Destroking a BOP 215

[Moderator]

Reducing stroke is a time-honored way to meet racing class restrictions whilst achieving superior volumetric efficiency.

Most famously, Jack Brabham won the 1966 Grand Prix World Championship with Buick and/or Olds 215s destroked and otherwise modified by Repco to 2.5 liters.

There's quite a lot of anecdotal evidence that destroking Buick and Olds 215s was common practice in multiple forms of racing during the 1960s. I've just done a quick Google search, and found mention of destroked 215s for SCCA sports car classes (probably to 3.0L), for Midget open-wheel dirt track cars (to 2.5L), and also for hydroplanes. But I'm curious to find more specific information.

-  Can you guys help me document any specific examples of destroked 215s from that era?  (By whom? To what displacement? For what racing series and class? Was the exercise successful? Etc.)

-  What are your thoughts about the technical issues? (How much can you realistically reduce a factory 215 crank's stroke via offset grinding? Would it be practical to "weld" the crank for even less? What other tricks or techniques might facilitate success results?)

[mgb260]

Curtis, I think the 62 Mickey Thompson Indy motors were destroked. The Brabham 3 liter Olds v8 was also destroked.  Looked everywhere for bore and stroke specs?

https://www.macsmotorcitygarage.com/featherweight-wonder-inside-buicks-1961-aluminum-v8/

[BlownMGB-V8]

If there was a forged crank available it was probably quite practical to weld up and destroke the crank to whatever you wanted. At 2.8" it was a short stroke crank to begin with.

Jim

[mgb260]

Curtis, If you offset grind to a smaller journal these rods would work. Have to figure compression height for pistons.

https://www.ebay.com/itm/363516988907?hash=item54a34d0deb:g:4pkAAOSw5BpdYxJg

[Dan Jones]

> also for hydroplanes

De-stroked (and sleeved for smaller bore) Buick 215s were used in the the 2.5L A-class hydroplanes:

 https://www.vintagehydroplanes.com/boats/media/media_spec_a150_cametoplay.html
 https://www.vintagehydroplanes.com/boats/directory.html#A%20Class%20Modified%20135ci/150ci/2.5%20litre

Buick 215s were much more popular in the 225 cubic inch N-class:

 https://www.vintagehydroplanes.com/boats/directory.html#N%20Class%20Modified%20225ci%20Div.%202%20&%20regular%20225ci

Stroked Buick 215s were also run in the larger displacement E and F classes.

> whilst achieving superior volumetric efficiency

Don't get hung up on VE.  It's a mathematical equation and largely irrelevant.  The only reason racers de-stroke is to meet class rules, often to get a more advantageous weight break.

> I think the 62 Mickey Thompson Indy motors were destroked

That's incorrect.  Mickey Thompson's Buick Indy car engines displaced 256 cubic inches (3.625" bore by 3.1" stroke).  The May 1970 issue of Hot Rod magazine ("Inside Buick's Back Room") states a re-worked factory iron crankshaft was used in the 1962 entry as there wasn't time to develop and manufacture a steel forging:

 http://wildaboutcarsonline.com/members/AardvarkPublisherAttachments/9990352744406/1970-05_HR_Buick_Experimental_Engines_1-4.pdf

Lance Reventlow's Scarab sports racer was also stroked (3.56" bore by 3.0" stroke) for 239 cubic inches.  The Brabham-Repco BT-19 engine was a 3.0L (183 cubic inches) with 88.9 mm (3.5") bore and 60.3 mm (2.374") stroke.

Dan Jones

[MGBV8]

Great info, Dan.  I was not aware of the hydroplane use.