Dry clutch conversion

[DrPMC]

I don’t know about you, but I’ve always liked the look of a dry clutch on a racing engine. There is also the noise, a la Ducati - exhaust is booming away, clutch is tinkling… I had wondered about doing this for ages and then saw it done by Allen Millyard on one of his videos (for a Honda 6 racer replica he made). I don’t have Mr Millyard’s engineering genius but thought I could just about stretch my skills and hobby tools to do it.

A disclaimer to begin with: this modification is certainly not ‘necessary’ and probably not even sensible. If your ire is roused by such things you should probably give this a miss. I should also add that I haven’t tested it on the road yet - so far it works on the bench and doesn’t piss oil everywhere but it may fall off in 5 or 500 miles time.

My conversion is of a XRV750 engine but I’d expect the NT650 would be similar. I bought a blown engine last year for the cylinder heads so also had all the parts to do this - none of the parts on my engine have been altered so I can easily revert to normal if required.

I’ll post some more pictures and some rough details in a bit.

[rpcraft]

I doubt I would go down the road to doing this but I am interested to see the engineering that went into it!!

[Henway]

Starting off with the accelerator pump "whoosh" and tinkling of flat-slide carbs might be a way to scratch your aural itch.

[DrPMC]

Well, here goes, step 1.

First of all, I drilled out the three rivets that hold the clutch basket to the big steel pinion that holds the cush-drive springs and the main clutch bearing. There are accounts of doing this on line, usually to replace sagging springs. The only small refinement I added was to make a little guide to go around the head of the rivets to make sure they were drilled off square. Looking at the pics, it seems that I also used a counterbore to remove the rest of the rivet head and then pressed the rivet out.

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The next act of wanton destruction was to remove the part of the crankcase cover around the clutch. I made sure this was done in such a way that the removed part could be later reused by bolting the cover to the rotary table of my milling machine. Its only a small machine and rotary table but I had already made a bigger bolt on top for the table so could support the engine cover. I did the cut with one of the small HSS circular saw blades that are sold to use with Dremel tools and the like, held in a home made arbour. This set up works surprisingly well and the thin blade means that there is only a small loss of metal.



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[DrPMC]

Step 2. To make this clutch run in air, we need to be able to insert an oil seal between the clutch basket and the steel clutch pinion. To do this, I found that a spacer of 10mm was the minimum that would be needed to accommodate an oil seal of 10mm thickness and 120mm bore.

This spacer is quite a complicated thing - essentially it reproduces the back of the clutch basket, with its recesses for the cush-drive springs and attachment lugs on one side and has recesses to accept the existing lugs on the back of the clutch basket on the other. Mostly this required basic work on the lathe and milling machine to do this but I struggled to find a way of cutting the barrel shaped recesses for the springs. In the end, I first cut these with a 16mm round end cutter and then used a 16mm diamond coated cutting disc in the mill to finish the ends of each recess. No doubt there are better ways of doing this but I couldn’t find them.

The clutch drum was a tight push fit onto the spacer.

The spacer was then attached to the steel pinion with three M10 low profile HT socket head screws that replace the rivets. Each of these screws has been drilled and tapped M6, so that the clutch basket can be screwed on. Three low profile M6 HT socket screws were used here. Obviously it’s necessary to be able to assemble the clutch basket assembly onto the spacer like this because you’ll be doing it with the modified crankcase cover already in place.

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[DrPMC]

Step 3. OK, we have our 10mm gap to accommodate the oil seal but the oil seal cannot run against an aluminium part without quickly wearing it. One solution would have been to make the spacer out of steel but I wasn’t confident that my machines could cope with that, especially cutting the recesses not to mention the amount of hard material to remove with feeble machines.

My solution was to make a sleeve out of a piece of scrap stainless and then press it onto the spacer. I picked up a suitable bit at my excellent local scrap yard. It was a little out of round presumably from being have been driven over but there was lots of material, so truth could be restored on the lathe. I cut a section off with an angle grinder and the used a carbide tipped tool in my long suffering mini lathe to machine it. Very small cuts and loads of lube were used and the lathe complained a great deal.

I didn’t machine the outside to the desired 120mm diameter but left it a little oversize. After I pressed it onto the spacer with my homemade press I assembled the spacer, basket and the rest onto the gearbox mainshaft and mounted that in the lathe to machine it to size. My reasoning being was that was most likely to give me something that would run true in the engine.

I don’t think there any chance that the stainless sleeve would ever move again but I added three countersunk M4 HT screws as an insurance. These threaded into the outside face of the spacer, with the heads screwing down against a internal shoulder of the sleeve.

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[6]

Holly shit dude!

Epic cool project. Can not tell you how many times I thought making a hawk with a dry clutch would be next level cool, but also know my limits well enough to not try.

You have my endless respect. Hell yeah.

Can't wait to see a vid.

