Clutch Nut torque settings

[Phil Parkinson]

I am in need of some help. I have an original Rev Grip dry clutch on my Mac91B and twice now the clutch nut has come undone. I have lapped the clutch to the shaft, fitted a new key each time but it still comes loose /off after a dozen or so laps. What torque settings should be on the clutch nut? and should I use a particular loctite? I have been told the key is unnecessary, but am unsure, although I never used keys on any direct drive Yamaha engines we ran . Any help greatly appreciated.
Thanks
Phil

[Rob Voska]

Sounds like you are doing things right by lapping etc..... Use a drop of blue & you can even add a jam set screw inside the nut. Don't overtighten as the thin nut will crack. I never torque them.

[Bill Hermsted]

Regarding the Max Torque "solid body" dry clutch: Mr Donovan provides either an inboard or outboard mounting option for a Mac. Question is- which way do most of you racers mount your clutches? Even though my pto has a left hand thread, so the nut hopefully won't loosen, I'm working on a simple idea for a mechanical safety that should keep a right hand nut tight. The only thing- the clutch must be inboard and the hub face drillable (is that a word)?
bill

[Jeff Campbell]

If you have a 91B with a key'ed PTO crank, then you must have Right Hand threads. Under power, the crank will want to unscrew those RH threads, so that nut really needs to be super tight. The newer Rev-Grip cluthes came with a weak thin walled nut that really can't be torqued over 225 in-lbs, this is barely adequate for a MC91 series motor .... so your older style clutch nut can handle more torque than that, you have to be higher than 225 in-lbs. The McCulloch torque spec chart in the 91-101 owners manual lists 260 to 300in-lbs for the PTO nut, but the PTO threads will handle much higher than that (if the nut can) .. for instance the new Max-Torque clutches have a torque spec of 430in-lbs. If you have one of the old style Rev-Grip clutches made back in the 60's or 70's, the clutch nut should handle something in the 260 to 300in-lb range.

No, you don't need the woodruff key, if you lap the clutch hub to the crank. Those keys provide very little help in keeping a clutch hub from spinning off, think about it, all that power and a tiny little soft steel key ... all the holding power of the clutch on that PTO comes from the taper and good torque on the clutch nut. If you use those keys, and the clutch spins off, you can damage the crank .. toss the key. Don't be afraid to use red loctite on that clutch nut, the clutch gets so hot, that the red loctite will get broken down by the heat and is removeable with hand tools. If someone has a LH threaded PTO, I would recommend against loctite at all, since the clutch can self tighten with LH threads, adding loctite prevents the self tightening.

FYI - the adapter shafts on my Foreign B-Bomb and BM-130's are the same size as the mac PTO, and with the nuts I use on clutches, can handle up to 700 in-lbs of torque ... I use no woodruff keys on those.

[Bob Towarnicki]

Phil,
I had the same problem with the Rev Grip outboard nuts so I had some made of 4140 pre-hard chrome moly. I had seen Rob Voska's recommendation for this material on this site. They have Class 3 threads machined to match the Class 3 thread on the crank. I have some available in right or left hand thread - $19 each, free shipping. If interested, send me a PM.
Bob

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