I am planning to do a T140 conversion to my b31 engine. I think I found al the info I need on the forum but I want to be shore I got everything right. I need a Triumph T140 piston complete with rings and pin? I would like to increase the diameter of the inlet valve. Is this the 500cc goldstar inlet valve that can be used? Someone told me to enlarge the intake as wel. 30 to 32 mm,is this the correct size? I also have been reading about rebalancing the crankshaft. I never done this before and I Always tought you did the balancing with the new piston attached to the crank. On the website that I have been reading they speak of 50 to 53 procent of the piston weight. Is this correct?
The 500 Goldie valve will not fit. 350 Goldie valves will fit but the stems need to be shortened, you will need good measuring equipment and a lathe. My 400 B31 has 350 Goldie valves, springs and retainers. Inlet port enlarged to 32mm with a Mikuni flat slide carb, Vernier DBD34 clubman cams, Goldie cam followers and delivering the goods via a Pearson crank with Carillo rod
Thanks fore the info. I am not planning to make it a racing engine but want some more power. Is there a bigger inlet valve that wil fit without shortening? I have a lathe,no problem there but are the valves not to hard to turn them? I have a small workshop and did engine revisions before but this is my first atempt to modify one. The crank and rod wil stay BSA. No budget to change them.
Tim, the most basic 400cc conversion I ever done was to a standard YB 31 small fin engine that I got the cylinder bored to suit a Triumph T140 piston. I had to make a 2mm thick shim to sit under the cylinder to raise it up so I had a tiny bit of valve clearance as the T140 piston has a domed head, the inlet port was enlarged slightly at the carb end as it is smaller there compared to further on in, more or less to get it the same diameter of the carburettor hole which is the standard AMAL 276, you do have to change jets and stuff but that comes when you start testing the engine. I put in two gold star fast road cams as well simply because I had them.
Standard crank, rod and valves all used and it works absolutely fine, it gives your B31 a bit more get up and go. Almost 800 miles on it so far with no ill effects and all working good.
Last edited by B31 Ally; 05/03/179:00 am. Reason: cant spell !
I did the same conversion over last winter to a 1956 swing-arm BB31 motor that I was building for scrambling. IMHO you need to spend your budget on what will best deliver your requirements. Head work is only really going to deliver top-end so if you are not going racing you want to concentrate on getting flexibility through the Rev range with good torque/acceleration characteristics. So you re-bore to suit T140 piston because it's a straight fit (you might want to look out for Goetze piston rings as they are top quality and approx €30 a set). Make a 1.5 to 2mm decompression plate to fit under the barrel otherwise you risk stressing the bottom end and/or getting valve/piston interference (bad thing). If you can, invest in 2448/50 touring cams (€100 ish) - again they are a straight fit.
You don't say what model you are converting but if it has the breather on the timing case you might want to consider fitting a reed valve as the bakelite disc fitted as standard just can't keep up over about 3000rpm. I got a Yamaha one off eBay for a few € and it cuts down on oil leaks due to crankcase over pressure - not a mod for the purists of course.
Good luck with your project.
Mike F Current crop:1946 BSA B31, 1955 BSA B31, 1997 Yamaha TRX 850, 2006 Yamaha MT03 Last year's project - Yamaha XS400 Street Tracker
I've been running a 400cc conversion on my plunger B31 for a few years now. I used a slightly modified low compression T140 piston with a decompression plate and kept the standard cams. It has noticably more go than before and the deco plate is thick enough to make it easy starting with my deteriorating knees.
I have a set of touring GS cams 2448/50 and, when I tried them out in the engine, either the springs went coil bound or the collar was hitting the valve guide. In any case the engine would not turn by hand so I took them out again. I have a few spare heads so I'm intending to modify one with shortened guides and maybe GS springs if necessary, as well as fitting a B33 inlet valve and opening up the inlet tract for a slightly bigger carb. Another winter project.
I seem to remember reading somewhere that the GS springs would require preloading in this application. Does anyone know about this, or if it is necesary in a B engine?
HI Pete, The GS valve springs are a smaller diamater and length than the B31 so will need spacers underneath You will need the top caps and collets to suit, which maybe problematic as the GS collets fit a semicircular groove in the valve stem, I think they are a different taper angle than the B31 as well? a possibility might be C15/B40 caps and collets?? The B33 inlet valve has a larger diamater than the B31 Recently I fitted a T120 valve to a B31 head that had a recessed seat, the slightly larger valve head brought the valve back to a good position in the recut seat Overall valve length can be corrected by taking a little off the valve tip or in this case leaving out the hardened cap