Category Archives: Engine Tuning

GSX-R COIL SPRING CLUTCH CONVERSIONS

Words & pics: Lee Workman

Before we start, I’m no journo, so you’ll have to forgive the lack of long and rambling intro and humorous asides throughout. I’m an engineer by trade and have been messing around with Suzuki’s big bore oil-cooled motors for ages and I’ve picked up a few tricks and developed a few of my own along the way.
I’ve done this coil-spring clutch conversion a few times on later GSX-R’s (and Bandit Twelve’s too). It’s cheap, very effective and it’s a piece of piss, so here are the facts.

There are the two types of clutch unit fitted to the GSX-R range and Suzuki’s first attempt, the coil spring type, is definitely the best. Fitted to all GSX-R Slabsides from 1986 through to 1988 (the 1052cc G/H/J models), it uses ten steel driven plates and eleven fibre drive plates and is clamped together with four coil springs acting on an alloy pressure disc.
With any clutch, the limiting factors are the unit’s overall strength, the surface area available and the clamp load on the drive and driven plates. The design of the Slabside coil-spring unit excels in all three areas – this unit can easily cope with even the most ham-fisted wheelie/burnout merchant and on the road it should handle over 160bhp as stock.

For the heroes or for strip use, it can be still further improved with the use of heavy-duty coil springs, and for the serious drag racers out there, there are a couple of aftermarket ‘lock-up’ type conversions, which bolt straight in and replace the existing pressure disc. The lock-up is the ultimate in super-strong non-slip clutches.
Then there’s the diaphragm type. Fitted to all GSX-R Slingshots from 1989 through to 1991 (the 1127cc K/L/M/N models), it uses a similar clutch basket and the same driven and drive plates as the earlier Slabside clutch (with the exception that there are only ten drive plates in this unit), but the plates are clamped by two diaphragm springs acting on a different pressure disc design.
This unit is fairly robust and is less prone to ‘judder’ than the coil-spring type, but it starts to struggle with more than 140bhp and if you like your wheelies and burnouts or if you launch it hard on the strip, it’s not long before it gives up the ghost .

There is already a conversion kit on the market from APE, costing around £190, which converts a Slingshot diaphragm clutch to a coil-spring unit like that of the Slabbies and, therefore, allows the fitting of a ‘lock up ‘ kit at a latter date if necessary. But I’m going to tell you how to do it (using quality components) the cheap way. You can either get the stuff from breakers for around £30 or, using NEW genuine Suzuki parts for about £80 – about half the cost of the over-the-counter unit. And in a later article, I’m going to tell you how to fit one into your Bandit Twelve motor too, so keep any eye out for that, okay?

Anyway, in preparing this article I’ve been down to my local Suzuki dealer to get the correct part numbers for you to order and latest prices (all are plus VAT and correct in August 2002). And to ensure that those of you using second-hand stuff from the breaker’s get the right bits off the right models, I’ve already checked to see which part numbers are superseded by later ones, and that the information given is correct to the best of my knowledge and experience etc. Aren’t I good to you lot, eh?

Firstly for those of you starting with a Slingshot GSX-R 1127 (a K, L, M or N model), you’ll need the following parts:

The parts needed for the conversion. 

Then you’ll need two extra components – a 10mm steel ball bearing from your local bearing factors, and a hub nut spacer too. You’ll need to get one of these knocked up by someone with access to a lathe or any local engineering firm should make you one for around a fiver. The dimensions are 35mm OD and 25.5mm ID and it needs to be 10mm thick.

