'77 GS750 hybrid question

[Anonymous]

ok, i gots a '77 GS750 that i'm thinkin of makin a hybrid out of with some 850 parts......question #1 is:

Will an '81 GS850 top end fit the 750? I'm thinking it will, but i want to make sure before i tear into the 750. I'm planning on running the pistons, cylinders and head from the 850 on the 750.

Question #2: Would using the 750 head on the 850 cylinders/pistons increase the compression? If so, i plan on having the 750 head ported and polished, just curious if it would make a bit more compression.

Anybody? Everybody?

TIA

[SqDancerLynn1]

I'm not the expert but I don't think it will work. Different crank stroke different piston pin height?? Bolt pattern may not be the same? I would compare the Cyl base gaskets for the 750 & 850 and the compression height of the pistons. The 850 is not a high compression engine, has mild cams. If you try it. Make sure to check piston to head clearence and valve to head clearence. You probably would be better off getting a big boar kit for the 750.

[Anonymous]

The 850 is not a high compression engine, has mild cams.

Sigh.....................I love my 850, but sometimes wish it had more get-up-and-go.

[humhead]

750 Hybrid

The stroke on 750 and 850 engines is the same. The bore is 2mm larger.
I know others have switched to 850 cylinder blocks, but some machining is involved. I just checked the clearances using a '79 850 block and the top half of a '79 750 case. The bottom of the cylinder block hits the sides of the case. Looks like a little case boring is needed. An 850 block can be bored to approx. 870cc. Keeping the 750 head and cams will result in higher compression and better hi rpm performance. An 850 head from 1980 or newer will need the CV carbs that came on the bike. (This is a good thing, they work better.)

Another option is the wiseco piston kit. All you need to do is the 2mm overbore, no other change.
Either way, some machining expertise is needed. Hope this helps

Terry

[Anonymous]

where can you get the weisco kit for the 750?

[humhead]

Wiseco info

wiseco's website is: http://www.wiseco.com If you click on "contact us" You'll see their office phone #. They should be happy to tell you who has them in your area. List price on the kit is ~ $440.00

Terry

[Anonymous]

thanx for all the info

appreciate it guys.....not sure if doing the converion is worth it now, being only 2mm....hmmmmm

thanx

[humhead]

would another 2mm seal the deal?

It appears I was wrong in saying the 850's bore is only 2mm larger than the 750. Looks like it's more like 4mm. 65mm vs 69mm. Anyway try thinking of it as an extra 95cc's. <g>

Terry

[Anonymous]

Thanx Terry

that makes it worth doin

Now all i gotta do is find a good gasket set, new timing chain (may as well replace that while i am there) and whatever else i can think of.

Thanx again!

[Anonymous]

Re: Thanx Terry

Wiseco's catalog lists an 840 cc big-bore piston for the 750 that measures 69 mm.

[AOD]

850 block, 750 head and cams. possible?

sounds good to me.

[KW]

All the info I've been given says that the 850 block will fit straight on - no machining required!

Am I wrong on this? I hope not coz I've got an 850 block and pistons ready to drop onto my 750!

[Anonymous]

Why not bore it out.

Why not put the 850 top end on but bore it out first. Having a 950 wiesco GS750 would be pretty nice. yes? Sounds like a good basis for a sleeper to me.

YOU WANNA PUT THAT CBR TI TLE WHERE YOUR MOUTH IS PUNK?

[Anonymous]

Bored 850

Wiseco doesn't list a big-bore kit for the 850. If they did, I'd buy one in a hot second for my GS850.

Vince

[humhead]

All the info I've been given says that the 850 block will fit straight on - no machining required!

I have the top half of a 2 valve 750 case & a 79 850 block. I tried to fit the block on the case. The block was about 1/2" from seating properly on the case. I flipped the case over & noticed that the 850 cylinder sleeves were hitting the insides of the case. It looked like some minor grinding would solve the problem, but I'm going to get a professional opinion.

Terry