91 A engine

[KiWinUS]

Friends I have just dismantled the first flathead I ever purchased a 91A that was 47 years ago. It was out of an old race car , I also had it in my 36 (avatar) many years ago & it would fly!!! 3 3/16 Merc engine factory relieved.
Question what is everyone's feeling (before I sonic check it ) that it will safely bore to ?
thanks
cheers
Tony

[V12Bill]

I removed the sleeves in my 81A and bored it to 3 3/16". This has always been a common re bore.

[Phil Gillespie]

Not all these 81A & 91A blocks are fitted with sleeves I take it?
Whats the bore size less sleeve generally?
Phil NZ

[Ol' Ron]

We don't have enough information here. the thin sleeves are .040" thick, so removing them will increase the bore by.080" amd you can by pistons for that, Some of the 99 series blocks are very thick and can be bored to 3 6/16, AS old as this is it could be anything. Another thing, when removing the sleeves there is a small recess at the top that make installing rings difficult . Start measuring and tack some pics.

[rotorwrench]

91A would be a 221 cid engine. 99A would be the 239. Relieved engines were generally slated for truck use. The deck characteristics will tell if it is a 239 or not. The 221 engines are kind of limited on how far they can be further bored from 3 3/16". A sonic check would be helpful whether it is a 221 or a 239 to get an idea about how much cylinder wall is left.

[V12Bill]

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Originally Posted by Phil Gillespie

Not all these 81A & 91A blocks are fitted with sleeves I take it?
Whats the bore size less sleeve generally?
Phil NZ

The 81A was Henrys "forever engine". He thought that by replacing the sleeves and/or the valve seats that the block would last "forever". This engine was in service with the sleeves from 1938 to 1942. The 41 and 42 blocks were the ones with the "raised intake" a slight modification from the earlier block.