vacation. I will be getting the piston and bore diameter measurements from machinist on Monday. I will post it here as soon as I have it. Thanks for the help! Steve

[Steve G.]

Re: 70 LS5 forged vs cast pistons

Duke,
I have the info below from machine shop. They indicated all pistons / bores were about the same:

nominal bore dimensions measured in the unworn area: 4.2505"

piston diameter at skirt: 4.2475"

bore taper: ~ .001 to .0015"

Head gasket thickness: I am still looking for the old head gaskets. I have them around the garage somewhere... They look like they were changed to composition type gaskets at some point. See picture attached.

piston casting numbers: 396355 3/4, which I believe means 3963553 or 3963554. They also have an 'F' on them.

Machinists observations: bores show very little wear, but pistons do have wear.

I am trying to 'keep it original as possible', but I don't want to put it together with problems either. Machinist said if I didn't want to bore it .030" over (and I don't unless its absolutely necessary), that I could put new rings and hone it and hope I don't get piston slap during cold start. Or, I could buy new pistons, then recheck clearance with feeler gage. If within 2 to 3 thousands, I might be able to get away with a hone, new pistons, new rings. Do you concur or have any other suggestions? Steve

PS - crank looked very good. He just needed to polish it. He is also doing a valve job and changing the valve guides.

[Timothy B.]

Re: 70 LS5 forged vs cast pistons

Steve,

If I am reading you figures correctly there is .003 between skirts and wall. Sometimes the pistons can be knurled in the skirt area just for this reason, talk to machine shop about this.

[Duke W.]

Re: 70 LS5 forged vs cast pistons

Bore measurements indicate the block has never been bored. I'm not sure about the piston numbers (I think Joe Lucia can shed some light.), but OE cast pistons are normally fit at about .001" clearance. I can't imagine how the piston skirts could be worn .002" with only .001-.0015" taper at the top of the bore.

The data doesn't make sense to me.

Check to see what Keith Black offers as an OE replacement piston.

Did the machinist check the deck clearances, so you can compute compression ratio?

If the piston clearance was in the OE ballpark, I'd say it's okay to use them, hone, and rering, but in this case I'd want to get to the bottom of why the piston-bore clearance is so high.

I'd like to here what Clem thinks.

Duke

[Clem Z.]

Re: 70 LS5 forged vs cast pistons

Bore measurements indicate the block has never been bored. I'm not sure about the piston numbers (I think Joe Lucia can shed some light.), but OE cast pistons are normally fit at about .001" clearance. I can't imagine how the piston skirts could be worn .002" with only .001-.0015" taper at the top of the bore.

The data doesn't make sense to me.

Check to see what Keith Black offers as an OE replacement piston.

Did the machinist check the deck clearances, so you can compute compression ratio?

If the piston clearance was in the OE ballpark, I'd say it's okay to use them, hone, and rering, but in this case I'd want to get to the bottom of why the piston-bore clearance is so high.

I'd like to here what Clem thinks.

Duke

find a shop with a pair of mics and a dial bore gauge as using feeler gauge is not the way to do it.

[Duke W.]

Re: 70 LS5 forged vs cast pistons

He did give piston and bore dimensions in post #9, which I assume were measured with a bore gage/mic.

So what do you think about the piston measurment. Could the skirts be worn .002"?

Duke

[Clem Z.]

Re: 70 LS5 forged vs cast pistons

Steve,

If I am reading you figures correctly there is .003 between skirts and wall. Sometimes the pistons can be knurled in the skirt area just for this reason, talk to machine shop about this.

the knurling will only last a couple of thousand miles before it wears down.

[Clem Z.]

Re: 70 LS5 forged vs cast pistons

He did give piston and bore dimensions in post #9, which I assume were measured with a bore gage/mic.

So what do you think about the piston measurment. Could the skirts be worn .002"?

Duke

my question is what is the bore size in the area where the rings run. the pistons could be worn .002 and it also changes where you measure the pistons. if he does not bore the engine and just hones it he had better use "file fit" rings so the end gap can be set to the proper specs. this is more important than a couple of thousands in the piston to wall clearance.