Will GS850 Head Fit my 78-GS750?

[larrysarg]

I'm looking for a little more performance. I assume the valves are bigger in the 850?

I'm also going to install a Wiseco 844cc piston kit.

Any thoughts?

[dardoonk]

Why not get a 850 block and oversize pistons too? It would allow you to maintain some thickness in the cylinder sleeves.

Not sure if any mild machining is required to fit the block though.

Pretty sure the 850 valves were the same as the 750 valves though

[TheCafeKid]

I think i read from BWringer that you have to do some shaving on the 750 bottom end to get the 850 top end to work, but its not that involved.

[Tarbash 27]

77-79 750's and 850's use the same exact head. 80-81 gs850 I believe do have bigger intake ports. But Im not 100% sure. Could somebody with a 80-81 gs850 measure there intake ports.

[bwringer]

77-79 750's and 850's use the same exact head. 80-81 gs850 I believe do have bigger intake ports. But Im not 100% sure. Could somebody with a 80-81 gs850 measure there intake ports.

80+ GS850 cylinder heads are different (different and bigger intake ports) than the 1979 -- they use the CV carbs instead of the earlier VM carbs.

Therefore, I believe what you need is a 1979 GS850 cylinder head. I've heard many times that this swap can be done, but I don't personally know anyone who has done this or if there were any problems.

I'd suggest spending some quality time with a microfiche site comparing part numbers. I'm pretty sure the head gaskets are the same, so I think this swap would work fine. I don't know if the camshafts are different, or what effect this might have.

[tkent02]

Early 750 and 850 heads are the same, and the early cylinders will fit into the 750 cases with little or no persuasion. The later cylinders can be made to fit, takes grinding out the cases a bit where the sleeves fit into them. This is from research I have been doing, have not taken my 750 apart yet, I may be mistaken...
We are trying to do a group buy on JE pistons, will give 894cc and 10.5-1 compression. This will work on any 850 and any two valve 750 with 850 cylinders. We need to get about three or four four more people into it to get a better price. JE make very good pistons, quality product.
Here is some info...

http://www.thegsresources.com/_forum/showthread.php?t=119385

http://www.thegsresources.com/_forum/showthread.php?t=119089

His torque and power numbers should be the same on a 750, except more of the power should get to the wheel, and the 750 has less power to begin with, so the improvement should be enormous.

Trying to get this thing going, anyone interested contact 49er or myself, it is his piston design as far as bore and shape, I will ship things to you guys in the states cheaper, since he's in New Zealand.

[SqDancerLynn1]

Use a complete 850 top end, Use the 750 cam sprockets.
I could be wrong ? Just using the 850 head will result in lower compression ??????? The 850 chamber is bigger for the bigger bore..

[tkent02]

Use a complete 850 top end, Use the 750 cam sprockets.
I could be wrong ? Just using the 850 head will result in lower compression ??????? The 850 chamber is bigger for the bigger bore..

Don't think so, should be the same head, the part numbers match. I know the sprockets are the same, not sure about the actual cam itself.
If it's the later head these things all change.

[49er]

Use a complete 850 top end, Use the 750 cam sprockets.
I could be wrong ? Just using the 850 head will result in lower compression ??????? The 850 chamber is bigger for the bigger bore..

Anyone planning to mod their 750 2 valve engines with my pistons
need to consider the following.

1. My pistons are 6mm O/S from stock 750 ones. This means that you will have no steel liners left to bore if you retain the stock 750 barrells.
2. I doubt that the 750 & 850 heads have identical chamber volumes. The 850 has 4mm larger diameter pistons than the 750. I suggest that mods be made by fitting 850 barrells and head to your 750 cases.
3. If the head chambers are identical, 750 stock pistons must have a greater squish band width to achieve the stock CR quoted by Suzuki.
4. A stock Vesrah 850 gasket gives 0.5mm clearance around the top of the bore with my 71mm pistons.
5. Camshaft profiles could be different. The 850 is reputed to have softer cams(timing wise) than the 750 has. You have a decision to make, sticking with the 750 cams, or changing to the 850 ones. I can't confirm the difference, only going on what I've read elsewhere!
6. My calcs for the 10.5-1 CR and the associated valve clearances are based on the stock 850 squish area, vesrah head gasket thickness/bores and the stock chamber volume in the 850 head. Valve pocket depths mirror the positions and depths on a stock 850 piston.

If there is sufficient interest, I will also keep a small stock of pistons here for any Kiwi and Aussie members wishing to do this mod.

[bowzer]

Sounds very interesting. If the valve size and combustion chamber are the same in both 750 and 850 (as almost everybody states) then all I would need is 850 jugs and "your" piston set-up. If they are not the same then I may not achieve a 10.5-1 CR because the squish area would differ. Is this correct? What is the projected cost of the piston kit?

[hampshirehog]

850 cams are 'softer' but the 750 cams swap straight in.

[49er]

Sounds very interesting. If the valve size and combustion chamber are the same in both 750 and 850 (as almost everybody states) then all I would need is 850 jugs and "your" piston set-up. If they are not the same then I may not achieve a 10.5-1 CR because the squish area would differ. Is this correct? What is the projected cost of the piston kit?

Yes, you are right. If the 750 chamber is slighly smaller, you will end up with a higher CR than 10.5-1, unless you were to fit a thicker head gasket or 2 base gaskets.

The present interest has been insufficient to enable me to buy the minimum 6 sets required to gain a significant discounted saving for all. As a result, I am still prepared to allow individuals access to these pistons. It will just cost more
PM me for details if you wish to proceed.
Ian