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240z overbore


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Hi guys/gals

I'm trying to ascertain how to go about overboring my 240z engine to take 280z piston, like rebello do. I'd be interested in any experiences re compression ratios (e31 head) and whether I need to change the conrods (I have the early thinner bolt ones) and any piston height problems etc

Thanks for any help.

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The L24 block can be overbored out to L28 specs, but thats about max.

As for C/R, should up it to around mid-high 9's, ie 9.5-9.7:1.

This will give an increased capacity of 2.6litre but what I like about this modification is you're keeping the short stroke, therefore the engine will rev harder.

I've helped build a motor once that didn't go the usual route of retaining the 240bore size and throwing in L28 rods and crank, this also gives 2.6 specs.

Rather we did what you intend doing and that is using a L28 bore with 240rods/crank, again 2.6 litres. We felt this engine had more power than the "popular conversion", we put this extra power down to bigger bore and higher revs (shorter stroke). Many V8s go down this route when talking "stroker vs non stroker", higher revs etc etc. It all depends on the end package and how you extract the most from it ie cam, valves, reducing weight, pulleys, ignition, g'box, diff ratio.

Mind you we also shaved the head .050", C/R was quite high, don't know what, it was never measured. Bear in mind I'm not saying this conversion would be better on the race circuit than a fully worked L28 ie L31 triple carbs, big cam etc etc!

Here's what we done;

N42 L28 block (86mm bore)

N42 head shaved .050" (bigger valves than E31)

L24 9mm rods and crank

Flat top pistons

240 SU carbs (modified needles and jets)

Camshaft provided by w'shop (not sure on specs)

280ZX elec distributor & coil

Electric fan

3 row radiator


2.5 inch exhaust

C110 5 speed

R180 3.9 open differential

This motor was fitted in a C110 Skyline, wish it was in a Z, the Z's handling would have loved it. Evidently was involved in a smash a few years back and is gone!

Anyhow it was a quick car, and no, you won't have piston height problems, changing the pistons don't affect height only con-rods and cranks on these motors.


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