heres the deal, bought the car with 73K on it, at about 93K i blew the clutch for the 3rd time. since it was aproaching winter i decided i would go through the whole car. rear/trans/and engine.

pulled the engine apart to find all the main and rod caps were already stamped with numbers... this thing was already rebuilt!!! pulled it apart anyway because i was going to swap to forged pistons. well after much debate i got another block through a trade i plan to make a forged lower for a turbo setup.

so now i want to slap the stock lower together with a 306 cam until i get the forged lower together, i expect another year at least.
well i began assembly and busted a ring compressing them.

so now i'm sending it to the shop for new cam bearings, and ordered some rings. i double checked the pistons today just to make sure it wasn't bored out...

the one piston i took to work to measure read 3.970 or something close to that.

is the stock pistons that sloppy in there????
seems like an aweful lot!

what gives??

 

3.070? Stock bore on a LT1 is 4.000". That would be quite loose

 

i know, that's what i don't get, i'm taking them all in today to check them over better. don't make much sense.

 

you sure it wasnt 3.970?

that would be about stock.

 

3.970" close to stock??

That's as close to stock as a 4.030" !

Stock hypereutectic pistons should measure ~ 3.997-3.999" . If they're forged in the stock bore they'll be ~ 3.995" or so.

-Dave C. '97 Z28

 

If its an LT1 and has the 327 written on the block then its 4". It could be an L99 which look like an LT1 but has a lower capacity of 4.3L but I'm not sure about the bore.

 

I hope you didn't get screwed and have an L99: 3.75" x 3.00" = 265ci. Uses a 5.94" rod. A stock piston for that motor would be pretty close to 3.70". Have the shop measure the bore.

Rich

 

lol, opps, yeah typo 3.970 was what i meant.

ok, made sense of it.

heres the deal, i was measuring up by the crown, above the first ring since i was using 6" calipers this is what i could reach. aparently they keep it well undersize at the top of the piston to deal with the heat expansion or something..... maybe this is why stock lt1 pistons have a rap for blowing the top ring land!

it's just under 4" on the skirt area. so everything is fine. block is getting new cam bearings and a 306 cam as we speak. i'm gonna clean out the ring grooves and get it ready to go back together.


what are the guidlines for ring gap? i want to keep it on the loose side, i may thow some boost to it.

thanks,
mike