[squirrelman]

OMG ! alot of work just for some noise, but with skills like these i believe you could convert a coffee maker into a nuclear particle cyclotron.

[eyhonda]

Step 1 already exceeded my limits. I'll just play my Ducati YouTube vids when I feel the urge. If I went down this route, the Hawk will be down another year. I gotta start riding my Hawk again. I miss it. Big skills Dr!

[DrPMC]

Step 4… is making the holder for the 120mm bore oil seal.

I intended to make this out of one piece of metal but the scrap yard didn’t have anything thick enough, so it was made in two parts. The first is a tube with a flange at one end to provide mounting points for the outer clutch cover; the second a base for the tube, machined to take the oil seal. I made these so that they slotted together with a manual push fit and used JB Weld to bond them together.

One critical consideration with the oil seal holder is the fit at the top of the aperture for the clutch drum in the side cover. There is very little clearance here, as shown by the way that the inside of the case has been machined at this point as standard. To ensure that there would be clearance for the holder, I reduced the holder wall to 1.5mm at this point and also relieved the cover a little. However, its not necessary to take much off and risk spoiling the seal of the cover to the crankcase.

The clearance of the bottom of the oil seal holder to the big clutch pinion was about 1mm. As an insurance i also skimmed a bit off the teeth of the pinion, this area of the teeth engaged with the anti-rattle gear thing on the crankshaft pinion, which I’ve deleted. This arrangement is specific to the XRV750 and removing it isn’t really a problem. Its hard metal but a carbide tool tip chewed it off.

Fixing the oil seal holder to the cover was done with more JB Weld. The trick here seems to take it in two stages. First stage, just use enough to bond the two parts together where the flange meets the cut edge of the aperture. I assembled my spare engine cases with mainshaft and clutch hub centre and new spacer all installed and the cover screwed down. Then I positioned the glued up holder with oil seal installed over the spacer. A couple of feeler gauge blades worked well to ease the lip of the seal into position. Assuming that there is a little clearance all round, this will position the oil seal holder in the correct concentric position. I left it with a weight on for 24Hrs to cure and then could remove the cover, turn it over and do stage two, completely filling the void between the two parts with JB. After that it cannot possibly leak from the join or move. With stage one its very important not to put too much epoxy at the narrow part of the joint at the top, to avoid anything that might prevent the seal finding its ‘natural’ concentric position around the spacer.

That’s about it for this part - obviously there’s many little checks and adjustments to be made as well but this is the main parts of the process.

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[DrPMC]

Step 5… Is adapting the chopped off clutch cover so that it can be securely refitted.

The main thing here was making a flange to add to the cover. This involved machining another big lump of scrap aluminium until hardly anything was left. I think I made it about 3.5mm at the thinnest part, which together with the 7mm of the flange on the oil seal cover made up for the thickness of the spacer. I made it so that it was a tight and interlocking fit with the original clutch cover. I also milled all the holes in the cover, which where positioned to avoid the strengthening ribs inside the cover so that it could be holey but still do its job.

This time I used aluminium brazing to fix the two parts together. I find this stuff works very well and with the right preparation makes a very strong joint. One thing I’ve found helps a great deal is to make a small hearth out of fire bricks - set up like this I can use an ordinary propane torch and it is quickly hot enough to melt the braze. Without the hearth a hotter flame is usually necessary and that also risks melting the workpiece which has to be one the more sickening workshop experiences.

Once it was stuck together i could mount it on the crankcase cover and then neaten up the joint with a Dremel and blend all the parts together.

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[squirrelman]

never saw a project here that generated so much swarf.

[DrPMC]

never saw a project here that generated so much swarf.

Yes, we now get aluminium chips with everything in this house. Even the dogs have aluminium chips on them.

[DrPMC]

Holly shit dude!

Epic cool project. Can not tell you how many times I thought making a hawk with a dry clutch would be next level cool, but also know my limits well enough to not try.

You have my endless respect. Hell yeah.

Can't wait to see a vid.

Hello Mr 6,

Thank you for the kind words... There is a link to a video here - https://youtu.be/Ncf4ioTyrYg I'll probably do another one when its all finished.

Actually, I bet you could do this thing - its not so much needing advanced skills as a lot of time and attention to detail. I'm just a hobbyist who didn't even do metalwork at school.

[6]

Hello Mr 6,

Thank you for the kind words... There is a link to a video here - https://youtu.be/Ncf4ioTyrYg I'll probably do another one when its all finished.

Actually, I bet you could do this thing - its not so much needing advanced skills as a lot of time and attention to detail. I'm just a hobbyist who didn't even do metalwork at school.

I think I'm missing a couple of key components to attempt that. Main ones being a lathe, mill and the skill to use either.

That said... Sounds like you could sell the needed parts as a kit.

I'm a capable fabricator with my saws, grinders and welders, but that's about it. Some day I'll get some of the precision tools and learn to use them. Until then I'll have to stick with living vicariously through threads like this.