Ian,just wondering which piston to overbore a B31 barrel to 400cc? Was it a 750 Trumpy, and what compression to use with Goldie touring cams. A friend wants to do it.

Hi, yes, it was a T140 one, but it had to be the low compression type

email (option): davmax@ntlworld.com

Hi Douglas..I use the low compression piston Dave describes..However, I further machine the piston crown to get nearer to a sensible compression ratio when fitted to the B31. Depending on what you are after you may also need to fit a decompression plate under the barrel. This can be accomodated at the thickness you are likely to use by the amount of adjustment at the tappets, without any modification. Compression ratio will need to be determined by fluid measurement..it is not possible to work it out by calculation with a domed piston.
A pair of Gold Star Scrambles cams (66-2446) are beneficial and ideal for a road engine. These generally fit with the standard valve springs but it is best to check clearance between the coils at full lift on each engine to be certain. I also trim back the tops of the valve guides back to the location shoulder to avoid any possibility of valve spring collar/guide contact.
If the cams are fitted a change to the inlet valve size, port size and carb size should also be considered neccessary..I run 30mm sizes on the latter two...Ian

email (option): ian@wright52.plus.com

i am slowly building my B31 from a kit i brought from ian one day i will get it running honest ian

Can we buy an 8.1 or thereabouts piston for the 350cc size instead? It's a ZB31 so could we put a goldy piston in it and can we even buy them anwhere.We were hoping not to alter barrel height and such so have now decided to leave it as a 350.We have plenty of Goldy cams to play with and am aware of the guide-spring thing.

Hi Douglas...I have a NOS high comp. ZB32 piston..just the one!..but I can't remember what size it is. I'll take a look tomorrow and add the info to this post...Ian

email (option): ian@wright52.plus.com

Thanks Ian.Anticipate the outcome.Am i to believe it would suit the longrod ZB31.

Hi Douglas..That is correct...as it happens (often these days!) I had forgotten to look...I'll do it today (Saturday) and post the info...Ian

email (option): ian@wright52.plus.com

Hi Douglas..Bad news I'm afraid unless you have a barrel for it...The piston is a STD size one.
It is NOS with the ringset and clips but no gudgeon pin (same as B31)..Manufacturer...Heplex....Ian

email (option): ian@wright52.plus.com

Hi Douglas..I am in the middle of a major sort out and have found another piston in my parts store...this one is +.030". NOS but is missing the gudgeon pin and the compression rings, the oil scraper is present. It is the type with a chamfer above the ring land but still a flat top..I'm guessing here, but about 7.5 to 8-1 I would think. Manufacturer...Hepolite..I can also supply a low comp Triumph piston with the crown mods already done if you decide to go down the 400cc route...Ian

email (option): ian@wright52.plus.com

Strange, i used the usual photobucket system and the picture didn't work.