74 Commando Leaking and Sticking Forks

Hi Folks,

I have a 74 Commando that I bought from a guy who did alot of refurbishing, reubuilding on this bike. He did a great job with everything but the front forks. I am getting ready to tear down the forks, but want to get some advice before I do.

First problem is that the forks are "sticky". By this I mean that when you try to compress the forks it takes quite a bit of effort until they will move, but then they move fairly easily, but won't rebound properly. I assume it's "sticktion" at the fork seal. I read somewhere, and now I can't find it, that some fork seals that are sold as "leak proof" cause lots of sticktion. I am assuming this is the problem. So, I am going to replace the fork seals. Can you all recommend a brand or part number for me?

Second the left fork leaks oil. It's coming from the bottom of the fork somewhere. Like I said, I have not taken the forks off, but I assume it's the bolt on the bottom of the fork. I can't see it yet because the axle is covering it. Anyhow, is this a common problem? Any other possibilities? Is there something I should know about the bottom of this fork that needs attending so I can order the proper parts?

Thanks much!

SquareHead said:

First problem is that the forks are "sticky". By this I mean that when you try to compress the forks it takes quite a bit of effort until they will move, but then they move fairly easily, but won't rebound properly. I assume it's "sticktion" at the fork seal. I read somewhere, and now I can't find it, that some fork seals that are sold as "leak proof" cause lots of sticktion. I am assuming this is the problem.

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I suggest you dismantle the forks, and not assume anything.

SquareHead said:

So, I am going to replace the fork seals. Can you all recommend a brand or part number for me?

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Why not use the standard Andover Norton fork seals? http://www.oldbritts.com/nor_comm_73_17.html 065483 (NM17713)

SquareHead said:

Second the left fork leaks oil. It's coming from the bottom of the fork somewhere. Like I said, I have not taken the forks off, but I assume it's the bolt on the bottom of the fork. I can't see it yet because the axle is covering it. Anyhow, is this a common problem?

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I wouldn't say it's a common problem, but it is a possibility if the damper retaining bolt is not fully tight or the internal sealing washer 067514 (NMT814) is missing or damaged. If you haven't really taken a close look at it, then the oil could just as easily be leaking from the drain plug hole?

Wear in the slider. common
scored sliders. common
scuffs on bush's
forks not aligned on assembly. slacken the pinch bolts and bounce the forks,
damage to yolks, bike dropped in its life can twist the iron casting.
bent stanchions , due to above
or just plain old bush's to tight.
just to start with :lol:

Most striction causing thing I run into on sat up forks is the springs and inside of stanchion rust together. The more the bushes wear the less resistance I got not more. I've had perfect assembled and function forks go so stiff a slam down needed to compress some and then stayed down even jerked up on, ugh. If so DPO may not be at fault but not servicing or working forks every season or so.

Bottom damper washer-seals are prone to leak or bolt get loose, new ones should seal it. Do roll the stanchions against each other for easiest definite way to find off of true-ness. A few 1000ths is common w/o an issue, much over that messes with seals lasting and some striction in parts of travel. A fair amount of simple bend can be easy corrected at many shops but if cork screw bent, candy cane like, best start shopping.

Check if the bottom hydro stop mod has been done, simple and worth while.

You never told us the mileage. TOTAL tear down. Betcha the springs are rusty old pieces of crappola. I think Old Britts has some kind of printout for front end rebuild made less painfull ?

The bike has 15,500 miles on it. I took the forks off today and completely tore down one of them. The springs are progressive and in pretty nice shape. I little bit of crud on them, but not much. Oil looks pretty decent also. There was some crud in the bottom of the slider, but again, not much. I am cleaning all of the parts up and putting the smaller ones, bolts, nuts, etc., as swell as the slider in my ultrasonic cleaner. I think I am just going to clean everything up and reinstall with new oil and see if I still have problems. I haven't tore the other fork apart yet, but it actually seems like it could be the one that is sticking. I want to get the left fork put back together before I tear the right fork apart.

If anyone wants to give me oil recommendation and amount, feel free. If not, I will be able to find it in back threads pretty easy, I am guessing.

Thanks!

SquareHead said:

If anyone wants to give me oil recommendation and amount, feel free. If not, I will be able to find it in back threads pretty easy, I am guessing.

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Or you could look it up in your workshop manual.

http://www.classicbike.biz/Norton/A_Lin ... ton-ad.pdf

(Factory manual, Section G 8.10 ).

It's been my experience that oftentimes people have come up with improvements over the older factory manuals. But, gee, thanks for the helpful advice :lol:

Did ya look down the stanchion inside yet? Other one could be the bugger. On factory forks with some wear might start with 20 to 30 grade engine oil. Search up the bottom damper hydraulic stop mod, block off old too low holes by weld JBW or Al pegs rammed in then make new similar or staggered holes 1/2-3/4" above the damper taper on tube. Stealing time is the hardest part for me.

john robert bould said:

Wear in the slider. common
scored sliders. common
scuffs on bush's
forks not aligned on assembly. slacken the pinch bolts and bounce the forks,
damage to yolks, bike dropped in its life can twist the iron casting.
bent stanchions , due to above
or just plain old bush's to tight.
just to start with :lol:

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This list would reflect my experience.

