7 replies to this topic

[Miniman Sam]

hi,

i race a classic mini in autograss class 4 and have been having trouble with the crown wheel stripping teeth, the gearbox is all new apart from the case, gearbox spec:

gearbox case dam 5626 3 bolt bearing retainer

swiftune roller drop gears straight cut ratio, 1:087

MED race dog box box gearset

swiftune Tran-x LSD diff

swiftune Lsd 4.7 cwp

all bearings new and quility RHP brand

 

the crownwheel strips teeth when I set of the startline, 1st gear, drop the clutch at no less than 5000rmp otherwise engine boggs and drops of the cam.

the engine is 1130cc, makes around 110hp at the flywheel, the engine is fitted with lightwieght reduced diameter flywheel sold by lynx ae, so reves really quick and to around 8000rpm (engine is all steel crank and rods )

 

the crown wheel issue has happend twice now using two different makes (MED and swiftune)

 

so I have just rebuilt the gearbox again and this time I have fitted a pinion support, to eliminate the pinion climbing up the crownwheel and going out of mesh, I have change the engine mounts from solid to rubber to eliminate any extra stress or flex though the gearbox as the  engine is mounted in a spaceframe not a mini subframe, I have ground out all the casing casting flash marks in the gearbox case to make it smooth to eliminate a potential crack starting.

 

engine is now back in the car but has not been raced since gearbox rebuild, any help will appreciated, as I don't really what this issue to happen again as I don't know what else I can do to strengthen or de stress the gearbox?

can any body please advise if they have had this issue before and how they solved it please?   

[Spider]

First thing I'd look for is a cracked gearbox case. They crack quite easily above the centre Mainshaft bearing, through the web. It's a very thin section here.

 

The 'action' between the Crown Wheel and Pinion when you drop the clutch is such that the Pinion wants to climb up the Crown Wheel and puts a lot of stress on the Centre Web of the gearbox Case. Being the material that it is, it has very poor fatigue resistance. Running an LSD also adds load to the Centre Web.

 

The next part is that with Ratios above 3.9:1, the Pinion becomes so small compared to the Crown that the Crown Wheel effective wants to 'Run Over' the Pinion. I'm not sure what Swiftune and MED have in regards to CW & P's but I do know KADs are very good and use the highest grade materials currently available for this application. Their heat treatment is also spot on.

 

Another problem is that the machining tolerances of these late gear cases is also pretty poor. The past few years for anything critical, I've been blueprinting the cases, but I find I'll go through about 4 or 5 case just on initial measuring before I find one I can actually work with. In regards to the problem you are seeing, the centre line between the Main Shaft and that of the Diff must be between 4.000 and 4.002". Trust me, that's a tall ask, however, most I've found are bigger, so you can re-tunnel the Diff Bearing Mounts to correct this. This is all to ensure full and proper mesh between the Gear Teeth and most likely, at the end of the day, it is because you are not getting or maintaining Mesh that the Teeth are stripping.

 

While you are at it, check the Centre Lines between these are parallel and you'll also find the faces on to which the Diff Output Side Coves bolt up to are not perpendicular to the Diff Centre Line, nor Parallel to each other.

 

I generally spend 1 to 3 days on the Mill sorting the Cases before I fit a part to them.

 

These will all help and possible 'get you over the line' with it all, however I have also beefed up a few cases by thickening up the Centre Web adding material by welding, but there's only so much you can do if you have a 1275 (which it appears you don't) as it will foul the crank / rods. When i get time, there's another 'avenue' I'll be trying here to cure this problem once and for all.

[Miniman Sam]

Hi, thanks for the information on the sizes, when I fitted the gearbox together I did blue up the cwp to ensure that the it was meshing correctly, and it was, on the subject of stiffening the casing I looked at trying to make a tie rod to put across the the centre web but this was not possible as it would hit the crank, so that is why I fitted the pinion surpport to try to keep the pinion in mesh with the crownwheel.

[Spider]

The stock Main Shaft isn't remotely strong enough for those Pinion supports to be effective. I've a couple of broken Main Shafts from trying them too.

 

Noted and good to see that you did 'blue' the gear teeth to check, however you'll likely find with the very low ratio you are running, that's well enough to flex the gearcasing, most likely the trying to 'push' the Diff out the back. I haven't broken a case here but I've seen a couple that have, which only shows to me that it's another weak point in them.

[Miniman Sam]

Hi,
The mainsfaft is specific for the dog box from MED all came as a kit, the pinion surport is billit aluminum also from MED made to suit the dog box main shaft.
The last time the teeth came off the crown wheel it pushed the whole diff housing out and cracked all around the top gearbox side of the diff housing and all so split the center web in two places around the bearing like the main shaft had tried to get out, the pinion must have climbed up crown this is why I fitted the surport to try and stop that happening

[Spider]

I think by the sounds of that, the first thing that happened was the Diff being pushed out the back of the gearcasing and then the rest happened following that.

 

Try to find an early DAM5626 Gearcase from about 1983 - 84. The alloy was better (stronger) and the machining while not great is considerably better then the later ones.

 

The last of these cases were real bad.

[Miniman Sam]

Hi, how can you tell if the case is early or late, the only difference I'm aware of is that late ones don't have gasket on diff housing and early ones do, my casing is one with out gaskets on the difff housing.
Also when I strip a gearbox I then bead blast the case's to clean them up for inspection, but I have noticed that some times the case comes out dull and sometimes more shiney is this the difference in material quality?

[mini13]

might also be wprth looking at getting the final drive and case cryo treated, Ive heard good things about it on steel, not sure how it works with alluminium though, any material expurts out there that can elaborate?