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Bent Pushrods

Posted By: Dave P

Bent Pushrods - 08/14/18 5:29 pm

Having problems with the valve train on my 1970 A50. Had the SRM roller bearing conversion and fitted a Spitfire cam at the same time. Put a few thousand miles on it no problem. Fitted a different head and barrels, did quite a few miles. Then out of the blue it packed up amid horrible metallic sounds and when I pulled it apart the right inlet rocker arm had snapped and the inlet pushrods were bent.
Rebuilt it and did about 500 miles before hearing some more nasty metal crunching sounds. The bike kept going so I made it home then found the top of an inlet pushrod had half cracked off, luckily there was enough of it left to still work the valve.
I fitted a new pushrod but only got about a mile up the road before more nasty noises and I limped home on one pot. This time both inlet pushrods had cracked at the top.
I've just got some one piece pushrods from SRM and am hoping maybe it was a dodgy batch of pushrods but that still doesn't explain the rocker arm breaking. I have found about 1/8" end float in the camshaft (manual says there shouldn't be any) and wondered if this could cause the cam to lift the wrong valve if it went out of line with the tappets? If so how can I eliminate this end float?
Posted By: gavin eisler

Re: Bent Pushrods - 08/14/18 5:55 pm

Cam is pushed to TS by a spring between the breather disc and the shaft end.maybe the spring has failed, quite unusual failures, perhaps their is a problem with the RHS inlet valve, bent or tight in guide.
Posted By: BrizzoBrit

Re: Bent Pushrods - 08/14/18 11:40 pm


I'd suggest coil binding I've had ongoing issues with that. In my case I've had buggered cams as the symptom. Potentially I guess spring cap could be hitting the valve guide.

Make sure you have additional spring compression available when valves are at maximum lift. NickL will be able to give a value. Best springs I've encountered are the SRM race springs.
To check the guide you can do a dummy install with a soft spring and push valve down to see how much movement you get. On my most recent install I was getting a minimum value of ~ 0.495". Again NickL knows the value off the top of his head. I'm not that smart.

Both issues have been discussed here before so maybe do a search. I use google, enter the topic and also britbike forum. There is a specific way to restrict to a website, but again, I'm not that smart.

Good luck
Posted By: triton thrasher

Re: Bent Pushrods - 08/15/18 5:55 am

Lifting the wrong valve would make a valve hit a piston. That would bend the valve, so you’d probably know if that had happened.
Posted By: Dave P

Re: Bent Pushrods - 08/15/18 12:56 pm

Thanks all, just had another look and the camshaft has to be pushed in to get the free play so the breather spring seems to be working. I'll check the valve springs when I put it back together.
Posted By: gavin eisler

Re: Bent Pushrods - 08/15/18 3:58 pm

Re coil binding, I had oversize inlet valves fitted, these sat further into the head, shortened the spring installed length to the point it would just coil bind at peak lift, Ended up getting the seats cut deeper to restore spring function, other options would be re cut the spring collet seats ( tricky ), lower valve spring land, risks breaking into inlet port.
There is a minimum dimension for valve tip to lower spring land, will dig it out.
Posted By: gavin eisler

Re: Bent Pushrods - 08/15/18 4:36 pm

Valve tip to valve spring pocket , should be 1.730" - 1.750".
Posted By: Allan Gill

Re: Bent Pushrods - 08/16/18 12:11 pm

Do you have the correct push rods?

A50, A65 ans 750 bigbore kit conversions all use different push rods! Possibly the first one failing was just bad luck and incorrect parts received since?

Checking the valve spring bind would verify that. But as mentioned above... The SRM race springs are very good for the money (Im using a set on mine with an Megacycle X12 cam), the Steel rods (im also using their A65 rods on this motor) are also a good purchase....

On a side note, I didn't used to live too far from the Wirral, and I used to know an 1970 A50, with Spitfire cam and end fed conversion.... Any chance it came with a duel carb head too? A50's aren't all that big on the ground and few people would have the extra work done on a model varient which wasn't known for breaking apart. If it is the same bike, It used to go like stink and my Honda 400/4 couldn't keep up.
Posted By: Dave P

Re: Bent Pushrods - 08/25/18 11:34 pm

Thanks Gavin I'll get measuring next weekend when I have another go at it!

Alan, no that wasn't my A50. I bought this in 2000 as a pile of bits and finally got it on the road in 2010. I had the SRM conversion done about three years ago. This pushrod business is a recent thing, I'll splash out on some race springs from SRM too.
The pushrods are for a big bore kit but are billed as fitting the A50 too - I'll check them for length against the old pushrods anyway.
By the way, I think I sold you some A65 barrels a few years back when you were with the Cheshire BSA folk.
Posted By: gunner

Re: Bent Pushrods - 08/26/18 12:25 am

I had a similar experience with bent pushrods when I first assembled and ran my A65 Firebird a few years back.

The problem turned out to be caused by the new valve springs becoming coil bound. Originally I purchased the valve springs from a reputable UK supplier but in the end they didn't seem to be the right ones as the spring wire was a bit too thick, hence the coil binding and bent pushrods.

I thought the problem could be resolved by using race springs and I rang SRM for advice. They suggested I measure the wire diameter of my springs and we then compared it to the standard springs they stock. Turned out that my springs were significantly thicker than the standard SRM springs and so I purchased a standard set from SRM. With the standard SRM springs fitted all was well with no coil binding or bent pushrods.

This issue seems to be fairly common as valve springs all look similar and its not easy to work out whether you have the right ones.

Suggest you measure the wire diameter of your springs and give SRM a call and see if your springs are different from their standard ones.
Posted By: Allan Gill

Re: Bent Pushrods - 08/26/18 11:59 am

Originally Posted by Dave P

By the way, I think I sold you some A65 barrels a few years back when you were with the Cheshire BSA folk.

May have done if it was around 2007? I think I ordered the first set from the star mag "for sale page", I fitted another set in 2014 as the first set ended up scorring and needing liners.

If your fitting a twin carb head to the bike (and going to ride it like you stole it) then i'd fit the race springs, the A50 in that setup will go really well and being lower geared than the A65 will rev harder keeping up (which it will)
Posted By: Dave P

Re: Bent Pushrods - 08/26/18 2:46 pm

Thanks, I've ordered some SRM race springs, will check the coil diameter against the old ones. Still baffled why it should run for a few hundred miles then suddenly get coil bound! Alan, it was some A65 barrels I got on ebay which I forgot to check the year of and they were the earlier ones for smaller studs. The barrels were painted red as I recall! I'm keeping it as an A50 for now with single carb, more looking for reliability than performance.
Posted By: Allan Gill

Re: Bent Pushrods - 08/28/18 3:54 pm

Ah yes, those will be the one I have fitted at present, although no longer red but silver.
Posted By: Dave P

Re: Bent Pushrods - 02/07/19 9:34 pm

Thanks for all the helpful suggestions folks! SRM Racing springs did the trick, it must have been the valve springs getting coil bound. Done a couple of thousand miles with no pushrod trouble. Battling a mystery intermittent electrical fault now!
Posted By: Allan Gill

Re: Bent Pushrods - 02/07/19 10:57 pm

Great news!!! I’ve been very happy with mine also.
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