anyone know for sure that- or if- the L69 pistons were different than the LG4 pistons- and know if the compression ratio increase of the L69 was due to not only the 350 camshaft useage, but to piston difference also ?


just checking

 

They were different, until 86. L69s all got flat-tops. LG4s got dished ones up through 85. 86-up the LG4 got flat tops.

 

The archives say that 85-up got the higher compression slugs. Never had an 85 apart, though

 

I have an 85 and they're Flat tops, this was banged around alot 85 up LG4's have 9.5:1 cr.

85-87 LG4's the same rotating assembly as an 83-86 L69

So Crank, rods, pistons are the same.

 

Not quite true. '85 was two-piece rear main seal, '86-'87 are one-piece rear main seal.

The difference in '83-'84 CR LG4 vs. L69 was the pistons (dished vs. flat-top). Cams can't affect static compression ratio.

 

knew there had to be a difference in piston heads, couldnt really see any other way the big difference in CR of an L69 to an LG4

i think im gonna put in L69 pistons in the LG4 block i have to freshen it up for the 82 T/A- and have crank fixed up for new bearings also sinc ei have some good shape heads ready for it.

but pistons changed in 86 ?

cant use 85-down L69 pistons in an 86-up block ?

if so then what changed that makes that so ?


thanks

 

Pistons didn't change except in metals and such but over all a piston from an 95 350 will fit in a 70 350, no problem, same for 305s pistons, as are pistons in that respect.

The 86-up referres to the rear main seal mid-86 and up it went to a 1 pc rear and 85 and back it was a 2 pc, this I believe is only for the block and crank. The only othe major changes, is a roller cam instead of flat tappet and center bolt heads.


So you should be able to use any flat tops for 305's in any 305 block provided the bores are the same of course

 

The LG4 pistons changed in whatever year it was, from dished to flat-top. There's nothing else about them that's different.

Speed-Pro makes a hypereutectic flat-top 305 piston, for about $13 a hole IIRC. I'm driving a set of them around right now, they seem to work just fine. Those will of course work in any 305 block, any year, under any heads, and will give you the 9¼:1 CR of the L69 & LB9 when used with 58cc (305) heads.

Don't go trying to find a bunch of used stock pistons, that's stupidity of the highest order.

 

13 each ??? that cheap ?? what brand and number ??

i dont see why there would be anything wrong with used pistons as long as are in good shape,



thanks

 

You could even say that putting all that money in a 305 shortblock is stupidity in the highest order, when the same money will get you a 350 built up.

 

http://store.summitracing.com/partdetail.asp?part=8468

They've gone up since then.... $14.39 now

 

some regions have no 350s left to find, none worth doing anything to anyways, its actually cheaper to go with a runnign 305 and just put these pistons in and new main and rod bearings and a new stock 350 cam and use good shape 58cc 22601 heads i already have and be done for alot less $$
than rebuilding a 350 mildly, its only a 2.56 geared trans am, nothing special, not like its gonna fly anyways

 

good deal, good price for getting 9.16 CR

thanks !

 

I believe the L69 had better rods than the LG4, as well.

 

Nope, Same rod. Same crank. Same cylinder head. Different piston. Different cam. That's about it for differrences in the shortblock.

--------------------------------

As for the question of 350's being hard to find, I have a 350 4-bolt main bare block complete with line bore, and ARP main studs that I cannot even sell for a measly $100. If the blocks were so scarce, I'd imagine that this block would be snapped up by someone. Everytime I've advertised it, no one calls or emails.

 

sounds about right to me, i agree with that

 

Actually, the rods are different, at least in some of them. My L69 came with "X" rods, which are not typical for a 305 at all. Those use to be the most sought-after stock rods for 350s, although nowadays there are much better ones available. Most 305 rods are much thinner in the beam than 350 rods, and are consequently weaker. I have never seen a LG4 with anything but the run-of-the-mill 305 ones.

But that has nothing to do with compression.

 

Yeah, that is what I had been led to believe, but I know Daniel generally knows his stuff, so didn't want to make an issue of it.

Oh well, no one's perfect

I think GM did it to ensure a rev to their stated redline at 5500 rpm.

 

Right...

It may have been a year-model thing too. My car is a 83, not too many of those running around; I don't know what was in the 84 or 85 L69, which are far more plentiful, but might have regular 305 rods. I simply don't know.

 

what number or markings are on X rods that are not on others ?

 

X

 

I didn't know about the rods, either. Maybe when the Minirammed 385 is finished, maybe then I'll pop open my L69 to satisfy my curiosity. I do know that it can rev to 7,000 without any grenading! Darned pesky 700R4 didn't want to shift to second gear once last summer. Seems fine now, though...