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Max oversize for prewar 350?

M20 on the back burner and currently working on my 1933 R33-4 (cooking version of Blue Star) and find I need a new piston. I intend to use a B31 piston and was wondering how far these barrels could be bored out. I understand there is not enough space to allow a liner to be fitted. I will get the barrel measured in a next few days but was hoping I could get a piston to fit without having to rebore. Did BSA offer oversize pistons, I can’t see any reference in my parts books.

Re: Max oversize for prewar 350?

BSA oversizes usually went to +.040"....Many of the barrels will take more than that, in some cases a lot more...The B33 for example can be bored out to 90mm from its original 85mm bore size without any issues...After market manufacturers back in the day provided pistons for many BSA models up to +.060" to give that extra rebore that extended the barrels life...So, in the case of your bike I would imagine +.060" wouldn't present any problems....Ian

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Re: Max oversize for prewar 350?

Many thanks Ian,
Good to know that +40 thou should be safe. Bore looks to be in good condition but I will get it measured when the engineering shops re open and hopefully it will be reusable with just a light hone and the correct size piston.


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