Crank Taper Dimension
Turbo Mini Forums

Hi,

Can anyone tell me what the standard dimensions of the crank taper are?

I've got a crank that needs some remedial work (its a one-off billet short-stroke crank so I hope to recover it rather than need to replace it) and I'm hoping to get it back to standard on the taper.

I've managed to find a local engineering works that can do the work on the taper, but would like to give them the exact dimensions required if I can.

Any info would be much appreciated.

Thanks,

Mike

Just measure it all up and cad draw it for them, or is the taper too goosed to to do so?

The dimension is usually given as a taper ratio. An easy way is to measure it is to cut two different known sized holes in a sheet of metal, slide them both over the taper then measure the distance between them obviously taking into account the thickness of the sheet.

You can measure the taper of the flywheel hub the same way, making up two plugs different sizes and measure how deep the plugs fit into the hub taper.

Obviously both of the above needs to be calculated into a ratio.

Failing both of them methods of measuring, most decent grinders will be able to use the flywheel taper as a taper gauge for the crank taper.

Edited by matty on 13th Sep, 2013.

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1/4mile in 13.2sec @ 111 terminal on 15psi

I had a verto centre CMM'd a few years back - I'll find the notes and report back. I am sure MED and other builders already know it though as many offer a tail re-grinding service

What is the maximum diameter possible to machine off a taper, say 1mm etc, before it has moved too far in and contacts the c-clip, not much by the looks of it.

I have just torn down an engine from a car that has not moved in 20 years, and the reason is the bloody flywheel has spun. It is not too bad, but certainly will want a nibble, so will take as much as possible off to try to clean it up well, it has picked up a little too so will want some material off to clean that up. Shame as the crank is perfect in every measurable way apart from that, journals and bearings are unmarked.

Cheers

James

Not a lot, because of the shallow taper. a couple of thou will move it up the taper 20thou. You are better off blueing the taper up to find the high spots, then wet stoning the high spots off. When you are close to a good fit, use some very fine grinding paste to finish off. Don't be tempted to use grinding paste before you have to as the taper will lap unevenly, as the larger dia tends to lap in quicker than the smaller end.

Edited by matty on 8th Nov, 2013.

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1/4mile in 13.2sec @ 111 terminal on 15psi

If the flywheels has genuinely spun the taper is fucked, it may be possible to weld said taper and re machine it ,but the cost would be more then a good replacement


carl

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Good replacements seem to be getting harder to come by that's the trouble, many now have been through a multitude of engines or have crap rolled fillett radius.

The damage is mainly located towards the nose end of the taper, I've yet to touch the high spots with any emery yet.

Edit: Curiosity got the better of me, so been out and cleaned it up a bit. 2/3rds of the taper is still in good condition, so will get rid of the high spots, lap a spare flywheel on it, and check the run out.

Edited by minimole23 on 8th Nov, 2013.

There is another option of having it hard chromed, but I'd only do that if it was a fully worked crank, where hard chroming would be more cost effective than a new worked crank.

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1/4mile in 13.2sec @ 111 terminal on 15psi

An hour of stoning, lapping and generally sorting has yielded this. I must say while an improvement it could probably do with 10 thou grinding off it. I read somewhere that it is a 1:7 ratio taper so should only move the flywheel in 2mm, still leaving adequate room between the C washer.

having concluded its not Fubar, it can now go under the bed for 20 years as I have a couple of better ones to use first.