So we have a pair of late type crankcases,and a new O/S timing cover, inner, and outer.

 I know that there are still C and B engines about with the timing side bush, and has you know I have made a conversion on the Scott Ellis bike engine.

But for this build we are going to set off and build the best engine we can with parts that are available, and to tune this engine to what we think would give the best power for a trials bike.

 The capacity of the engine will probably be near to 280cc and we are going to try and build in a bit better bottom end torque. To this end, I am getting a pair of steel flywheels machined up and will probably use a Alpha big end and rod, I see know point going for the more expensive rods available, as in the end this is only a trials engine.

Well that has started the page, and I just hope someone comes up with a camera for me this Christmas?


Still no camera,but it is my birthday next week!

Well I thought I would publish this letter from Martin as it will give us all something to think about while I am collecting the engine parts,or making them (Flywheels) etc.

 I spent hour's yesterday trying to track down parts that I know exist,but it is not easy! I think I have said before you think everyone is on the Net, but they are NOT, especially the backstreet engineers that turn these parts out. But that is my job isn't it? to find them.

Here is what Martin  sent me.

Hi Charlie,

               I sent a comment through the web site but it limited the number of characters I could use. My comment was that you have given us great info on the frame and cycle parts but to do justice to the frame and the work that goes into building such a bike, we need to know how to make the engine work properly for trials. The standard engine isn't very good and I understand that although there are a number of engine builders in the Midlands, they keep their secrets close to their hearts. I assume the engine modifications are possibly different for the C15, B25, and B40.


Some of the answers to recommended engine mods are already well known through Trials Central such as


-What ignition system                PVL most popular (any others?) (Any adjustment to PVL's recommended timing for the above engines, e.g. slight advance or retard?)

-Gearbox internals                     set of trials gears (very rare so what else?)

-engine sprocket                        18 tooth

-G/box                                       13??

-rear wheel                                 ??

-Carb                                         Concentric type and jet sizes (any other carbs??)


Other questions I can think of for which I haven't seen answers in TC are:


-What compression ratio for each of above engines. (I heard of Triumph pistons in B25s, but is this as much to get the ratio right as increase engine size?)

-The B25 flywheel is smaller than the C15's, should it be increased to match the C15, should it be made as big as possible, (I increased mine by 1/2 inch in dia), or can a C15 crank fit a B25?

-What relative benefit is there in reducing both inlet valve size and inlet port diameter?

-What is best cam and valve timing? R. Bacon book says C15Ts had different cams in different years. Which is best?  How is the opening point defined, is it just when the tappet clearance is zero or at some specified amount of valve lift? The standard B25 inlet opens around 52 deg BTDC, I thought a trials engine would need closer to 30 deg.  What are relative benefits of soft cams with high compression versus hard cams with low compression?

- Recommended exhaust length and silencer type to keep engine quiet without strangling it?

- Any improvements to the lubrication system other than in-line filters? 


I am sure your readers could think up more questions.


Regards Martin de Vries 




Has I have said on the news page  I have been studying what the old works riders said about the best BSA C15 trials engine. And most of them thought the older motors had better softer power, for a trials bike. But we have our "G" type bottom end, so I need to find other ways of softening the engine. I have been told that if you use an industrial engine piston, this can help, I am researching this at the moment. And has I have said again on the news page, I am getting the pattern for the alloy barrel  started again. I can tell you that a small valved C15 cylinder head will be used, and experimental camshaft, and followers, will be used. The profile for the camshaft will be based on the camshaft that I have, taken from GOV132 by Don Morley. So It may work with a similar profile on a BSA or it may not.


Here is  a bit of information from one of our Canadian friend's. 


Hi Charlie,

 I was just reading your page on c15 engines, I live in Canada but I am a Brit. We have vintage trials here and one of the guys rides a B25 but he fitted a C15 crank and top end to it. He is an engineer so had the tools to do the job, he converted the big end oil feed to quill type and made up a spacer to raise the barrel. The bike was transformed and would plonk and pull from zero revs. I talked to Jeff Smith about this and he reckoned the conrod length and ratio had a lot to do with the C15 being a better engine. Just to let you know we enjoy reading about your exploits and your skills with frame building and machine work. Thanks, Derek Cherry.


Thank you Derek.

I have this past couple of weeks been scanning the pages of e-Bay to find the bits I need to assemble the engine. I am amazed at how difficult it is to get BSA  B and C engine parts, and how expensive they are becoming. Anyway parts are on the way, and I already seem to have spent the budget, but has I said the project was never going to be cheap. Looks like I should use one of Pete Kirby's belt drive clutch units, if I can beat him down a bit on price? ( Look at all the free advertising you will get on this site Pete!!).

Any way I will start to build the bottom end up this week, and from advice from someone who knows, I am now using a crank assembly from a BSA unit but not what you would think for a C15.

Pictures will accompany the build. 


OK Call me thick then,

I have just spent some time today going through old email's from you guy's, and a new one.

And have now realised that this engine build page is not live well , sorry I must have thrown the switch so to speak, and I hope it is back on now.

 Well a quick update, I now have all, of the parts to build this engine and the other two that I am rebuilding. That is the one from the Nick Draper bike, and this engine is interesting inside, and has shown me the way to go with the NEW engine. the other rebuild is the Scott bike engine, I have obtained a better pair of crankcases for this motor, along with an updated clutch. The New motor will have a Pete Kirby Diaphragm clutch if he will sell me one? Dave Wood'sbike also has one of these fitted. The bearing's I have been waiting for are now here, so if the weather gets a bit warmer next week I will get the spanner's twirling, and the camera clicking !!