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Discussion Starter #1
I'm looking at building an big bore B20 with a 2130 overbore. Is there any reason I shouldn't (or couldn't) use the marine B21 Penta pistons for the build?
 

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If the B20 is a '74 or '75 (flywheel held on with eight bolts), B21 pistons will swap right in. If it's a '69-'73 B20 (six bolt flywheel), there's a mismatch in the wrist pin size, 22mm diameter vs. 24mm for the B21. What you can do in that case is bore the rods for .0006" running clearance and install the pins without bushings. Sounds iffy, but works just fine.
 

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Discussion Starter #3
Thankfully, the engine I plan to swap in will be from an 8-bolt '74.

So the bore is really the only thing that -needs- modification when overboring, correct?
 

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I did just this. Picked up a set of used pistons and Beefier M rods from a boat outfit. Much easier to come by than flat top auto slugs.
 
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