Dear Members, I am in the process of rebuilding my 61 Bonnevilles 389 Engine. I want to change the rotating asymbley and make it a 421. The car has a Hydramatic transmission with the taraus/ hydro coupler rather then a converter. I am having a problem locating a aftermarket forged crank with the correct bolt circle and spacing for the non ring gear flex plate which is used on the 61 Engine. I would appreciate any help or guidance with this project. I want to keep the Hydramatic transmission. Best Regards Gmil60
The best guy to talk to is Steve Barcek at Pontiac Heaven - HAMB member "applekrate". He's into the early stuff, and has a lot of sources.
You might have a problem with the crank as the 421 unit has 1/4" larger main bearing journals.Not sure how much of an overbore it will accept but .060" will give you 414 c.i.IIRC.
The large journal is not much of an issue, and can be machined down to work, but the flywheel bolt pattern may be a problem. You can buy a new aftermarket 455 (4.21") stroke crank to drop right into the small main blocks for sure, and possibly even a 421/428 (4.0") stroke crank. Check with Steve on the bolt pattern.
The Roto-hydramatic will not stand up to the 421 engine, I'd get a post '63 block and better transmission.
Thank you all for your help, I just sent the flex plate to Butler Performance so they can match it up to aftermarket stroker crank. xlr8, I asked this question on the Pontiac Peformance Forum regarding durability of the Hydramatic with a 421. your comment raises my concerns. I did not intend to beat on the car however , 44 years ago I said the same thing to my Dad!! when I borrowed the family truckster . Happy New year to you all and thanks for your help Sincerely gmil60
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