1981 GS850G to GS1000 crank and barrels swap project

[Klob]

G'day from Australia everyone,

I've recently completed a project GS850G where I managed to squeeze a GS1000G crankshaft and barrels into something they were never designed to fit. I've learnt quite a bit from this site and thought I should share some of the details in case anyone else is thinking of the same.

Much of the internet said it couldn't be done, so that was enough of a challenge for me. For the project I started with a a stock 1981 GS850G engine that was in need of a rebuild and went from there. It's currently running:

• Crankcases - 1981 GS850G upper and lower with the standard clutch and gearbox and secondary drive.
• Crankshaft - 1981 GS1000G.
• Barrels - 1981 GS1000G with Wiseco 73mm (1085cc) pistons.
• Head - 1981 GS1000G
• Carburettors - BS32s from the GS850G with 125 main.
• Exhaust - custom 4-2-4 with Cone Engineering mufflers.

Most of the project was a straightforward exercise in ignoring myths, comparing part numbers and measuring accurately.

The GS1000G crank dropped straight in and mated perfectly with the clutch. The upper crankcase needed some relieving around the stud bosses to prevent interference, and the major work was providing clearance on the rear of the No 4 cylinder crankcases. In the GS1000s they removed this web completely and made the clutch cover to remove this issue.

The barrels went straight on with no issues with the oil passages; however, there are parts of the casting that protrude much more than the GS850 barrels and look slightly different.

I had the same with the head going straight on, and confirming valve clearances. The cams were another story. The changed absolute height of the GS850 head on GS1000 barrels threw the timing way off, and degreeing the cams was the stuff of nightmares. It seems to have worked out well, but if there's one part of this journey that's sketchy it's cam timing and the camchain tensioner.

I'm still adjusting the carbs but it has a huge amount of torque and sounds great.

I'll post some pictures and put some commentary in as I get a chance.


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​​Here's what I started with.
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[Klob]

Here's some photos of the journey.

 

[Klob]

[No message]

 

[Grimly]

Interesting stuff.
I'd long harboured the notion that a 1000 crank might fit the 850 cases, but didn't have one to try it with. I also suspected there would be clashing areas, and if it were a straightforward parts-swapping upgrade, it would have been done commonly by now.
The necessary relieving hole in the cases and the resultant welded cover are pretty good, though.

 

[Nessism]

I've seen threads where guys put the bigger engine into the 850, but I can't remember any threads like this. Hope it all works out for you in the end. KEEP GOING...

 

[93Bandit]

Very interesting! I like it when someone takes on a challenge that all the forum gurus claim is impossible.

Although I think most forum gurus on here don't mean that it is entirely impossible to do something. Rather I think their point is that sometimes these sort of projects require a special level of engineering capability and tools as well as financial investment to get the job done properly. That's something that is beyond the capability of a lot of average joes working in their garage, and often not worth the cost.