Fork stanchion problem?

gammakeith · Post by **gammakeith** » Sat Jan 16, 2016 6:28 pm

I don't seem to be having much luck with this bike. Alongside the engine problems it seems like I also have an issue with the forks (early style with pre-load only). They had to come off the bike while I checked the loose steering head bearings and noticed that one of them (the right one) was not right when pumped - it just gurgled with little/no damping. The left was fine so I took the tops off and carefully set the oil level to the correct figure (it was a little low). Unfortunately, this made no difference so I stripped both legs down. The oil is nearly new so they have clearly been apart recently and all looks well - even the teflon on the bushes is in good condition. However, the bottom of the right fork stanchion is to a different design. The picture above should show the issue (the new right tube is on the left in the picture). This stanchion appears nearly new unlike the other one which has some pitting so I assume that is the original. The only thing I can think of is that the internal bore of the new tube is too wide for the damper rod piston?

Any suggestions/advice much appreciated. I guess one possibility is that newer tube is for the the later style forks that probably has a different damper rod design? Either that its off a different bike completely. I don't suppose anyone has a spare original "early" stanchion? I don't mind rust damage as I can get rechromed and my current left one is a little pitted anyway.

Thanks!

Keith

Keith

richyrd5 · Post by **richyrd5** » Sun Jan 17, 2016 10:37 am

Hi Keith..if that IS a late type stanchion can I purchase it as I'm in desperate need of just one..i don't know if it I or not unfortunately but im sure someone will be along soon to say what's what..could it be a cbr 400 leg as they have preload only legs and are similar to early nc30 forks??

Post by **Neosophist** » Sun Jan 17, 2016 3:37 pm

later forks are cartridge design.

i dont have pictures to hand but they probably fitted the wrong tube or even worse a chinese copy tube, some of which are known to be unsuable from the factory.

either way, looks like your problem, both tubes should eb the same.

later forks tube - 51410-MR8-781

early forks tube - 51410-MR8-003

if you cant find a cheap stantion i would highly recommend fitting later type forks to an 89 nc30.. upgrading the front and rear suspension is one of, if not the best thing you can do to an 89 bike.

rebuild the front of my 89 suepnsion and it was sttill awful

Trasnformed my type 2 89 to fit rebuild front and rear later suspesion

gammakeith · Post by **gammakeith** » Sun Jan 17, 2016 7:28 pm

I've just had another look tonight and if it is a Chinese copy it seems well made and is dimensionally near identical. What is interesting is that the damper rod is actually a tighter fit in the "new" tube cf the other side so the oil is probably not leaking past the piston. Rather it seems the problem is with the damping assembly at the bottom of the stanchion. This is held in with circlip (unlike in the other leg) so I'll have a look on my next free evening and see if I can work out if something is obviously wrong. The bike is for my daughter who is training for her test at the moment. It would be good to upgrade the forks at some point but for now I just want to get the bike together and working right! It will take a while before she will get the benefit of any upgrades :-)

Keith

Post by **Neosophist** » Mon Jan 18, 2016 11:50 am

gammakeith wrote:I've just had another look tonight and if it is a Chinese copy it seems well made and is dimensionally near identical. What is interesting is that the damper rod is actually a tighter fit in the "new" tube cf the other side so the oil is probably not leaking past the piston. Rather it seems the problem is with the damping assembly at the bottom of the stanchion. This is held in with circlip (unlike in the other leg) so I'll have a look on my next free evening and see if I can work out if something is obviously wrong. The bike is for my daughter who is training for her test at the moment. It would be good to upgrade the forks at some point but for now I just want to get the bike together and working right! It will take a while before she will get the benefit of any upgrades :-)

Keith

Indeed, just worth bearing in mind as you might be able to pick up a decent set of late forks for nigh on the same price as refurbing the old ones (been there done that)

gammakeith · Post by **gammakeith** » Tue Jan 19, 2016 5:41 pm

OK, I've never stripped the "gubbins" at the bottom of a stanchion before but the "new" stanchion is now stripped (the original is peened over so not easily dismantled)! It seems the assembly is a simple restrictor plate coupled with a crude one-way valve. There is a thick plastic washer at the top that the damper rob slides through and acts as the one way valve. The one on the "old" stanchion is a fairly loose fit around the damper rod. The one on the "new" stanchion is quite tight as its actually oval! The spring washer that keeps it (gently) pushed down had partly popped out of the locating groove probably as it has been hammered up and down much harder probably due to the washer being a too tight fit on the damper rod? I will try easing the fit a little then partly reassembling the leg with some oil to see if that restores damping. Can anyone confirm what the issue was with the Chinese stanchions? If it is a fundamental design fault on the valve I will try making an alternative on the lathe.

Thanks!

Keith

richyrd5 · Post by **richyrd5** » Tue Jan 19, 2016 6:10 pm

Not sure on what the Chinese problem is but just to say I have sorted a stanchion now but good luck with forks chap

gammakeith · Post by **gammakeith** » Thu Feb 04, 2016 4:50 pm

[quote="Neosophist" Indeed, just worth bearing in mind as you might be able to pick up a decent set of late forks for nigh on the same price as refurbing the old ones (been there done that)[/quote]

Well I ended up going that route in the end! I played with the valve in the "new" stanchion tube but failed to get it to exhibit any meaningful damping. I managed to strip the thing completely and persuade the valve back to being circular but even then there was no joy. I've just managed to win a pair of dismantled late forks fairly cheaply off ebay so will clean them up and fit those. The chrome is not brilliant but they should hold oil for a year or so with new seals. I might then consider a re-chrome at that point - at the moment I don't want to spend too much more. I got the tubes on my RG500 re-chromed 15 years ago and they still look like new but its not cheap to do

Thanks for the advice!

Keith

Fork stanchion problem?

Fork stanchion problem?

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