Author Topic: front fork oil leak  (Read 289 times)

Online RDfella

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front fork oil leak
« on: 26.08. 2018 20:13 »
Never known a bike with so many unusual ‘issues’. Here’s the latest:
Front forks: New bushes, oil seals and stainless seal holders. Stanchions are in good order. The holes for the axle clamp haven’t been drilled into the leg. The bottom weld and mudguard bracket lug weld are fine. But the N/S leg leaks oil.
Removed the leg and stripped it again. All OK. Even filed away the counterbore around the drain plug to ensure it was flat for the copper washer. Re-assembled with thread seal on the drain plug and ptfe tape on the thread for the seal holder. Correct amount (210cc) of oil.
The leak seems to be coming from the top of the leg, but why? Oil is not under pressure there and it’s a new seal. Only thing I haven’t checked is whether the recess in the seal holder is shallow enough to ensure the seal is clamped. However, when tightening it, it doesn’t seem to be getting thread-bound (it comes to an abrupt halt) so I presume it is clamping as required. Any ideas? Never had one of these legs leak before (currently have four bikes so fitted). If it wasn't for the fact the oil gets on the tyre, I'd leave it until it runs out.

'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online Jules

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Re: front fork oil leak
« Reply #1 on: 27.08. 2018 01:24 »
There's a no. of posts in here about front forks which I'm sure you have looked over, but you may have missed the discussions about oil leaking past the thread on the seal holder?? there are a few different ideas in here but the consensus always seems to be that the old "string thread" works best  *eek*…..

Online RDfella

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Re: front fork oil leak
« Reply #2 on: 27.08. 2018 12:35 »
Most of the old forks I've disassembled over the years appeared to have a string-like material at the foot of the thread. Presumably plumber's hemp plus a little white lead. I've always put my forks together without sealing the thread and never had a leak - until this one.  Don't want to use thread seal, as those holders can be a bitch to undo at times (even with the correct tool). As it's a fine thread, I'll give the hemp a miss and try more ptfe tape. I'll check the recess depth too, because if the holder is clamping the seal, there should be no oil around the thread to leak out.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online wortluck

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Re: front fork oil leak
« Reply #3 on: 27.08. 2018 15:46 »
My N/S fork started p*****g oil out all over my jeans.  Whipped it apart and it was a knackered oil seal.  The replacement I bought had a metal outer which made it easier to fit and seems more robust than the old one.  Having said that the R/H is still functioning well.  Don't know if the premature failure was down to misaligned forks ('trouser legs' rubbing on fork seal holder0.  I found I didn't need PTFE or hemp tape, just belted it up and all was good.  Incidentally, I always put an extra 10ml of oil in each leg to stiffen the forks up a tad.  Using the recommended amount is a bit too soft under heavy braking, I find.
'62 Flash
'49 B31
'59 BMW R60
FS1-E, YB100, RS100, RD200DX,250DX,350B, XS750



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Online RDfella

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Re: front fork oil leak
« Reply #4 on: 27.08. 2018 15:51 »
wortluk - 'under heavy braking'.  Smile. I don't have that problem with the current set-up. As I bemoaned in another post, the front brake wouldn't skid on gravel.
'49 B31, '49 M21, '53 DOT, '58 Flash, '00 Firestorm, Weslake sprint bike.

Online wortluck

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Re: front fork oil leak
« Reply #5 on: 27.08. 2018 16:00 »
Heh, perhaps I should have mentioned the two hands on the brake lever and this  *eek* sort of look on my face (plus  :o).  I remember first riding a 1949 B31 up to a roundabout, applying the front brake, and ending up apologising to a car driver as I sailed out in front of her.  There is a 'cause and effect' equation in here somewhere in that the time it takes to get the B31 up to a meaningful speed is equal to the amount of time one has to apply the brakes to get it back down to a stop! *smile*
'62 Flash
'49 B31
'59 BMW R60
FS1-E, YB100, RS100, RD200DX,250DX,350B, XS750



"I may disagree with what you have to say, but I shall defend to the death your right to say it."
Evelyn Beatrice Hall