Answer to Bryans crank question

[L. Hargett]

Bryan Joplin asked;

Larry,
I am curious what happened with your Leopard. What is the story behind the crank failure. If you prefer to dicuss offline, contact me at 281-356-6438 or b.joplin@sbcglobal.net. I have been doing a little research on this poblem.

This is an 08 Leopard. It’s got 3.4 hours on it.
It appears the crank failure happened while practicing at Katy. Joshua was on the track for the first time@ Katy in his TAG jr. On lap 20 he came in and said it just quit running when he lifted in turn 1.

He rolled into the pits. The micron read as follows: 157 water temp.
RPM- 15947 except for the last 3 laps we show a spike of 18457 RPM.

The clutch key sheared (had not pulled the clutch yet, only drum maintenance) and the clutch spun and friction welded itself to the crank. In the process the shaft turned black, got very hot and tried to remove itself from the motor.

That spike was I believe near the entry to turn 1, my driver said the rear hopped off the ground in the braking zone when he said he hit that bump in a bad place and would kick up the rear, He also said he was playing and was trying to drift into turn one without lifting. I told him quit playing were trying to get some work done here
.
Motor is going back together now with the addition of a EGT in the header.
Piston burn pattern looks very good. Cylinder is still round and looking new, even the piston skirt looks almost out of the box. Plug reading looks very good, may have been a tad lean on the bottom but not by much.

[Bryan W. Joplin]

You are not the first to have this type of clutch failure on a 125 Leopard. It is common discussion on the internet and a number of us have had it happen at Gulf Coast. Not sure why you had the overspeed, but could be caused by the loose clutch. I don't believe the overspeed caused the clutch to turn on its taper. Two reasons for slipping on the taper are:

1. Improper installation of the clutch, even by the factory. Based on discussions with different shops and my own experience the factory torque spec is insufficient. You will find mechanics have their own preference for installing the clutch on the taper. All will clean the taper with alcohol or acetone. It must be absolutely clean. Some mechanics do no use a torque wrench, but tighten the nut as tight as they can get it (touch method). Others prefer an impact wrench, feeling the hammer affect seats the clutch on the taper. I use a torque wrench and tighten the nut to factory spec. I put a witness mark on the nut. I then give a short burst with my impact wrench. The nut easily tightens another 1/8 to 1/4 of a turn. Use loctite. I check the witness mark the first couple times on the track.

2. Second reason for the clutch slipping is a stone hit. Was the kart off the track? Debris can get inside the starter ring gear and lodge between the crankcase and gear. You might have an immediate ring gear failure, which is obvious, might take out the starter bendix and bend the crank. It is possible to put a slight bend in the gear, which puts it out of balance. The gear will eventually fail due to centrifugal force. Keep a close eye on the gear. Inspect it any time the kart is off the track. Check it carefully during clutch maintenance.

I have asked the Club to allow after market parts to these engines to improve reliability. You might have seen my previous post on this. I think this is in the works including the Regions race. It is my understanding that the 09 engines have improvements to avoid your type failure, at least that is what I was told by the importer. I can't wait to see one of the 09 engines.

Good Luck, and hope this helps.
Bryan

[L. Hargett]

Thanks for your imput.
Have been racing karts since the 80's and have never run across such a popular motor that is sooo-fragile.
Motor came out of the box with the clutch installed. I knew this was a different clutch than the one we ran on our comer 60. Lots of experience there, we would consume no less than 12 clutches per season. They even came out with a"New Batch" that a fellow racer and myself split an order of 10 and only got a max of 83 laps on the best clutch, and as little as 35 laps on one. They would brake one ear off or wear the lining completely off but never spun on on the crank. The Comer 60 Tag motor did not have a key like the higher hp motor such as the Leopard.

I made the mistake of assuming the clutch was properly installed and it may have been, and my mistake was not to check the mount more closely. Only did my normal between race maintenance with the drum & bearing and an inspection and a light de-glazing of the clutch material. Thanks again for your input. I will loctite and mark and keep an eye on it.

Also my driver never dropped a wheel and we were disappointed we could not make the race this past Sunday, when my son looked at my laps and saw the times he was bummed out because he was running some low 41's his first and only session on the Katy track in this kart.

Thanks,
Larry Hargett / Joshua Hargett

[Mark Reid]

Larry - Bryan is right. Same thing happened to me after replacing a clutch. I torqued to IAME spec and the nut back off in the first race, sheered the kay and welded to the crank. I follow a similar procedure to Bryan's but with the added step of 1) lube threads lightly, tighten, back off a half turn, tighten, back off again and tighten a final time (as tight as I can get it). Of course, with a little loctite.

The other thing you should consider (even stright from the factory), is remove the 3 screws that retain the drive sprocket to the clutch drum and blue loctite them. I have had the screws reverse out on a new engine within the first test. Do not plan on removing the screws with an allen wrench henceforth. You will need to buy a good star socket (I think t20) to tap into the screws and gently remove with a ratchet while holding downward pressure.

[Rick Miller]

Good point by all. Heres something else that is important when you are installing a clutch on a tapered shaft with or without a woodruf key. Even though we are dealing with "new" parts ie: clutches, keep in mind these items are mass produced to very strict tolerances but all are not a perfect fit. Prior to installation, pick up some lapping compound, yes, the same stuff you use when you are lapping valve stems to their seats in a motor. Cloverleaf brand is very popular and it is readily available in corse and fine blend. Generally the fine stuff works well. Smear a slight amount on the taper of the crank and slide the clutch hub on the shaft and work it back and forth, and rotate it at the same time, with pressure to insure a good contact pattern. do this for 3 or 4 minutes, remove, wipe off the excess and you will be able to see the "grey" area of the contact patch. Keep working it until you get an even patch all the way around. After that CLEAN BOTH PIECES VERY WELL with brake cleaner. This stuff is an abrasive so remove all excess, then install with loctite. Hope this helps and happy motoring!! Rick

[Scott Madison]

did you learn that in flat trackin!!!!!!

[Rick Miller]

Many years of working on bikes, both 2 stroke and 4. Years of racing, you figure out a hundreds of things that wont work, then whats left are the things that do. Rick

[L. Hargett]

I did the lapping deal on the Comer 60'S after noticing the clutch had shown some slippage on that tapered shaft w/out a key. It became a reg. deal with us because the KWE 60 needed a new clutch almost each race day, and at times I had to change to run the final. An unmodified KWE 60 clutch would stall around 5K RPM. When you machined to clutch to stall @ 7K per TAG rules it was only good for 58 to 85 laps.
Had a few that wouldn't last past 35 laps. After a while you would end up with an inventory consisting of a practice clutch (stk.) a qualifying clutch ( one with a stall of * ), and then another for the heat and main machined to stall at *. Had to do this to be competitive with the Gazelles after they put a manifold restrictor and an extra 10lbs on this motor. Really changed the motor and we had to get all we could to keep up.

Things like this make you change your program for evermore.

I will pull the clutch and inspect the fit to shaft after each race day, and you see me checking torque between rounds. At least they have improved the lining. With aprox. 265 laps on the clutch it measures out to show very little wear. I am pleased to see a good stall # and liner wear on this Leopard clutch. Hope I can keep the clutch gremlins out of my pit.

Thanks all for your imput and good advice to help keep my son on the track and out of the pit.

Larry Hargett / Joshua