Author Topic: BSA B25 Breaking Pushrod Caps  (Read 300 times)

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FirebladeDreamer

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BSA B25 Breaking Pushrod Caps
« on: 08 November, 2021, 21:28:55 »
I have a B25S that I've recently purchased. It has been fully rebuilt from the ground up by a very experienced engineer who specialises in this particular model.
It barely done 100 miles but suffered from an inlet pushrod cap breakage after some 50 miles. Both pushrods were replaced by the original engineer.

But now after another 30 miles or so I removed the side panel to check the tappet clearances only to find that the top of the exhaust pushrod
is cracked around the top with pieces missing.

The replaced pushrods were new BSA originals so I hope the quality should be good.
The clearances after replacement were set to perfection.

Has anyone any idea what might be the issue here?

DAVE BRADY

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Re: BSA B25 Breaking Pushrod Caps
« Reply #1 on: 09 November, 2021, 08:28:52 »
Hi,

A strange one but probably not totally without precedent.   It looks like contact between the pushrod cap rim and the rocker.
Try turning the engine until the valve is fully open and see if there is clearance between the remaining bit of the push rod cap and the rocker.  If there is contact or even very close then I would be thinking that there is something not quite right with the angles when the valve is fully open.  A bit far fetched but could the cam be opening the valve too much and so allowing some contact?  Are the inlet and exhaust rockers the same - they have different part numbers?  If not and have been swapped, could a slightly different geometry cause this problem?

Dave.
« Last Edit: 09 November, 2021, 08:33:17 by DAVE BRADY »

FirebladeDreamer

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Re: BSA B25 Breaking Pushrod Caps
« Reply #2 on: 09 November, 2021, 18:38:44 »
Thanks for the reply Dave. I've had a look and they dont appear to be too close. What would your definition of too close be though.
After the failure of the inlet rods cap the builder came down with a set of replacements which were allegedly BSA originals.
They were of slightly different length and the exhaust one had the red cap. When all is correct I'm wondering just what forces
these caps have to sustain. I dont believe the cam is anything but a standard B25S ietm.

DAVE BRADY

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Re: BSA B25 Breaking Pushrod Caps
« Reply #3 on: 09 November, 2021, 23:23:42 »
Hi,

I suppose allowing a bit for expansion they would just have to not touch.  The only other thing is if the rockers have a slightly different geometry for inlet and exhaust.  The part numbers are different but that does not necessarily mean that they are not interchangeable.  Someone on the Forum should be able to answer this.  The push rods being slightly different lengths does perhaps imply a difference in the rockers.  Probably not easy to see when fitted to the head.  I do not have any to compare.

Dave.

cdsdorset

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Re: BSA B25 Breaking Pushrod Caps
« Reply #4 on: 10 November, 2021, 07:13:38 »
Might be worth having a word with RTS racing. He is an expert on these engines.

ducati2242

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Re: BSA B25 Breaking Pushrod Caps
« Reply #5 on: 10 November, 2021, 07:57:40 »
I know on the b44 there were two different lengths of push rod . One for the square barrel and a longer one for the round barrel . There was not a great deal of difference but if the wrong ones were fitted then binding on the caps could occur . Only a thought but could they be the wrong length causing the ball to be at the wrong angle and breaking the lip off .
1956 bsa GS DB500
1968 mk1 Rocket 3
2006 ducati 999R .

FirebladeDreamer

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Re: BSA B25 Breaking Pushrod Caps
« Reply #6 on: 10 November, 2021, 17:32:15 »
Thanks for your replies gentlemen.

Dave - apologies for misreading your first reply. Where you wrote different rockers I stupidly read different pushrods.
So I can try to research that a bit more.

For those interested the difference in pushrod length is about 1/16 of an inch.

cdsdorset - RTS Racing - do they have a presence on this site?

I do have a friend also with a B25 of similar build date so next week I'm going to have a peer at his rods and rockers to see if I can see/measure any obvious differences.

cdsdorset

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Re: BSA B25 Breaking Pushrod Caps
« Reply #7 on: 10 November, 2021, 20:24:17 »
No. Just look them up. Roger Taylor Racing.
Builds probably the fastest, most reliable bsa single grasstrack engines around.