CB250 Nighthawk 1997 Fork Seals

[BusterGonads]

Hello again bike mechanic gurus... Been a while.

Bit of a puzzle here - more below.

Recently, I sold my Street Twin and got the old bargain (£650) Nighthawk 250 out of the garage where it had been sitting for couple of years under a cloth, and got it mot'd in July. It has a leaking fork seal, but not bad, just a slight weep really, that would leave a thin line on the chrome of the stanshion so I just did a cheeky wipe of the fork outside the MOT place and the geezer passed it A1 no advisories. So, I've ridden it about a thousand miles this summer, around the winding lanes here in Northumberland, while I was deciding about whether to pull the trigger on another bike.

Meanwhile - over that thousand miles the leak got a wee bit worse, so I undid the top nut on the fork and pumped out most of the oil to see how it affected damping. Using one of those Lidle oil change oil pumps, I extracted all I could reach, but there is a minimal amount left in the bottom I think. The damping seems not affected at all. The fork is just about empty, but damping is fine as is handling. I got about 80ml out so I don't think much can be left.

So now - I can go for a ride and there is next to no weeping in twenty - twenty-five miles. I raise the dust seal after such a ride and the top of the seal is clear of pooled oil. HOWEVER - over the next few hours, oil is seeping upwards into the space above the seal. Maybe half a ml or more. How can this happen????

I am wondering of somehow the fork inside is becoming pressurised during the ride and pushing residual splashed oil out. How does it happen after the ride and not during it? It definitely isn't coming from anywhere else than below the seal, but the bike is sitting in the garage inactive, on its side stand, and up she comes.

PS - I know the solution is to replace, and I have ordered a seal kit on ebay.

EDIT: just noticed that you can see the oil line on the right front fork in the picture if you expand the image. That would be what I used to get out in about twenty odd miles before I took the oil out.
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2016 Triumph Street Twin; 2000 Honda CG125; 1997 Honda Nighthawk CB250

[stinkwheel]

Well, the oil will be hot and frothy after being compressed through the holes in the damper rod. Even more so if you've only got a small amount of oil in there.

I'd imagine riding along will air cool it a fair bit then when you stop, the heat would let the air and oil froth inside the shock expand. A bit like a coolant system that pukes a little coolant after you park up because it's topped circulating.

But yeah, new seals. Use OEM ones if you don't want to do them again in 6 months. Also may be worth replacing the bushes, worn bushes are a common cause of seal failure and it's also not unheard of to wreck them taking the fork apart.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[BusterGonads]

Good answer Stinkwheel. Thanks.THat all makes sense. Unfortunately, I already ordered ebay seals rather than oem..... Hope it comes apart easily. I am not a great spanner man.... Cheers...

By the way - I know you are a keen fan of the 350 bullets. I'm quite tempted by them. Been looking on line for one. Plenty about but not often good ones near me in Northumberland.
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2016 Triumph Street Twin; 2000 Honda CG125; 1997 Honda Nighthawk CB250

[stinkwheel]

[BusterGonads]

PM sent to Stinkwheel.

Thanks for the advice.
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2016 Triumph Street Twin; 2000 Honda CG125; 1997 Honda Nighthawk CB250

[Robby]

Genuine seals are a must. I've never had a set of genuine seals leak, and I've never had a set of pattern ones work.

Even if the fork leg chrome looks good, I tend to give them a rub with a finishing pad, to smooth down any invisible pitting or rough edges that could harm the seal.

Bushes are also a good idea, the bike was cheap when new and is now old enough for things like that to need doing.

You might get lucky and get away with just chucking in pattern seals and some new oil. Pulling forks apart is a messy and annoying job, so I prefer to get all the bits I might need and do it properly, once.

While you have the front wheel off the ground, check the headstock bearings for notchiness too. They have a habit of wearing out about the same time as the fork seals.

[blurredman]

Contrastly, I've never bought genuine seals, only generic ones and i've never had them leak.
I just get the size and order the cheapest generic seals I can. £3 to £5 for both, delivered normally.
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S.
Current: 1973 MZ ES250/2 - 17k. , 1979 Suzuki TS185ER - 9k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 38k, 1989 MZ ETZ251 - 49k

[BusterGonads]

Thanks to Robbie and Burred man.

Well - I already ordered the generic seals and will use them - or at least one - I only have one leaking leg. I've never done this job, and suspect that when I try to undo the bolt at the bottom of the leg which holds the telescoping parts together, it will just spin around as the internal part of the damper rotates. Not sure how it will pan out.

The affected leg has a tiny pit in the chrome which is probably the cause of the seal failure. It is VERY tiny, but I can just feel it with a finger nail. I could try and dress it with very fine abrasive - not sure whether it might ruin the stanshion. Any comment on that would be appreciated.

I won't do it quite yet... The leaking is minor and tolerable on this leisure bike which I ride around the country lanes at low speed when I fancy a ride out. It is rural and quiet around here, so I just toddle along and often hardly see another vehicle in a half hour ride. The fork will be dealt with during the winter when I doubt I will be motivated to ride in the bad weather.
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2016 Triumph Street Twin; 2000 Honda CG125; 1997 Honda Nighthawk CB250

[Robby]

Minor pitting will get some oil past the seal, but going over it with a finishing pad will help.

You want the seal to wipe oil off the fork leg. The seal is rubber and can deform a bit to follow peaks and valleys on the fork leg. Smoothing the peaks give the seal less work to do.

Clean assembly is important too. The thing that can really kill seals is dirt getting stuck inside them. A combination of clean assembly and good dust seals will help. Gaiters will help even more, but they also make it difficult to check if your seals are leaking.

[Bikebolt]

Honda cb250 nighthawk 1997 USA ?

A grey import ???

Get a manual

Visit CMSNL.com.....

Put the spring and cap back into to undo that recessed Allan bolt....

I just know you will return with questions like "htf do I get these sheared off mudguard bolts out ? " !!!, Not to mention " which way do these seals go ? ".....

Yes, any pitting/wear will encourage seal problems. Any "sharp edges" in the seals stroke area will rip and ruin the seals in one stroke...

Enjoy.

[BusterGonads]

It isn't a grey import though like most 250 bikes, they are not much favoured by the UK Bike importing industry so there were never many of them seen here. To be fair, I don't think our bike licensing laws help here. They were very popular in other domains.

Yes - very fine dressing of the tiny pit in the fork leg will likely help. The bike has quite low mileage. Currently just under 16000, but it was 12k and a few when I picked it up for buttons in (a far as I can recall) 2018. This kind of problem is likely on an older bike that spent its early years as a commuter. Winter riding, I expect account for the issue.

I have the Clymer online manual. It is not very helpful as far as techniques of dismantling are concerned.

I am not expecting broken bolts. Penetrating oil, no rush, gentle spanner use, and if need be, a careful use of heat will be basic strategies with small diameter bolts. Broken studs and bolts? Been there; done that half a century ago. Don't want to return to that kind of grief.
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2016 Triumph Street Twin; 2000 Honda CG125; 1997 Honda Nighthawk CB250