Early 101 and 91 blocks question...

[allan savins]

Hi,

My question relates to the early 101 and 91 blocks with bolt in boost port covers.

Did milling a window into the side of the block for the bolt in boost port significantly weaken the block?

Regards,
Allan

[Sonny Gerber]

Allan, speaking of the 91 and 91A blocks, I have never seen any problems with the blocks warping or being weak in any way. I've had 91's that run very very close to my best 91b's or 91b1's. I personally think they are good blocks.

[steveohara]

Allan,

The first generation 101 blocks were determined to be too flimsy with the dual exhaust ports and the big single window for a boost port. It was common to see them fail by cracking through all the ribs between the ports and pop the top of the cylinder off. When McCulloch released the 101A the writeups in the magazines reported that the engine design change back to the single exhaust port was primarily a structural consideration rather than a performance decision. The single port layout leaves a lot more material between the sides of the exhaust ports and the first transfer. Same for the change to the boost port.. lots more material remains to provide strength.

Oddly, they did not feel the 91 series needed to go back to the single port like the 90 blocks but they did beef up the 91B and later models with more material behind the center rib in the exhaust... a very good move for heat transfer from that sensitive area.
Steve O'Hara

[allan savins]

Hi Steve and Sonny,

Thanks for the feedback.

I had a feeling a little too much material had been removed from the blocks. High Comp Alcohol and modified level of tune, with a tuned exhaust would be asking for tears?

Allan