Primary gear floating bush fail
Turbo Mini Forums

I have again problems with my primary gear bush. Last year I fitted 2 standard bust with good clearances, they worked so time ok but then I was impossible to select gears. Then I fitted floatig bush, I worked very well but Then it Failed. I opened the clutch and the floating bush was ruined because it was cutted to the oil holes and after that it was blocked the oil holes. Now the question is that should you use cirlips with the floating bush so that the bush doesn't hit to the oil holes? I have Jack Knight straight cut drop gears.

I always use a smear of lithium grease when I assemble the primary gear onto the crank tail. In the past I have still found grease there after the engine had been stripped down.

There is an article on Minispares web site that talks about a similar problem you have, and talked about adding a slightly larger chamfer on the edges. A standard primary gear usually has a slight ridge at the point where the holes are stopping the bush covering them. I'm assuming JKD have been efficient and removed this lip?

What is it that causes the bush failures? Is it heat/rpm/torque? (or all three)

I think its the heat that is the real killer. Ultralight flywheels don't have much mass, and a powerfull engine in a car driven a bit hard will dump a huge amount of heat straight into the crank tail and primary gear. One reason why I think balls out turbo motors are better off with an RTS clutch on a light weight verto flywheel. Its heavy, but will soak up more of that heat and disipating it slower, and shouldn't be a problem on a motor up to 7500rpm occassionally

I think mine will end up RTS once I've tested the cover I'm trying out for a clutch place. I don't expect it will cope. So hopefully no bush problems (said the actress to the bishop)

Those bushes are the wrong size for jack knight gears, you'll need to have one made up.

sprocket you are correct buddy with the heat soak

I think it the heat that causes the problems. But is there anything that I can do with my JKD primary gear so that I can fit the floating bush? For example Swiftune sells drop gear sets that has floating bush and cirlips that holds the bush in place. But I really don't want to buy a new set of drops because the JKD gear set is almost new.

here is pictures of the standard bush failure.

Have you polished up the tail of the crank after it had been hrdened?

I had a similar failure with a standard bush in very few miles. I suspect it was down to the finish on the crank after it was hardened, since it wasn't polished like the bearing journals. Same crank with the tail polished with a floating bush, and its been fine.

Incidently, is that a crack in the bush I can see?

If I recall Sprox, the bush is a 'split' type so that when you press it in, the ends close up, if that makes sense.

Richard.

Minivanless, but reluctantly happy living with the decision. There'll be another one day.
http://www.youtube.com/profile?user=minimadmotorman#p/u

Yes this bush is split type, but the non split type did the same. Crank is MED one.
So should that end part where the bush contact be polished???

Funny, the crank I had the bush fail on was an un polished MED crank. Coincidence? Polished up the tail and not had a problem since.

Edited by Sprocket on 28th Jun, 2011.

Interesting because I know other supercharged mini which has also MED crank and he has also same problems. But he changed drop gears to swiftune set and after since he has no broblems.

By the way are you sprocket now using non floating bush?

Sorry I may have confused things.

I ran the MED crank with a Minispares straight cut gear, with a standard bush, and it failed. Before I did anything more with the crank, I asked my machinist, and he said it was likely the coating left by hardening.

Before re using this crank, I had the tail polished, and I then used a helical gear with the floating bush and everything was fine (the noisey straight cut gears now on the shelf, complete with failed bush, for future use)

So, in a way, I cannot be 100% sure that the hardening coating was the real problem since I changed two things, but there is more than a good reasoning to say that it was the coating that is THE problem

Edited by Sprocket on 29th Jun, 2011.

Looking at it noticed that 1 you have a new rear bush. 2 that the heat is toward the front of the front bush. This points to the primary gear not being concentric on the crank measure the crank if the small end is 1.499 then make the bush have 1.502 (.003 clearance) the forward larger bush should be 1.624 and a pressed in non split bush id will be 1.627 (.003 clearance)this will get the gear to have full contact on the crank. Make sure you have a good machinist do all this so the gear is true. I had a bit of troubles with a primary gear bushes but careful machining fixed it.

Racecar spelled backwards is Racecar

Thread resurection but.

When fitting the primary gear should the bush remain un lubricated, or have a smear of grease or engine oil applied?

Got to do mine tomorrow and the previous bush was shagged for no obvious reason so want to ensure this time it gets the best chance of success.

Cheers

James

Haynes says nothing sorry,checked mini and metro manuals.
As the parts would be subject to oil from the engine I would of thought very light oiling would be ok at the very least but others may know/have better exp

I've been having problems the last year with my bush 😃I mad sure the crank was spotless and polished it up with autosol to try and prevent it from picking up. I just fitted it with oil first seems to have done the trick!

I always fit mine with a bit of engine build lube, seems to do the trick,

its probably a very short time till they get oily from the crank main so std oil should be fine as above, the nose one is more of an issue though,make sure thats well lubed.

Now back together spotlessly clean, and with plenty of engine oil.