Hi All, I completely lost the left cylinder yesterday when returning from a 90 mile round trip.
Stripped the bike this morning to find a broken inlet pushrod end (see photos).
Anybody else had this?
I would normally assume it was just a faulty pushrod but a few months ago the rocker on the right hand inlet snapped so I am wondering if there is some other reason?
Any ideas?
Ian
Your photo shows a single but you are referring to a twin
Which one? Last time I saw damage like that was when the piston hit the valves.⁷
Definitely a twin, BSA A50.
When the rocker broke (on the other cylinder) previously the piston looked ok see my video https://www.youtube.com/watch?v=SorZeuvQamA
but I will take the head off again just to check.
Quote from: idie on 10 Jun, 2024, 11:32
Your photo shows a single but you are referring to a twin
Which one? Last time I saw damage like that was when the piston hit the valves.⁷
What photo shows a single? The head is clearly from a unit twin.
Since your getting failings on the inlet side, are the pushrods fitted correctly and in the right positions? Have you got the rockers in the right places (though it should be obvious if you haven't). What valve clearances are you using?
I'm sure pushrods are fitted correctly, don't think it's possible to fit them them in the wrong position.
Valve clearance inlet .008 and exhaust .010 as per spec.
Valve timing is also correct, I did videos of the rebuild and checked back that I had lined up timing marks. I am going to check valve opening/closing with a degree wheel once I get the new pushrods.
Hi,
I suppose that as our bikes are getting on a bit the occasional failure is to be expected especially when the history is not always known. The rocker could have been subject to unusual forces in an engine blow up hence a weakness. Ditto the push rod.
Is it possible that the broken push rod was on the other side when the rocker failed? This could have caused the start of a hairline crack that has ultimately failed.
Dave.
Failure on one cylinder may just be bad luck. Both seems suspicious. Could the valve springs be coil bound, making the pushrods and rockers the weak points between cam and valve. The smallest restriction of movement might, over time, hammer enough to cause the damage you have experienced.
Hi,
Good idea re springs. Have you fitted new springs recently?
Dave.
Yes, new springs, valves & collets. done about 250 miles.
Hi,
If the head is still on and the other pushrods in place turn the engine until a valve is fully open and note if there is still a gap between the coils of the spring. Take a photo if possible.
Dave.
I will try that when I get the new pushrods and put it back together
No Gap! or at best very little.
I could just about get a .0015" feeler gauge between one of the coils, the rest were zero gap!
Taken the head off again and removed the springs to check free length again (originally measured on rebuild)
Inners all around 34.5mm (spec is 36.5mm)
Outers all around 45.9mm (spec is 44.5mm)
I also measured the coil diameter
Inner 3.02mm
Outers 4.07mm
I couldn't find any spec for this but I did keep the old springs, interestingly the wire gauge is thinner.
Inners 2.98mm
Outers 3.93mm
Even more interesting is that the new outer springs are more tightly wound (smaller gap between coils) than the old original springs so I think this could be the problem.
Compressed length outers old=22.7mm new=26.23
I do have an A65 camshaft fitted but this should not be an issue as the A50 Wasp used this cam, longer opening but don't think the 'lift' is any greater.
Should I just refit the old springs?
They are not far off spec: inners ~34.5mm outers ~44mm
Any other suggestions?
Hi,
The old springs look good enough do another turn and at least they would not be a problem. Most of us ride our bikes below max. flat out so they should be fine.
It may be worth returning the springs to the supplier as not fit for purpose and get a refund. I recently did this with some A10 springs.
Do you know what brand the dodgy springs are?
Dave.
New valve springs were Hepolite
Hi,
Hepolite is a name used by Wassell and whilst many will have no issues with such products there seems to be quite a few who do.
SRM do A50/A65 valve springs, UK made but by who?
This is their info.
Dave.
Srm used to use their own springs and they are very good. They now sell the Hepolite ones, same with the cams. The cams are good quality, however I was lucky enough to be building two engines with two new cams at the same time. One of the cams had a larger base circle and overall diameter on the lobe (though the lift was correct) it didn't fit one of the engines, but there was enough clearance in the other. So I could mix and match.
The springs work which I bought for the second engine and haven't been an issue (yet) though they were not a good fit in the lower retainer. I did check for coil bound and they seemed fine.
Old springs often look fine but can be weak or randomly break. When one breaks you'll find a flapping noise. Keep an eye out for some old terrys springs or get the srm race springs (which will be fine).