A pal of mine in France wants to use a T140 piston in the B31 motor he is building, to up it to 400cc. He has heard that a barrel spacer is needed, but gets two answers when searching the web, some say 2mm whilst others say 3. Has anybody here had any experience in doing this mod? Cheers.
I did this modification to my B31 back in 2000 and have covered many thousands of trouble free miles with it since. I've also converted quite a few engines for friends without any negative feedbacks.
I've always used a 2mm spacer (decompression plate) but must admit that I showed concern when I first fitted Goldie touring cams with the high compression T140 piston. However, there was no need for me to worry as nothing fouled and that engine has been running fine for quite some time now.
So, I would suggest that a 2mm spacer should be fine but once the job is done I would advise to turn the engine over very slowly before trying to fire it up, just in case!!
If I'm converting an engine for somebody else I ask if they have a preference and act according to their wishes.
I generally don't use gaskets in my own engines. Instead, I use "Granvilles Instant Gasket". This is a specially formulated silicon based substance that comes in a squeeze tube. It's available in various colours, including clear (my preference). Most of my riding is doing long distances and this stuff is ideal if I need to undertake repairs on the roadside or on a campsite miles away from anywhere. With no worries of damaging paper gaskets or trying to make replacements. I'm just more careful of using it around oilways.
I put a smear of this each side of the decompression plate.
Before you do anything decide what compression ratio you require. Temporarily assemble barrel head and piston onto the engine, position with the spark plug hole uppermost. Turn engine until piston is at TDC on compression stroke, or leave pushrods out Using a graduated syringe fill the combustion chamber with thin oil until it reaches half way up spark plug hole and note exactly how much oil has been added Your compression ratio is the cylinder capacity + the quantity of oil / the quantity of oil.