6 bolt VS 8 bolt B20

Rod journal diameter and width is not the same on 6 and 8 bolt cranks. 6 bolt crank rod journals are slightly larger (54.102 mm vs. 54.0 mm) and slightly wider (32.05 mm vs. 30.05 mm) than 8 bolt crank rod journals. The main journals are the same.

hiperfauto said:

Rod journal diameter and width is not the same on 6 and 8 bolt cranks. 6 bolt crank rod journals are slightly larger (54.102 mm vs. 54.0 mm) and slightly wider (32.05 mm vs. 30.05 mm) than 8 bolt crank rod journals. The main journals are the same.

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Hmm, interesting... well, then press that pin in the small end and ditch the bushing.

That would be the easiest way out of his predicament. Another option is to locate an 8 bolt crank and rods as it will bolt right in to his block. Early B21 cranks have a removable seal sleeve and are identical to an 8 bolt B20 crank. Later B21/B23 cranks have an integral seal sleeve and won't work.

Another option is forged pistons. Sure they cost more but we have these made in 3.6" (91.44 mm) diameter so that a stock head gasket can be used which saves ~$100. They're also a lot more forgiving than a cast piston. Displacement is a little smaller than IPD's kit (2101 cc vs. 2130 cc).

http://hiperformanceautoservice.com/product_info.php?cPath=511_513&products_id=456

JohnMc said:

Last time I replaced all 4 pistons with a new set, I have 3 'good' pistons sitting around. I have a half a mind to fix the PV quick, dirty, and cheap by just pulling the motor apart, swapping a piston (having them all balanced), and putting it back together.

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FWIW, that's what I ended up doing with the PV motor. It ended up being an almost identical failure on the piston, in almost the same exact spot on the piston, on the same cylinder. Weird. I used one of the 3 pistons from the previous set, had the machine shop install it on the rod since the interference fit isn't something I think I can do at home, and then balanced it myself at home to within a gram (big end and piston end separately) to the others. Spins nice and smooth, I'm back to abusing the PV like I stole it.

There is a 3rd option. But might take a little bit more maths, machining, and a finger crossing.

Have the 8 bolt rods opened up by .004" (.102mm) bringing them to 6 bolt spec. Then measure, maths, and fashion some shims on the piston side to give you a piston guided setup.

And another interesting tidbit, my 6 bolt rod measures 58.04, and a maxpeeding rods h beam I have for a b21/b23 measures out at 58.08 on the big end. So seems to me the difference between the b20(8 bolt)/b21/23 might be shell thickness and width rather than big end id.

Been reading and searching and came across someone saying that early ('75) B21 rods fit the 8 bolt B20 crank? If so I located an 8 bolt crank and need to get it ordered before it's gone...getting rare even in Europe

The 8-bolt crank is basically the B21 bottom end. They just did the big redesign in two stages. Bottom end first (8-bolt B20), then the top end (OHC head).

Any B21/B23 rod will fit an 8 bolt B20 crank.

Early B21 cranks were identical to 8 bolt B20. The seal sleeve that is installed outboard of the crank gear on a B20 is shorter and installed inboard of the timing belt pulley when used on an early B21. Later cranks are machined with an integral seal sleeve.