I thought the TPI motors all had flat top pistons, but when I took what I thought was the stock TPI 305 out of a 92 Z28, I found dished pistons.

Does anyone know if the 92 came with these or is this a replacement engine?

It does have 081 heads and this is a 5 speed car so it has an L98 cam and dual-cats. In 2006 it ran 14.3@96, so the performance seemed right for a stock LB9 T-5 w/G92.

I did a search and couldn't find an answer, so any help will be appreciated.

 

Can't say for sure but I'll let you know in a few weeks as I'm getting ready to pull/work on the original LB9 in my 'vert. It's an auto, though. I'd be thrilled with a 14.3 when I'm done!

 

All earlier ones had dished for sure, I know for a fact 92 L98s came with flat tops. IF it's the original engine the stamp pad should have the correct partial vin in there.

Late flat tops look like this:



The dished ones have a clear quench ring around the perimeter with a notch facing the FWD side of the engine.

 

My 85 LG4 had flattops like the picture above.

 

My 87 350 had ones just like the picture

 

87-up 305s had dished pistons, but not a very deep dish. Maybe 10cc at the most (including valve reliefs). They were used extensively after the change-over to 1 piece RMS engines happened. From that point on the short block assembly was essentially identical between all 305 passenger car engines whether they were TBI, TPI or the last years of the carbureted engines.

The only 305s that came with real flattops were the L69 HO 4bbl carb engines and the 85-up LG4 carb engines. Plus (probably) the 85-86 305 TPIs.

You'll notice the advertised compression ratio on the 305s changed from 9.5:1 to 9.2:1 in 87. The difference is the use of (slightly) dished pistons from that point forward.

These pistons should not be confused with earlier 305 dished pistons which had a much deeper dish and an 8.5:1 compression ratio (for example 84 and earlier LG4 carb engines).

Specs on 350 engines were different than 305s so don't try to compare what you find in a 350 with what you expect to find in a 305.

 

Thanks for the info, Damon.

I figured it was stock because the VIN was still on the pad and the bore is standard size. I have owned it since 98 and put 150,000 of its 196,000 miles.

 

My 305 had dished pistons with no valve reliefs i just pulled the head. and i had the same casting numbers this was in a 91 z28. not sure this was the original engine tho i heard mine had an engine change

 

My pistons did have the 4 valve relief cut in them and I'm. Sure they are original. There is a pad on the front right edge of the deck that has the last 6 digits of the VIN. If your engine is the original motor that should still be there. If not, then it was decked and rebuilt or replaced. If they don't match then obviously it isn't original either. Again, mine is a 92 LB9 T-5.

 

I know the guy replaced the engine i found the paper work they put the lo3 engine in my car I just checked my heads and they ended 187.and my pistons are dished a good size dished.

 

403thousandths bore gm rebuild 350 bored 355 little gray skirts under neath
gm tw or wiseco bored 30 thousandths flat top forged corvette pistons
LOL
got same thing in my garage probably rebuilt american plant
about 87 z28 or corvette engine

 

dude, dude, you replied to a nearly 8 year old thread. awesome.

 

least he used the search function....

 

I don't even get what he's saying. I think he says I posted a pic identical to
030 overbore pistons? Those are 40k mile stock pistons in a 4 inch stock bore block.

And rebuilt... it's from an export car. It left tge factory and no engine builder ever touched it.

As for corvette pistons. The forged ones in that era are the l82 pistons. Later vettes all had hypereutecticd. 464664 is the trw l82 piston and it has a single large valve relief.