andy m Posted February 6, 2014 Report Share Posted February 6, 2014 I have a distributor BSA C15 engine I intend to build for trials, I have a WD low ratio box, I want to keep it 250cc, also intend to keep the bush bearing having a new phosphor bronze one made to suit. Having removed the top end all looks good so far, not run for a while but sound. PVL or Electrex ignition will be used. Has anyone any more tips for a soft 250cc build?? Thanks in advance... Quote Link to comment Share on other sites More sharing options...
davetom Posted February 6, 2014 Report Share Posted February 6, 2014 I took 40thou off the top of the piston of my stock C15, makes it plonk better and less fussy to opening the throttle. Harder to stall too.. 1 Quote Link to comment Share on other sites More sharing options...
woody Posted February 6, 2014 Report Share Posted February 6, 2014 My old C15 was on 250cc (I didn't notice any difference when the bigger Triumph piston was in to be honest) It was on standard road cam, head and valves, followers, low compression piston (ie: flat top piston for standard trials CR that is about 6.5:1) PVL ignition with a Rex Caunt CDI unit with a softer pick up - this was one I asked him to do for me before they came on the market a few years ago, not a current one. I think the one change I made that gave the biggest difference to pick up from idle, was spacing the carb back using two of the tufnel inlet spacers (forget how thick they are - 3/4"?) This made pick up off idle very smooth and predictable and virtually eliminated coughing/spitting back in all but the harshest of throttle opening. I used to run it with no tickover so that when I shut off it would come to a virtual stand-still with no clutch and it would pick up again off the throttle cleanly and with no hesitation. Carb was a 22mm Amal I'm not sure you can put a WD gearbox in a distributor engine as a straight fit as I think the shaft sizes differ. 1 Quote Link to comment Share on other sites More sharing options...
andy m Posted February 6, 2014 Author Report Share Posted February 6, 2014 Thanks so does gearing down just on primary do the job if the WD box won't fit? ( using std road box?) Quote Link to comment Share on other sites More sharing options...
woody Posted February 6, 2014 Report Share Posted February 6, 2014 Yes, reduce it on primary to whatever it should be (I can't remember) and then fine tune with gearbox/rear sprocket combo. Again, can't remember but someone will have what the 'standard' set up is I ran mine on a normal road box and had no issues with the gearing / ratios for sections. I managed to get a proper trials box for mine eventually but I'm not sure it made a huge difference and I'm sure it felt slower on the road. Quote Link to comment Share on other sites More sharing options...
davetom Posted February 6, 2014 Report Share Posted February 6, 2014 (edited) Mine uses a 622 Amal too, with a 3/4" tufnol manifold spacer Edited February 6, 2014 by davetom Quote Link to comment Share on other sites More sharing options...
the yukon sizzler Posted February 7, 2014 Report Share Posted February 7, 2014 You could do no worse than invest in the book -BSA unit single engine manual by "Rupert Ratio". Literally worth its weight in gold. Tells all about parts interchangability down the years Good luck with the build. 1 Quote Link to comment Share on other sites More sharing options...
andy m Posted February 8, 2014 Author Report Share Posted February 8, 2014 I have the book, not had a chance to study it yet. Apparently the WD box fits with machining, ( BMCA have done it ??) Quote Link to comment Share on other sites More sharing options...
woody Posted February 8, 2014 Report Share Posted February 8, 2014 The book is a decent read but doesn't offer much to trials versions specifically I think to make the WD box fit you have to machine the inner casing on the distributor side to take different size bushes as the shafts are different diameters. There may be more to it than that though, I don't know. I don't know of anyone who has done it, so can't point you to anyone. 1 Quote Link to comment Share on other sites More sharing options...
charlie prescott Posted February 8, 2014 Report Share Posted February 8, 2014 Hi Guy's. Hi Andy. If you fit smaller engine sprocket you will have to machine of the oil. Seal lip and then fit a thinner seal. Look on the BSA Otter web site!! You may have problems with the kick start by fitting the WD gears ,unless. You use a latter inner timing cover and machine off the crank oil feed boss. You will then need to use a points on side camshaft and make up a one off bush for the crankcase side cam bearing. Basically you need to convert the engine to "F" spec . Using the Rupert book has a guide. Regards Charlie ..www.bsaotter.com 1 Quote Link to comment Share on other sites More sharing options...
hencam Posted February 8, 2014 Report Share Posted February 8, 2014 hi all, i don't wish to hijack anyone's thread here but i am in the process of rebuilding my brother's Cotswold Bsa and Andy's post is a similar question to what i was going to post. i have found him a C15G motor, and i have some WD gears left over from B40's when i fitted victor boxes in them for motocross. my main query is the Piston subject. the bike is in desperate need of rebore but out to +0.060" already, i assumed i would need to get it linered but it seems from this thread you can fit the triumph piston, can somebody please enlighten me with exactly which triumph model had the flat top piston it should be, the only ones i have come across previously have been daytona pistons fitted to C15 scramblers etc, obviously this defeats the object of low CR to achieve that soft torquey plonk!! i am running standard cam, small inlet head, 622 carb etc etc Quote Link to comment Share on other sites More sharing options...
andy m Posted February 8, 2014 Author Report Share Posted February 8, 2014 (edited) After reading what Woody and Charlie have said it looks to be a pain to use the wd box and for my ability the std box will be fine. Thanks for the info. Charlie, I was looking at your site this morning and noticed the engine build article. Is the piston used different to the std road type ?? Edited February 8, 2014 by andy m Quote Link to comment Share on other sites More sharing options...
charlie prescott Posted February 8, 2014 Report Share Posted February 8, 2014 Hi Guy's Look I am using 9-1 Triumph T100 pistons on these builds , standard C15 heads and valves. It may not work, but this is what the last of the works boys did, and it was said to soften the motor even more. Cotswold BSA C15 is what I am copying has the Nick Draper bike has this set up, and I thought I would build these two engines to that spec. The Blue C15T was on a low compresion set up when I took it to bits. It was using a piston from a B25, and C15 head, and that mix just does not work?? although the bottom end power is good. It ran out of steam at the top end on Long climbs. They still use some of these in the IOM you know!!! Andy I am just doing a page on the BSA Otter site about the convertion from a standard "Dissy" engine to the "F"type spec. and it is not that easy. Hope this helps Guy's. Regards Charlie. 1 Quote Link to comment Share on other sites More sharing options...
andy m Posted February 9, 2014 Author Report Share Posted February 9, 2014 Thanks Charlie, looks like I'll stick with std gear box, gear down on primary and sort sprockets. Not worth messing with if std box can be used as Woody said. I can keep the WD cluster in stock Quote Link to comment Share on other sites More sharing options...
jon v8 Posted February 9, 2014 Report Share Posted February 9, 2014 A while ago I chatted to George Greenland about using unit BSA motor's. He uses a Kawasaki Z1000 piston to run them at about 280cc.He says they go really well at this bore size,plonk AND pick up better than either the standard 250 or the 350cc B40.They also last well,the Kawasaki piston and ring sets seal well and don't burn oil etc. He also talked about a mod to the gearbox to make the shift into a one up,three down pattern - good in deep ruts where you don't want the box to get knocked up a gear,but changing down one wouldn't be quite so bad.I seem to remember he also cut large holes into a gearbox casing to view the cluster as an assembled unit.I think he found that most of the selection or reliability problems of the earlier boxes were down to poor machining and too loose tolerances.It may well be worth getting the cases machined accurately to accept the WD gears,for better selection and ratios. 1 Quote Link to comment Share on other sites More sharing options...
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