Crankcase chop

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ajb235

Crankcase chop

Post by ajb235 »

I'm considering a lightweight short wheelbase Gt750 racer. Looking at the crankcases, could the engine be shortened by removing the rear housing where the kickstart mechanism sits? This would cutting and welding the cases, and also a modified or new clutch cover. Has this ever been done?
I want to use a GSXR 750 k2 swinging arm, but this will lengthen the wheelbase by approx 4 inches, so if I can lose this else where so much the better.

Just a thought!

Jim
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BATTS
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Location: Lancashire

Re: Crankcase chop

Post by BATTS »

Others will know better but that end of the crankcase also contains the oil pump drive (presumably on a racer you're going premix?) and the slightly more complex gear-change linkages. The latter may be problematical to re-engineer.
DSC00007 (2a).jpg
Chris
76 GT750, 78 GT550, BMW K1600gtl
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ajb235

Re: Crankcase chop

Post by ajb235 »

Yes, premix. Looking at your helpful photo,and considering the gear linkage, the chop might have to be behind this. Some big bits can go, Kickstarter, shaft,gear,oil pump and drive, and a hefty gear on the lay shaft.
Would still lose about 3inches on the length of the motor.rear engine mount to be considered too.

Jim
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ajb235

Re: Crankcase chop

Post by ajb235 »

Oops, lay shaft gear has to stay, otherwise no gears. I thought it was just for the kickstart.
Kettletimes3
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Location: Sunny Wales

Re: Crankcase chop

Post by Kettletimes3 »

I have taken on a few difficult projects in my time,
But i think i would use a different swingarm,One that's shorter,
And leave the motor alone,You could get huge distortion problems with the welding.
teazer
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Re: Crankcase chop

Post by teazer »

It's absolutely possible and a very sensible upgrade, but not without some issues to consider.

Need new rear engine mounts.

Need to bolt a heavy steel plate to the crankcase halves when they are welded to minimize distortion, but be prepared to machine the mating faces and then line boring the crank and gearbox bearing holes.

Need to move the swingarm pivot forwards in the frame to get that wheelbase reduction you are looking for.

You could also dump the water pump and drive shaft and use an electric pump with thermo switch. VW/Audi OEM Bosch are pretty cheap but a little bulky or use a Davis Craig pump. For a race bike you don't need the electric foot either, so that's even more weight saved.

And you could go with either a NOVA close ratio transmission or just fit a GS750 or 1100 trans to get a higher first (1100 higher than 750).

Don't forget that to convert to pre-mix you will need to modify the crank slightly and the crankcases to be on the safe side.

Please post pics or links to your build.

edit: if you are serious about cutting and welding the cases, you may want to consider cutting the cases right behind the crank and weld on a GSXR 6 speed transmission. It would be a little more difficult in terms of say crank gear to clutch gear tolerances, but would give you a modern light shift effort 6 speed. I chopped up a set of GSXR cases with that in mind before I reluctantly had to accept that it was way past my skill level or pay grade. It is possible for someone with the right skills, equipment and budget though. Sadly I was a touch short in all three areas. :?
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ajb235

Re: Crankcase chop

Post by ajb235 »

Thanks, some good tips there. I would entrust the welding to a specialist, but it's good to know that someone has been thinking along the same lines.
I like the idea of mounting the swinging arm pivot on the back of the engine gearbox unit, but this would mean removing all the rubber engine mountings, and making the engine a stressed member of the frame. However with engine mounts from the cylinder head bolts, the frame down tubes could be removed. The major consideration is whether the engine casings are strong enough for this.
I was also considering a thermo symphon cooling system, without a pump. I had a Scott Flying Squirrel 600 cc two stroke twin many years ago which used this system successfully.
Chopping off the gearbox and welding in the GSXR box is a dream for me, way beyond my ability too, but very interesting.

Not sure what to do now, but the stressed engine unit with integral swing arm pivots does appeal

Thanks

Jim
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