Nut start Macs?

[ted johnson]

I'd like to hear from guys who've nut started Mac 101's from the clutch side. I've got a 101 with a 101B left hand threaded crank. I'm using one of Jim Donovan's latest Max Torque clutches, and it has the set screw that dogs down axially to the end of the crank. My concerns are that the starter will be trying to loosen the nut, and that the PTO end of the crank is only 3/8-24. Any thoughts? I've nut started plenty of Macs from the flywheel side with the Hartman adapter, but never from the other end. I don't think I have room to get to the flywheel side with a nut starter. Thanks. TJ

[Tom Luttenbacher]

Ted,
I tried it a few years ago with the old M-T dry clutches and the new Rev Grips, the nuts are very similar. I wrecked 2 so I quit trying and re vamped my starter for the belt pulleys on the flywheel side. The nut, if you look real close is very thin between the bushing outside and the thread inside and just won't stand the twist from the starter or the jam nut. Tom

[steveohara]

Here is the hot ticket for starting off the PTO side with the left hand thread cranks.
The nut that hold the clutch on should be threaded for left hand threads out to the point where the crank threads end. The outer end of the ID of the nut should be threaded with right hand threads in to the point where the left hand threads end. Install the clutch and torque the nut, then run a right hand threaded bolt down through the nut from the outside and tighten it up agains the end of the crank. Use the hex on the end of the bolt as the drive for the starter.
For Rev Grip and Max Torque outboard type clutches that use the very thin wall nut the material used to make the nut should be high quality stuff or it may still fail.
Steve O'Hara

[Phil Parkinson]

I have just had a Clutch nut machined for my Mac 91b fitted with an Rev Grip dry clutch.
The clutch nut has a 19mm head which is used on the majority of clutched motors used in 'modern karting' in Australia. So starters are readily available and light.Plus I had one and it eliminates the fan belt dramas.
It works perfectly well and has stopped my original style clutch nuts coming undone.

[ted johnson]

Thanks, Tom. Steve and Phil. You echo all of my concerns. I like Steve's solution of the left/right threaded clutch nut where you're actually starting off the bolt that's dogged down to the crank end. This whole thing was just so I could run the old plastic flywheel screen and the easier-to-use nut starter. I may stick to the belt start pulley I already have on it! If I do decide to go with the Steve-nut, I will CAD model it, draft it up and ask Jim Donovan if he'll make it. Of course, the obvious solution is to replace the LH crank with a RH one and start off the "dog-bolt". I'd do that if I weren't so darn lazy! Hmmmm, maybe just replace the whole engine...Hmmm... Thanks again. Ted

[steveohara]

Ted,

One thing I didn't mention about my right/left jam bolt setup.... On the bolt I run down in there to butt against the end of the crank I have the threads turned off the first .150" just to make sure there is no chance the right hand threads can run into the lefties.
Steve

[ted johnson]

Thanks, Steve. I figured that one out. Wouldn't do to jam RH threads into LH ones! TJ

[Tom Smith]

The new Max-Torque barrel nuts are not thin walled like the original style. On the new Max-Torque clutches the drum bush rides directly on the barrel nut. The hub sleeve and the thin wall barrel nut are no longer. Me I'd just install a right hand crank in the first place for PTO starting.

[ted johnson]

Thanks, Tom. I looked at the barrel nut, and it's plenty beefy. If I do nut start off the PTO, I'll wait until i tear the engine down for refresh, and install an RH crank. Then I'll drive a 3/8-24 hex head bolt down to the end of the crank and start off the 9/16" hex. For now, I think I'll find a Hartman flywheel side adapter and start from that side. I appreciate all the help! Ted

[Jeff Campbell]

Lot's of good ideas here. I'd say the opposite tread bolt in the nut is the most used solution on LH mac PTOs and on foreign motor nut starters. Yes, the new rev-grips that came out around 6-7yrs ago, have a very thin and weak nut .. it will only handle 225in-lbs .. .the new Max-Torque clutch nuts can be cranked down to Twice that much, I highly doubt a LH Max-Torque nut fully torqued would spin off during a nut start. Some of the gear reduction starters do pack a hard kick when turning on, like an impact driver, personally for nut starting, I like the old non-gear reduction starters. For what it's worth, my LH BM-130 rotary valve motors use a RH thread starter nut. Those nuts can not take much torque, probably around 200 in-lbs, I put red loctite on them ... and have never had one spin off (starter turns CCW), and most of the starts have been from gear reduction starters. I have had the whole end of the crank break off! ... but that is a different issue and is the issue you should be MOST concerned about. Make sure the starter drive does not go too deeply into the engine nut (or hex bolt head), the fit should always allow the starter to be really mis-aligned without stressing the crankshaft end. The most common way to do this is to only allow a short length of engagement (design that into the nut on your engine, in case some elses starter must be used). Some of the old starters used to have a U-joint with rubber hose covering on their shaft, still a good idea today IMO. During a start-up, things can happen, the person starting it may rest the weight of the starter on the crank by hanging the starter, or wrestle off the starter if it sticks in the motor nut, all HUGE NO NOs! Always maintain alignment and pull it straight off.

Jeff