11 replies to this topic

[PaulB]

Hi

I've been trying for some time now to get some details on enlarging the capacity of my B25. It's had several mods already done, including changing the cam, and it now plonks and pulls pretty well. I would like to know the easiest way to get it out to say 280 ish ideally without having to bore the crankcases. I did hear a 650 triumph piston would do it? But which model and compression, is there enough on the standard liner to go out that big, if not what liner will I need?

thanks Paul B

[charlie prescott]

Hi Guy's,

Hi Paul,
Yes faced with the same problem , I have a triumph part Number E6884/10 piston at 69 mm and have checked the barrel and found there is not enough meat left on the liner if it is bored to this size. Yes you can get a liner from Westwood trading that would do the job,Can't find the liner number at the moment will post it later. But it is not an easy job and you will need use of a well equipped workshop.Hope this helps.

Regards Charlie.

[bobtlr]

Hey,

would it be possible to put a b40 cylinder and piston on the engine?
I 'm looking to increase my B40 to 390 cc but i don't want a long stroke engine so a bigger piston will be the way to go, are ther kits to do this (mayby for a scrambler) ?


regards bob

ps nice looking bike that b25

[PaulB]

Hi Guys

Just stripped motor. As B25 cylinder has large alloy sleeve to support the exposed part of the liner, the crank case mouth won't need opening.
Looked on Westwood's website, I think the easiest liner requiring the least machining looks to be WCL 12A FL . Is that the one you decided on Charlie?

Thanks for your help

Let you know how it goes

Paul B

[charlie prescott]

Hi Guy's.
Hi Paul, and Bob,

The liner I was looking at was WCL13DFL. But then Bob said about using B40 barrel, which got me thinking. Well just read an article in a magazine and I quote the following.[Basis of the 350s engine is B25 crankcases BSA one piece crank and Triumph Daytona alloy con rod ETC, and a standard B40 piston]. So I looked up more information in the Rupert book, and thought well then you could use a B44 VE barrel ,push rods ETC, and I think you could mod the head(Stud holes ETC) from the B25 which would be better for a trials bike. So perhaps that is the way to go. Don't forget I am working on the Scott Ellis replica bike at the moment, so have the C15F engine as a basis. that is why I have decided to cast my own barrel. But I have a new Faber fitted with a TR25 engine, so may try this exercise on that later. (So Many things to do I hope I have the time)!


Regards Charlie.

[PaulB]

Hi Charlie,

Following your coments, Just been looking at WCL 13D FL again, as it has so much more to meat to play with, I have also found a triumph 650 piston of 71mm available from Burton bike bits which would give me a reasonably straight forward conversion with a capacity of 277cc. I have been toying with the idea of seeing if a C15G end fed crank could be made to fit. Which hopefully would be heavier and have the roller bigend.
However I think the liner and piston option will be easier to achieve with the machinery I have and quicker, as I start to get withdrawal syptoms if I cant ride every other week!!

Thanks again

Paul

[PaulB]

Hi Guys,

thought I would give you an up date.
Used the above mentioned liner. Which didn't need much machining to form the top flange, and then cut down to length. I used a 71mm low compression Triumph 650 piston, which just required a bit off the skirt and the crown machining to get the compression down to 7.5:1. It gives you 277cc without too much expence. Can't wait to ride it now!

[charlie prescott]

Hi Guy's.
Hi Paul.
Let us know how much better the power is, or any faults,etc. Was it easy to press in, what interference did you use? hot or cold? At least you have got on with the job, well done mate.

Regards Charlie.

[PaulB]

Hi Charlie,

I used three thou interferance, heated barrel until liner just dropped in, then put under pressure in the press until cool, to stop it moving under contraction.
At 10:1 it used to regularly cough and stall just as you opened the throttle off tickover in a section.
On starting, the big differance is that at now at around 7:1 compression the motor is so soft off the bottom, and will now tick over really smoothly and so slowly. Hoping to ride on Sunday so I will be able to better judge the power gain now it is 280cc then

Regards Paul B

[charlie prescott]

Hi Guy's
Hi Paul.
Yes that is what we thought 3-4 thou max to the same method, I am glad the job as gone well for you, and what you have said about the engine is exactly what we predicted, Hope it goes well this week end, I predict you will get more grip and a much more pleasant ride even if you do need adjustment. Good luck. Let us know won't you?

Regards Charlie.

[PaulB]

Hi Guys

Hi Charlie,

Rode it today, spot on! nice and torquey more power throughout. plonks along really smoothly, and pulls cleanly from nothing.

I did notice after I built mine that although I used a 71mm piston, there is 76mm (Bonneville I think) piston available. I'm sure, the liner I fitted still has loads of meat left to bore to 76mm, which would give you around 320cc ? I don't know how the characteristics of the engine would change going this size with a 70mm stroke?
I have a contact number for a guy that has base spacers of varying thicknesses for this engine to drop the compression without having to machine the top of the piston, if you need it

Regards Paul

[charlie prescott]

Hi Guy's.

Hi Paul,
That's brilliant then, Just what we thought. I have just checked my liner for the 76mm and it is possible just but it would make the liner thin again. I would be in two minds to touch it again for a while If it is running so well, It may make the engine too over square, and upset the balance. I will keep the spacers in mind thanks. Keep posting a progress report wont you? I will let you know how I get on with mine.

Regards Charlie.