Got all them? Good, so here’s how you actually do it. Firstly, shift the transmission into first gear and remove the nine screws from the clutch cover. Then take off the clutch cover and remove the large circlip in the centre of the hub before you take out the pressure disc lifter and the push piece and its bearings. Hold the pressure disc from moving using the special tool no.1 (see below), undo the big holder nut and remove the diaphragm springs and the spring seat and then the pressure disc.
Remove the drive and driven plates, followed by the wave washer and its seat, then hold the hub with special tool no.2 (see below an’ all) and undo the hub nut and remove the nut, washer and hub assembly. Important – ensure the long (steel tipped) alloy pushrod and the thrust washer between the basket and hub is still in place or you’ll be buggered. Now fit the new hub.
Problem number one – you’ll notice that the hub nut and washer will not now tighten down because there aren’t enough threads on the countershaft. This is because the Slabside hub has a thinner base and the Slabside countershaft is shorter, so this is where the spacer is needed on the driven shaft. Fit it over the shaft, then you can secure the new hub using your original nut and washer using special tool no.2 to hold the hub while you tighten the nut to the correct torque (140 to 160Nm or 102 to 115ft-lbs).
Now re-fit your original clutch plates (as long as they’re not fucked, check the fibre ones for the correct thickness – 2.52 to 2.68mm / 0.100” to 0.106”) and check the steels for warpage using a feeler gauge and a piece of plate glass (max limit 0.10mm / 0.004”), starting with a fibre one, then steel and so on (you should finish with steel, if you’re not a complete wazzock). Now fit the extra fibre one you’ve just bought.
Problem number two now rears its head – because of the differences in the length of the countershafts, your clutch push rod is now, effectively, too short. This is where you fit the ball bearing – it will take up the gap between the alloy pushrod and the push piece. Fit your push piece complete with its thrust bearing and washer and then fit the new pressure disc using the new spacers, springs, washers and bolts and secure the bolts in a criss-cross pattern to the correct torque setting (11 to 13Nm or 8.0 to 9.5 ft-lbs).
Re–fit the clutch cover, check the oil level and you’re away! You’re now the owner of a coil-spring clutch – told you it was easy!
Now, extra hints and tips.
Tip 1: When you do the job, put the bike on its sidestand and carefully lift the front wheel and chock it on a brick – this will stop you losing any oil when you remove the clutch cover.
Tip 2: When buying new clutch plates, use only genuine Suzuki parts – I know they’re more expensive, but it’s false economy to put cheapo plates in. I know, I’ve tried ALL the super-trick/heavy-duty ‘performance’ clutch plates and they just don’t compare with the genuine stuff. You have been warned!
Tip 3: With this particular conversion you fine-tune the biting point by changing the ball bearing – if it drags too much (assuming you’re using the correct grade oil and the steels aren’t warped), then you need more travel so try fitting an 11.5mm one instead. If it slips (assuming the fibre plates and/or the springs aren’t worn), you need less travel so pop in a 9mm one. It really does make a difference!
Also take the time to make the special tools, they’re a piece of piss to make, and really make life easier.

Special tool one: You’ll need two pieces of steel bar that are 200mm long, 25mm wide and 5mm thick (ish …), drill two 6mm holes in each bar with the centres 165mm apart then, at one end of each bar, fit a M6x30 bolt and secure it tightly with a nut on the underside. At the other end, open out the 6mm hole to 8mm and join the two bars together with a M8x70 bolt and loosely secure it with a nut on the underside.

To use it, the M6 bolts will neatly fit into the slots on the diaphragm pressure disc and the M8 bolt will fit straight into the swingarm spindle – once the slack is taken up, you’ve got both hands free to undo/tighten the big 50mm holder nut!

Special tool two: First get one 400mm long piece of square section (20x20mm) bar and two old GSX-R clutch driven (steel) plates. Put the plates on top of one another and drill three holes equally around the diameter and secure them to the square bar using two M6x60 bolts and 20mm spacers and a couple of M6 nuts (this way, you can support the basket really well and reduce the chances off slipping, as the plates will be deep inside the unit). Finally put a small M6x10 bolt and nut through the remaining hole to secure the plates together. This is a wicked tool to have, when you’re trying to remove/secure the hub nut at 160Nm!