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#879612 05/05/22 4:01 pm
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I’m replacing the exhaust rocker on my B44 and have shaved the 3 o-rings I thought I’d need for this.
I’ve been using a flattened pick and a small screwdriver to try to persuade them and it’s the first time I’ve attempted this.
The o-rings are square section that fit perfectly on the spindle and that I bough from Peter at BSA Unit singles so I know they’re "right". It’s me.

So my question is, are square section o-rings inherently more difficult than circular o-rings to install because the leading edge is blunt?
I need to buy more o-rings hence the question.
Although I am tempted to smear hi-temp sealant on the outside and call it a day. Not really.
Thanks,
Adam

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Are you greasing/applying a smear of oil to the o-ring and shaft before assembly ?


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I’m using P-80.

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consider chamfering the edge of the rocker spindle hole and heating the rocker box to 250 degrees or so. Of course use some lube. Triumphs use a tool to compress the seal so it enters the bore more easily. Don't know if a similar tool for BSA's is out there.

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Thanks for the reply Stuart.
There is a chamfer, courtesy of a PO. The spindle and o-ring were in the freezer overnight and I used a torch on the head.
I’m pretty sure clearance wasn’t an issue, and I used P-80 lube.
Since I need more o-rings, my question is if there any practical difference, in this instance, between round and square section o-rings?
Thanks all.

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The O rings all get a bit of a shave when inserting them. Nothing to worry about. Are you experiencing leaking oil at that location?

Peter


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Originally Posted by Stuart Kirk
Triumphs use a tool to compress the seal so it enters the bore more easily. Don't know if a similar tool for BSA's is out there.
Trust me, that tool doesn't work much better, the round O rings on Triumph end up getting torn up anyway. I end up using black silicone to seal them.


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The rocker box is not yet back on, so no leaks yet Peter. I was waiting to hear real world experience, thanks.
I kept the rocker box assembled after the last attempt, and I smeared hi-temp silicon on the end of the spindle.
My assembly can now continue.
Thanks all,
Adam

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I would say a square section "O" ring is the wrong way to go.
A round "O" ring should fit the base of the groove in the shaft, it should be a slight interference on the outside diameter in the hole and have axial clearance in the groove.
As the shaft is pressed into the hole the "O" ring will distort into an oval cross section. If there is no room to distort axially the ring will be damaged


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The reason for the shaving is the groove s not sized correctly, as Andy says an O ring works by being compressed on the Id and od by having a shallow groove but the groove is wider to allow the sides to expand. But both BSA and Triumph designed a groove with less X sectional than the O ring. Result the O ring has nowhere to go when compressed.

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I've rebuild a load of unit single rocker boxes, like at least 40 or so. Funny thing is that using round o-rings or square cross section o-rings ultimately doesn't seem to make a difference as far as leaking oil at that location goes. None leak. Meaning that a fresh o-ring of any type seems to do the job even when they are shaved off a bit. I do not seem to have difficulty with shearing on the square section o-rings I have been using. They do have a bit of an interference fit so sealing does occur. Where leaking might be an issue is when the rocker shafts have been pushed in or out of the box too many times and the shaft is no longer an interference fit in the alloy box. Meaning it does not have to be pushed in with a press. Then the o-ring can be expected to be doing a lot more work. By the way I never use lubricants on my rocker shaft o-rings. Others may have different opinions and experiences of course.


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