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Bsa B40 Engine Rebuild


duckwizard
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Help required.

After stripping down my B40 i noticed the following.

The engine has a high compression piston.

Should there be a compression plate fitted ?, as the piston has touched the valves.

Any advice on thichness of plate.

The bike was always difficult to kick over, which maty explain this.

Any advice appreciated.

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Does your bke have a cast iron barrel or an alloy one? Some alloy ones are machined from B25 barrels and may require a plateunder the bottom. If its a std cast iron barrel then a high comp piston shouldnt be a problem unless the cam has been mistimed or oversize valves fitted. Had aB40 myself and sometimes the marks on the cam are out. May be worth checking. One tooth is all it would take. Then again could just be tired valve springs and a bit too much revs? I used a std piston with an alloy barrel and two paper gaskets on the base. Std copper head gasket is fine.

One other thing to check are the pushrods. Make sure they are not bent after the valves met the piston also that they are the correct ones as a lot of these engines have been rebuilt in their time from various parts which all look similar but vary slghtly i.e. C15, B25, B40, B44, B50 etc etc

Best source of information i found was Rupert Ratios book which comes up regularly on e bay.

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If there has been no machining to the crankcase, cylinder barrel or cylinder, head even with a H C piston and MX cam shaft there should be plenty of room between the valves and piston. Check that everything is standard, including the valve timing. A "physical" check on the valve timing may be needed to verify that the dots on the camshaft pinion & the half time pinion are correct, ( they have been known to be wrong.) Does it seem that the valves have been "kissing" the piston or been giving the piston a big whack.? When you find the problem i would be interested to know what the problem was. Regards, Slinny, Pat.

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