I own a sand rail with a L20B motor. It has a turbo from a Ford Probe. I

SIMPLE, use L28 Turbo pistons from an 81-82 280ZX, std. size for them is 86.00 mm where as standard bore for an L20B is 85.00 mm. Obviously your block will need to be bored, but other than that it's a straight bolt in item, this should net you between 8.0 and 8.5 compression ratio.

How much boost are you running?
Stock head gasket now?
What kind of fuel system are you using?
What part of the motor didn't last more than an hour an a half?

if you are looking to lower the comp big time I would look into getting some z20 pistons and install them on your rods this will lower your comp with the open head to 6.5:1 and 7.5:1 with the closed head. 20 to 25psi is not out of the question with that comp ratio

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L20B stock pistons have bigger dish than 280ZX turbo pistons.

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Absolutely not. Low compression ratio is one goal but other goal is high turbulence wedge combustion chamber. L20B stock pistons are great with closed chamber A87 or W53 head with normal high flow modifications in combustion chamber, in normally aspirated engine problem is to get comp high enough without exensive high dome piston buty with turbo setup comp ratio would be just right 7,5-8:1.

If you had well made high compression head use it with naturally aspirated engine or sell it to somebody but with turbo setup head flow is secondary, good combustion chamber is primary goal and even stock closed-chamber head is superior compared to good flowing head with some 4mm head gasket or Z20 pistons which won't reach deck level.

hello I'm deans brother thanks for all the replies.hear is the info we are working with;
bore 86.5 already over1.5mm or.060
stroke 86
head 36 cc's this a SSS head from nissan
deck hgt .018
stk gasket is 1.5mm
dish piston .120 deep 2.710 dia
CR looking for is 7.5-1\8.0-1
15 psi boost with intercooler max adv of timming of 24 deg.cam is 112 * special turbo grind
this melted the edge of the pistons within a short time no other damage was done fuel sytem is a weber progesive 36\42 with 100 octane fuel holley blue fuel pump with
if you more info contact me please we are eager to get this running correct thanks doug

could you not just, for cost and simpicitys sake, just make a spacer plate with a head gasket each side between the block and head. Rudimentary but effective in old style engines. It will get you that 4mm space you need.

what is your initial timing set at? how quick is your advance curve? what grade of fuel are you using? what plugs and heat range are you using? is the cam timing advanced?

it sounds to me that your spark plugs are the wrong heat range and retaining too much heat in the combustion chamber, or possably thhat your advance curve is too quick there by causing preignition, or maybe a combination of both.

Says he's using 100 octane fuel. Sounds like rgrinder's on the right track but also sounds like it's not getting enough fuel and running too lean. And like he said it could be a combination of things.