Jampot plastic bushes
Posted: Sun Jun 07, 2020 9:50 am
Morning all,
This has probably been covered before but a search didn't throw anything up. The rear end of my 1951 18S is very stiff, pressing down on the rack would give little or no movement then, 'clunk' it would compress and then release with a similar clunk. I removed the Jampots and found the swing arm to be free. If I pressed down on one of the Jampots against the bench it would compress but 'stuttering', this is the best description I can give. Pressing down on the other Jampot gave no compression at all.
I've stripped the first Jampot, the very stiff one, and discovered all of the resistance is between the inner tube and plastic bush, so much so I had to use a drift to tap the inner tube out. My guess is the other Jampot will have the same problem.
I wondered if the outer tube was oval and therefore deforming the plastic bush but measuring the outside diameter of the outer tube in the area of the plastic bush only shows a few thou variation around it's circumference. I don't have a suitable tool to measure the inside diameter to see if it is oval.
Assuming for a moment both inner and outer tubes are OK is there any history of the plastic bushes swelling over time, perhaps because an unsuitable oil has been used? I can rebuild the Jampots easily enough but I'm uncomfortable with what I've found and would like to know why this has happened before proceeding.
When rebuilding the Jampots do I just go for 85cc of 20W or is a heavy fork oil preferred these days?
Any ideas or thoughts would be appreciated.
Thank you in advance,
Chris.
This has probably been covered before but a search didn't throw anything up. The rear end of my 1951 18S is very stiff, pressing down on the rack would give little or no movement then, 'clunk' it would compress and then release with a similar clunk. I removed the Jampots and found the swing arm to be free. If I pressed down on one of the Jampots against the bench it would compress but 'stuttering', this is the best description I can give. Pressing down on the other Jampot gave no compression at all.
I've stripped the first Jampot, the very stiff one, and discovered all of the resistance is between the inner tube and plastic bush, so much so I had to use a drift to tap the inner tube out. My guess is the other Jampot will have the same problem.
I wondered if the outer tube was oval and therefore deforming the plastic bush but measuring the outside diameter of the outer tube in the area of the plastic bush only shows a few thou variation around it's circumference. I don't have a suitable tool to measure the inside diameter to see if it is oval.
Assuming for a moment both inner and outer tubes are OK is there any history of the plastic bushes swelling over time, perhaps because an unsuitable oil has been used? I can rebuild the Jampots easily enough but I'm uncomfortable with what I've found and would like to know why this has happened before proceeding.
When rebuilding the Jampots do I just go for 85cc of 20W or is a heavy fork oil preferred these days?
Any ideas or thoughts would be appreciated.
Thank you in advance,
Chris.