Oil level in GT forks (fitted to BSA!) - no data seems correct

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t20racerman
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Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by t20racerman »

Hi all

I'm not a Kettle owner - sorry! - but I do have a couple of kettle 4LS brakes fitted to two of my bikes, a 1967 Suzuki T20 race bike (on which I won a fair amount of stuff) and my lovely old BSA A10. I fitted the GT forks and brake to my BSA a fair time ago, had race linings fitted and have had fantasatic braking ever since - and annoyed a whole load of BSA purists at the same time, so win/win! :D

Anyway, fork rebuild time and I'm struggling to find a source for the fork oil level that matches my experience. The forks and hub were bought together, and must be 750J or K ones - is that right? The fork oil levels I've seen quoted vary a lot, but I have seen many sources telling me 260cc per leg in the earlier ones. I put 260cc in mine when I fitted the front end and the forks barely moved. I emptied some out and ended up with around 190cc per leg. This worked, but seems too high a level as the forks were still a little unresponsive. I've read that the level should be 135cc per leg in the later ones (which mine aren't) but also 160cc a leg as suggested here: https://www.louis-moto.co.uk/magazin/r ... gt750/0756.

I'm also wondering now if maybe they are the early GT550 forks - were they the same as the early 750 ones?

I was going to experiment and go for around 160cc per leg, but because my forks are in BSA Yokes with BSA RGS headlamp mountings, the end cap is hard to access and its a pain to do (long story...) and I wondered if anyone here could offer any advice. I'm happy to experiment, but would really like to know what was actually recommended.

Anyway, lovely forum you have here.
All the best
Adrian
BSA A10.jpg
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Alan H
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by Alan H »

Early 550 and 750 forks were similar, but differently sprung, fork capacity details here - page 22.
https://www.oldjapanesebikes.com/mraxl_ ... ice/page-2

Nice looking bike btw, the 4ls from the 550J and 750J were (and are!) highly sought after by the classic racer lads and aren't cheap! Better than the discs of the time too.
I have 20w50 mineral oil in the ones on my 550J which just stiffens them a tad from standard. When you next remove the caps, drill a hole through them (then tap it 10mm and put a short blanking bolt in each) so you can fill them with a syringe without all the faff.
Proof that four strokes are over complicated
t20racerman
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by t20racerman »

Thanks for that. I'm surprised its as high as 235cc per leg though, and amazed that you ran 20/50 in yours! I'll try 235cc of SAE5 as a starting point and see where it ends up.

Re cap modification - its not that easy - I have large chromed bolts screwed into my end caps that hold the BSA clocks in place as BSA originally had the RGS top yoke. Looks lovely. I'll just have to get on with the job in future and stop moaning at it being a pain :lol:
teazer
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by teazer »

J forks are an old bottom valve design and don't have the damping rods of later models. The oil holes are quite large and they need pretty viscous oil. Start with 30 wt and work from there. 5 wt is great in modern forks with small orifices, but a little light for these old dino bones.

Without fork springs with the legs fully compressed, start at say 100mm down for the oil level.
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Alan H
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by Alan H »

I got my first GT550 in August 1972 and at the time, I was told it was the first privately registered one after the 'official' Suzuki bikes. Crooks raced one as a production bike at Oulton park that year and I went to Barrow in Furness to find out what they did to it. Unfortunately ending up in North Lonsdale hospital there with a broken leg and the bike went to Crooks for mending after writing off an Austin Cambridge!
Frank Whiteway's advice on the forks was to use 20w50 at the time to stiffen the fork action up and I still use that on all my Suzuki triples.
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t20racerman
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by t20racerman »

Thanks for the help and advice guys.

I'm still puzzled though at why mine are so unresponsive, even with less than the recommended oil level. They are all lined up properly, with the front end moved up and down to settle in position before tigtening up - and there is a little space either side between the brake plates and the fork legs, so its not that.

I had these end caps made up over 20 years ago, and wonder if they are too long and preload the spring too much? Has anyone a picture or dimensions of the original fork top bolts so I can see what the preload was originally.
Thanks again for the advice.

By the way, just as an aside - I have a GT750 rear wheel fitted to my BSA too. Way lighter than the cast iron BSA one - and as an added bonus, the brake actually stops you! :D
teazer
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by teazer »

Time to go back to basics perhaps.
First I would check that the legs are straight and triple trees (yokes) are dead straight. The easy way to do that is to take a pair of tubes and place them across the forks, one between the yokes and one closer to the hub and look down from above. It is easy to see if they are off.

Next I would make sure that each leg slides easily up and down the staunshion. There are ways to do that. For example, remove one spring and bounce the bike up and down. Repeat on the other side. Or remove the fork legs and test them.

If that seems OK, next up is to check and set static sag. Fully extend the forks and either measure the extended length or use a zip tie on the fork at the top of the sliderthen allow the bike to settle down and see where the zip tie ends up. Measure the difference which should be somewhere around about 35mm +/-5mm. If the forks don't compress much, it's probably because of too much pre-load or the springs are too long. Early and late forks used different springs IIRC. I don't recall the stock lengths but they do tend to sag with time and they are often replaced. Fortunately springs are easy to source now and pre-load spacers can be made out of PVC tube.

It's possible that someone had brazed some of the damping holes closed or the shuttle valves might be rusted in position. If that's a possibility, the forks need to come apart, but with the correct sag, you should be able to push down firmly on the front of the bike and it should compress the springs easily and smoothly and should return slower.
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Alan H
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by Alan H »

Another one to think about would be to measure the distance apart the forks are where the front wheel axle goes through, then take the front wheel out and measure again. This will ensure that the forks aren't being bent slightly inwards (or outwards) at the bottom of the legs with the front axle nut tightened up as this will stop them working.

Just reread your last comment. The brake plates shouldn't have gaps between them and the fork legs, or the wheel will 'slop' side to side. The axle nut being tight should stop that.
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Kettletimes3
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by Kettletimes3 »

I think something else should be accounted for,
And that is weight.The A10 weights 170kg.and the Suzuki 219kg.
I think you said 160cc was to stiff,If that’s the case then I would be putting 120cc in and check stiffness.
If it’s still to stiff it leads me believe it’s an internal problem.
t20racerman
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Re: Oil level in GT forks (fitted to BSA!) - no data seems correct

Post by t20racerman »

Some very useful stuff here folks.
The alignment is all spot on - I've checked many times - and the space I mentioned between the fork and the brakeplate is just a small 25 thou or so gap each side. The BSA yokes put the forks much closer together than on the GT, so I had to machine a fair bit of metal off of each brakeplate to get it to fit.

The weight issue is interesting, as is the springs sagging as they age. Definitely something I'll be looking into. A lighter spring seems a likely way to sort this out. Do you guys know of any sources for different springs?

Plenty of food for thought here - thanks all for the help.
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