6 replies to this topic

[chichestermini]

Hello all,

I have stumbled across a problem that i have also noticed many others have faced on here, and have done a lot of research to little avail.

I have a full set of 1275 A+ straight cut drop gears, for use on a 998 crank. I know they aren't directly compatible, but from what i've gathered you can have a bush machined to fit into the primary gear? I've seen people have them made up, but i am not highly knowledgable of machining so it becomes a problem.

Is anyone selling a modified bush of some sort to rectify this problem?

Thanks in advance!

[Cooperman]

Funny you should mention SC drop gears today.
Yesterday I went to a classic car and country show about 45 miles from where I live and I took the rally Cooper 'S' with the SC drops. Despite wearing ear plugs the noise was still horrible and I resolved to take the engine out and put the helical drops back in. I fitted SC drops after 29 rallies with the helical ones after a drop gear failed, but just find that whining noise spoils the enjoyment of the car on any journey over about 5 miles.
SC drops are not normally used in a 998 as they simply don't have the torque to cause a problem. The only time they would be useful would be to change the overall gearing in a race car to suit different circuits without having to change the FDR.
And yes, you can sleeve a 1275 primary gear down to fit a 998 crankshaft. A good local machine shop could turn one up from phospher-bronze to suit your gear and crank.

Edited by Cooperman, 28 August 2012 - 09:11 PM.

[chichestermini]

Cooperman, thanks for such a helpful reply.

It's a 998 miglia engine build, but i have certainly taken on-board your advice.

I will approach a machine shop and explain the situation, hopefully the matter will be resolved.

Thank you very much for your time

[johnnyblaze]

so the std helical drops are strong enough for a good race engine then?

[Cooperman]

Helical drop gears will allow more end load to go into the drop gears, but my personal opinion is that helicals are fine for engines with up to around 120 bhp, especially as a race engine is taken to pieces quite often. In my rally 'S' I did 29 rallies with the same set and that's a lot more shock loading than a race car will get.
The big problem with SC drops in a rally car is that on the road sections the horrible whining noise gets very wearing. That is not an issue with a race car, or a rally car on the special stages as a crash helmet is being worn. The big advantage on a race car is the ability to slightly change the overall final drive ratio (FDR) without removine the engine to get at the diff. All that is necessary is to take off the clutch cover, flywheel and transfer case to raise or lower the FDR a bit to suit different circuits.
With SC drops on the road sections of events, or simply on the roads, it really is strongly advised that ear plugs are always worn. If you don't, then be prepared to suffer with tinnittus in older age, as I do. It's not much fun!

[cooperrodeo]

The big advantage on a race car is the ability to slightly change the overall final drive ratio (FDR) without removine the engine to get at the diff. All that is necessary is to take off the clutch cover, flywheel and transfer case to raise or lower the FDR a bit to suit different circuits.

So how do you measure/set the idler gear endfloat?

[Cooperman]

You measure the initial set-up by doing a dummy fit of the transfer gear case to the gearbox before the engine goes on. Then you set the float using feeler gauges and the required thrust pads. If you change the transfer gear with the engine still on the gearbox you can only measure acrtoss the transfer gear faces with the thrust pads on and set the replacement transfer gear and thrusts to the same measurement. That assumes no wear on the thrusts or casings, but if it's race application the whole lot will be coming apart very regularly.
The problem arises when an 'after-market' cheapie transfer casing gasket is used which is thinner than the 'original' type. I've actually had that happen and now I will only use a Payen gasket which is a 'pink' colour. If you are concerned you can check the transfer gear for free movement by taking out the 'red' seal to gain a bit of access to the gear.