BSA A50 Pushrod Fail Possible Causes and Fix

Found (I hope) and fixed the cause of my pushrod failure and probably also my previous rocker arm failure.

[@oompah_your_dupa]

Nice job troubleshooting that! Gives me yet another thing to check while I rebuild my own A50

[@InTheShedUK]

Just goes to show that you can't trust new pattern parts.

[@rotax636nut5]

Back in the day I lost a cylinder on my own BSA A50 (VKN46J) while on a touring holiday with my girlfriend right up in the top of Scotland, I didn't have the tools to do much other than remove the tank and rocker cover and I found that I had a broken outer inlet valve spring which had jammed up and bent the pushrod. Using a rock I straightened the pushrod by beating it against a kerbstone and by setting the pistons at TDC so the valve couldn't drop into the cylinder I got the retainer off and removed the broken spring, then put it all back and continued my tour just on the inner spring and reduced RPM, I rode the old girl over 1000 miles like that without issues and fixed it properly once I got home, Roland, HT Racing Ltd

[@InTheShedUK]

A great bit of roadside engineering there!

[@denerumsby6789]

Hi Ian very interesting video I remember you had a rocker arm break a while ago and had to replace it - is this the same valve whose arm broke, or a different one?

[@InTheShedUK]

That was on the other cylinder but also the inlet side so I can only assume caused by the same problem with springs.

[@gpo746]

One possible cause is that you are using A65 pushrods which are longer than A50 pushrods . I have just checked the ACTUAL spring wire thickness for you. I have some NOS Terry VS641 Springs in the box ready for my A50 Royal Star rebuild The measurements are : OUTER : 3.83mm (VS641) Inner : 3.00mm (VS451) Measurements were taken from 2 middle coils of each spring , readings were bang on every time . Looks like the aftermarket Hepolite ones are too thick and your rocker 'balls' are too low and the rod is crashing into it . Hope this helps

[@InTheShedUK]

Definitely A50 pushrods, A65 ones are about 5mm longer so I doubt I would be able to get any gap on the tappets. My old original springs are about the same coil thickness as your new Terry ones so yes does look like the Hepolite ones are made incorrectly. Hepolite springs are not compressing enough causing stress on pushrod.

[@gpo746]

@@InTheShedUK Yes CBS did a youtube vid explaining about "new" valve springs , Thats why I ended up going for NOS Terry springs from the 60's/70's

[@retromechanicalengineer]

Strange indeed Ian. Just as an afterthought, are your pushrods a good match for the cams you are using? Best wishes, Dean.

[@InTheShedUK]

Yes pushrod/cam combo works ok, according to parts book the Wasp uses A65 cam and A50 pushrods. I can get correct gap on tappets so fairly confident it's just the new springs are not made to the correct spec i.e. too many coils and too thick wire diameter. I emailed Hepolite to let them know but they just implied that I had bought the wrong springs! The packet was definitely marked VS641. One of the guys in the bike club had the same problem with his Rocket three.

[@retromechanicalengineer]

@@InTheShedUK typical, blame the customer!