Has anyone tried putting a a12 crank in a a15. I been thinking about it and it should pull the piston down from the top off the block which would decompess it suitable for a charger and make it rev a little harder. Thoughts?

Talk to member Bige (Earnie) he will probably be able to fill you in on what you;re looking for...

Assuming it all bolts together the piston will be 6mm down the bore, that's a pretty massive decompression! you'd have something like 55-60cc of clearance volume or a compression ratio below 6. Planning on running 60psi boost?

Hahahaha thats awsome yea why not its a datto itl take it. Lol cool thats why i thought id ask first. Thanks lamb_

Great Topic warner01..Is your a15 Stock?

I would be keen in doing that to my a15 if its a bit better at performance?

Yea motor is near enough stock but looks like 2 do that i would need 2 get longer con rods or a taller / really domed piston 2 get the comp ratio back up to somethin thatl run then i think that would be a great combo with the short stroke for rpm and the long con rod and big bore of the a15. I may have 2 try this be4 my ca18 conversion. Maybe.

Also i doubt this would work as a direct power increase just cos itl rev easier in fact itl be a decrease in power, until its forced inducted then i think it would work pretty good.

An E15T piston would take up 3 of those 6mm. With a H75 head (open chamber) it might be a half decent combustion chamber. Lots of dish though and I guess a CR sitting around the 6 mark. Have a look at the ACL Piston Catalog for options. Try find the pdf which is still floating around the internet in forums.

Perhaps a thick head shim would be a better way to go about creating a lower compression ratio.
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Resorting to crank and block combinations seems to be alot of effort for what you want to achieve. I had one made out of 3mm plate.

Of course the next major obstacle (following the countless many) would be blowing head gaskets and piston or conrod failure.

Once you sort those issues it'd be interesting to hear how it goes.

Its actually common to speedway Datsun racers.

A14 or A15 block with A12 crank with modded crankpins
for E16 140mm conrods and Mazda pistons @ 29mm comp height
or 1mm milled a15 77mm pistons gives 0.100 deck clearance.

1304cc @ 2:1 rod stroke ratio with good springs should rev well.

Off the wall 2:1 rod stroke ratio 1515cc
Full sleeve it to 83mm using milled 1mm off ca18de pistons
with worked raised section to suit 8v A series head

Ca18det alternative for boost
As above yet ca18det 83mm slugs fully sleeved block, race head,
prepped CA18de rods, A15 crank stroked +1mm to 83mm = 1796cc

the big block A would would pass engineering even with boosted
T25BB yet look tiny and very oldschool. There have been a couple of these big block A series one recently for sale in the US and another close to completion in QLD.