Gearbox and Clutch Issues - Corvette

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Oneball
Posts: 581
Joined: Mon May 06, 2019 12:33 pm
Location: Worcester

Gearbox and Clutch Issues - Corvette

Post by Oneball »

Apologies for the non-Mini technical query, but there’s very few places that seem to have the wealth of engineering knowledge as here. Slightly long winded but it gives the full story:

My 1969 Corvette that I’ve built as a race car has a 355cu in small block Chevrolet engine. The block is a Chevy one, crank is a new forged Scat one. Bellhousing is original Chevy. Gearbox case is original Chevy with a new gear set, internals are basically all new with the exception of the mainshaft shaft.

Engine run in for 200 or so easy miles and oil changed. Pretty much the first time I gave it any stick when I lifted off it’d pop out if 3rd gear.

I took the gearbox out and stripped it. There was nothing obviously wrong but I changed the detent combs and spring as they were about the only original things left.

Went to Shelsley at the weekend, after getting the box in last week. 4 runs up the hill and it was popping out of 1, 2, 3 on the overrun even at low revs coming down the hill.

Pulled the gear box yesterday and there’s a lot of up/down play now at the end of the input shaft but no in/out. So the input shaft bearing has worn.

It has also eaten the clutch and burnt the flywheel and pressure plate. But I didn’t notice any clutch slip.

The pilot bearing is good. I’ve checked the centring of the bellhousing to the crank and it’s within 2 thou. I’ve checked the box mounting face of the bellhousing and it’s within 4 thou.

It’s been suggested that the clutch can’t handle the torque (although it’s rated 630ftlb) or that the input bearing has caused the problem. Both clutch and popping out of gear.

My concern is that without identifying the cause properly I’m going eat another clutch/box.

Any thoughts?
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andy1071
Posts: 233
Joined: Thu Mar 24, 2011 9:04 am
Location: Sweden

Re: Gearbox and Clutch Issues - Corvette

Post by andy1071 »

A few thoughts...:

If the clutch was dragging, or there was a problem with the spigot bearing (such as too much clearance), then I would expect you would feel that when changing gear....

If the flywheel is a 'race' one, maybe the driven plate is "too thick" -being a 'road' type plate...?
-A quick check is that the Diaphragm spring is normally approx flat (parallel to the flywheel face) when the clutch is bolted up.

I would normally expect the popping-out-of-gear to be something not right in the gearbox....

A thought though.... is the new crank pushing the input shaft into the gearbox... (you say the input bearing has worn...)
-How is the length from the new crank -thrust-face to the flywheel mounting-face / spigot bearing, compared to the original crank...?
(Or flywheel mounting-face to bellhousing mounting-face...)

-It is 'normal' that there is some longitudinal flex of any crank ( "panting") If the crank is pushing the input shaft, then maybe that's being transmitted to the gears...

Is the spigot hole in the crank in the centre....?
(though if it isn't, I would expect you to feel some judder when changing gear)

/Andrew
Oneball
Posts: 581
Joined: Mon May 06, 2019 12:33 pm
Location: Worcester

Re: Gearbox and Clutch Issues - Corvette

Post by Oneball »

Thanks.

If anything the spigot bearing is a little short of location on the input. So I’m going to replace the needle roller with a bush as I think that’ll give more support.

I’ve found there is some radial play in the roller bearing where the input and mainshaft meet. So I’ve got a new mainshaft to fit. The old one is about the only thing in the box that’s not new.

I’ve also got a set of torque lock sliders that have shaped dog teeth to positively engage the heat dogs. They’re normal a fix for worn boxes but I might just fit them. They do have downside such as a slower shift and increasing wear.
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As for the clutch. I think that might just be me not bedding it in properly a lot of what I thought was cooked metal is actually friction material. So I’ve got a ceramic paddle clutch plate to give a try. The one that was in there and the new one are designed to go with the flywheel and pressure and plate.

Will check spigot centring.
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Oneball
Posts: 581
Joined: Mon May 06, 2019 12:33 pm
Location: Worcester

Re: Gearbox and Clutch Issues - Corvette

Post by Oneball »

Old vs new mainshaft. No really difference in diameter of the input shaft/main shaft bearing. But the new one has got a much smaller radius on the end as well as a sharper step behind. It also looks like from the polished surface the needles were moving backwards down the mainshaft. So hopefully this’ll fix any movement
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andy1071
Posts: 233
Joined: Thu Mar 24, 2011 9:04 am
Location: Sweden

Re: Gearbox and Clutch Issues - Corvette

Post by andy1071 »

You won't get much clutch slip with the ceramic plate.....
Oneball
Posts: 581
Joined: Mon May 06, 2019 12:33 pm
Location: Worcester

Re: Gearbox and Clutch Issues - Corvette

Post by Oneball »

I don’t think the clutch was slipping as such. More that I was slipping it.

Went to put a new bearing in the mid plate and there’s some axial play, tilt and it’s free to spin. Not a huge amount and not enough I’d have thought to cause an issue but maybe all the little things add up so I’ve ordered a new one. It’s the only thing that locates the main shaft fore/aft
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