My C15F (1965) ever since I bought this bike 6 months ago, has had a knock that I could hear, but others at the local BSA club, said they couln't and not to worry about it.
The knock is getting louder and due to the icy weather I haven't had chance to get over to the club, some 30 miles round trip away, for them to have another listen. Apart from that I really don't want to ride the bike with this knock.
so anyone willing can browse over to youtube and hear the noise I refer to. The video is really dark and crappy, for which I apologise, but its the noise coming from the engine that is the important bit.
Hoping somebody can advise me as I am really nervous about stripping the engine. This is all new territory for me.
Sounds like a normal C15/Unit single clatter to me.
Grab a big screw driver and touch the engine with the pointy bit and stick the hangle to your ear, this will magnify the noises where the screwdriver touches, move it around until the knock is loudest and report back here what that is.
It may be big end but that is loudest under load not idling so probably something else like the alternator rotor etc.
I did quickly try the screwdriver tip when I made the video. I couldn't hear the knock when I touched the primary case or the bottom of the barrel area, where they stamp the numbers, but at the rocker area I could hear a knock but not certain if its in sync with the knock.
The knock becomes really evident when you roll off the throttle after taking taking the engine up to half throttle. I'm hoping and praying it's piston slap or an upper end problem as I might have the bottle to attempt that repair myself. Just wish I could find somebody in the Manchester area who could help me, for a fee of course.
I'll try and make another video as the one I just made doesn't reveal the knock very well.
BSA Pete, i watched your video and i only ever heard a noise like that on a C15 once and it turned out to be a small end in the conrod had got worn. Im not saying thats what is wrong with yours but the bike in question i worked at when you accelerated it was slightly quiter untill on the over run when you throttled back the noise was lethal as the piston was now hitting the head because there is no explosion of fuel and air to keep it pushed down. When i say hitting the head its only just hitting it but it can make sum racket.
If it were my C15 then I would take the head and barel off. This is a simple and straightforward job (it is on my '68 B25 which is essentially the same) and you can then answer your own questions by checking the little & big ends as well as the bore/piston clearance.
The weather is good for doing jobs like that (because its horrible out on the roads just now!) and you can then either put things back together or dismantle the engien to get done whatever needs doing: and you will be ready for the new season (assuming we get one!) early next year.
Sorry for the delay in answering but I've been really busy.
Thanks for checking out the video and you are spot on when you say the knock gets worse on the overrun. It's scary.
Using the screwdriver tip I can detect the knock at the upper head area. At the bottom of the barrel or crankcase I can't hear any knock. I would try removing the head but the 2 head nuts on the off side are awkward to get at. Do I need a special spanner to ensure I don't knacker them?
If the little end is going will there be bits in the crankcase which would mean a full engine strip down?
Hello Pete The fist thing you need to do is buy a service and parts manual. The only way you are going to find the problem is tear down part or all of the engine. If you don't feel confident there are some engine rebuilding dvds out there. They probably won't specifically be on the C15. But they will show you proper shop technics on engine rebuilding. Even try u-tube. Good Luck and keep us posted. Jeff
You need a whitworth ring spanner and it may need thinning to fit, opened spanners do not give enough arc and also come off the nut too easily rounding the nut. Have you checked the valve clearances, an adjuster nut may be coming undone.
Also a worn small end will not need a full strip to fix, a quick look in the sump will show what has been circulating.