You will pick up the easy ones in the list but bent yokes and bent stantions are more of a challenge.

I first check the stantions are straight on a large flat bed.And then asemble the stantions and yokes and check they are also straight and parallel on the flat bed. You will need to find an engineering shop to locate a suitable flat bed. The yokes can be bent straight again provided they are not too bad.

If you assemble the forks without springs and then move them up and down with the wheel or a dummy axle fitted you will feel any places where they are sticking.

I have fought a set of forks for weeks before getting them into reasonable shape. !!!

Rolling the stanchions together, ie: against each other, is even better than a flat edge or table. If some bow detected that way, the bows can be marked and stanchion installed so the bows are at front of back for least striction and normal function. Of it will reveal how much off and weather to fix or trash.

Once assembled check for slight slack sense back/front at axle, likely the slack in the top bushes, but not noticed on the fly.

Can someone remind me why they make a solid clunk when they top out,lack of oil perhaps ?

Damper piston hits the damper tube top cap underside
If the total damper lengths are the same this is reduced, as the piston /cap contact will be shared, instead of one side taking all the load, most fork assemblies have different lengths . It's dead easy to get the lengths wrong,as the final top nut/rod locking can be a guess. i have cured this klonking for ever with the MK 11 lansdowne Units, the final 15mm of extendsion is a hydraulic cushion.

Time Warp said:

Can someone remind me why they make a solid clunk when they top out,lack of oil perhaps ?

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john robert bould said:

Damper piston hits the damper tube top cap underside
If the total damper lengths are the same this is reduced, as the piston /cap contact will be shared, instead of one side taking all the load, most fork assemblies have different lengths . It's dead easy to get the lengths wrong,as the final top nut/rod locking can be a guess. i have cured this klonking for ever with the MK 11 lansdowne Units, the final 15mm of extendsion is a hydraulic cushion.

Time Warp said:

Can someone remind me why they make a solid clunk when they top out,lack of oil perhaps ?

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No top out spring as stock.
Looks like another set of Lansdowne dampers will be on the cards for the 850 sooner than later then. :lol:
(I don't remember the 750 making that noise)

Time warp;
Not all forks have the loud clonk, it depends on the mileage, and servicing history, i would say the oil in most forks is the last thing owners consider.
Water develops in the fork due to pumping ingress,this water rusts the rod,with a "round file" action this rough rusty rod rubs the top cap bore larger, with an ever increasing enlargement!
The manual states a hydraulic cushin in the stanchion design,this is rubbish, Peter Crisp as wrote about this total lack of extension damping, I have found this to be true,
Remove the damper and work the slider in and out...100% zero damping..the diagrams in the original manual fail to show anything that would cushion the extension.
The oil is supposed to be traped between the lower-top bush...as there is a 8mm hole it's just impossible!

There's two popular ways to get silent soft fork stops at extremes Lansdowne's high end race proven kit and cheap hobot's kit sold by Greg Fualt that's gives 6" travel and proven on and off road security with silent stops too. The top out issue arises because Norton installed too short of springs so to keep springs from bouncing free inside so they limited upward-expansion potential by too short of damper rods that clank on damper cap. The damper rod clearance in the cap is a big source of damping action and can be waisted a bit in the inch or so motion range for mild bump take up but progressive dampening on larger bumps or jump landings. I found ATF a bit too thin in hobot mod off road, but power steering fluid was just right but no rust protection as real fork or engine oil. I like 20/50 in my factory sloppy forks on Trixie, but she's got un-tamed isolastics so can't be ridden to point to need better fork action so i hardly ever get clanks at extremes but when jerking up on forks to lift over something backing up. The Fauth kit uses alloy rods and with the alloy damper tubes also available after market the forks end up over a pound lighter if that matters. Fauth's kit also allows mix-matching of 2" spacer spring to get progressive spring rates or retain factory sag stance.

SquareHead said:

It's been my experience that oftentimes people have come up with improvements over the older factory manuals. But, gee, thanks for the helpful advice

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If that had actually been part of the question, then my reply would have been different. :wink:

Thanks!

Okay, so I have torn down both forks completely. The reason for the left fork leak was the previous owner forgot to put the fiber washer on the bottom of the fork slider!

However, the issue of sticking is not yet fixed. The left fork, which was the leaker, went together well, with no sticking, etc. The right fork tube and the bronze bushing don't slide together well. It is a very tight fit. I tried to take the right fork bushing and slide it over the left fork tube and it didn't fit either. So, I assume the right side bronze bushing is out of true since it will not slide easily over either the right or left fork tube. I ordered a new bushing from Old Britts and it also doesn't fit over either fork tube. Now what??

I believe I read somewhere that the bushings are very "hit and miss" as to fit. Do I need to order several bushings at a time to see which ones will fit?

Any other suggestions would be greatly appreciated.

Thanks,

Chris

Chris, is there a local machine shop that you can take the bushing to be slightly reamed to fit?

bring the slider to, I had to do this some years ago and it took just a couple of minutes