850 jugs on GS750

[TeamDar]

It is still a work in progress. I have started it and it sounds great. It is in a drag bike. I used 29mm smoothbores. .429" lift cams with shim under buckets. With 13:1 compression I use a remote starter. It has a welded crank, pro cut transmission, and a reinforced clutch basket.
The previous 1000cc motor had 75mm pistons that were made for a GS750 with the correct wrist pin size. A home ported head with stock valves, and .395" lift cams. It ran well but it had poor ring seal. The previous owner of the pistons ran stock cams with high compression and detonated them enough that the ring land was partially collapsed and would not let the rings seal.

[7981GS]

So, you are using the early 750/850 cylinder head with the smaller combustion chambers?
I did that on one of my 1000's with over-sized valves to help bump the C/R.

Daniel

[TeamDar]

Yes the early 750 head because that closely resembles the combustion chamber of the KZ1000 engine. Same size valves too and that is why I used KZ aftermarket Wiseco pistons. The latest head that I have yet to install has 1.5mm oversize intake valves and 1mm oversize exhaust valves (37.5 & 31) along with a port job from Babor Performance Motors.

If I wanted more compression on a GS1000 motor I would just mill the head. The stock valves on the GS1000 are 2mm bigger on both sides than the 750/850.

[7981GS]

Custom made 38mm Intake valve and GS1000 30mm exhaust valve in a '81 850 head.

When the cam chain broke @ about 3,000 rpm, I immediately hit the kill switch and pulled in the clutch.
Apparently, the intake and exhaust valves had a dispute as to just who belonged in the chamber at the same time:



For comparison, '81 GS1100GL head:



The camera lens does not show clearly that the 850 combustion chamber is 65 mm in Dia. and the 1100 combustion chamber is 70mm in Dia.:
(fish-eye lens.)





I forgot just how many cc's difference there is between the two.

The valve reliefs in the 1000 Wiseco pistons were cut 4 mm over intake and exhaust and .040 deeper to give a little breathing room with the .392/.392 camshafts.
When the chain breaks, GAME OVER.

Daniel

[TeamDar]

When you install larger valves on these 2 valve heads you must maintain a minimum of .185" distance between the valves when they are on the seats. If necesary you have to sink them deeper. This worked out for me because The KZ oversize valves are shorter than the GS750 valves, and I needed to sink them to get in the shim range.

[7981GS]

The bike had 22,000 miles on that engine when the chain broke.
I added more clearance than what was recommended by Wiseco.

Daniel

[gssuzukixxx]

Just to wrap this up; I got an 850 cylinder block and pistons from a very generous forum member, and new rings and gasket set from Bike Bandit. I had to do the minor chiseling on the inside of the crankcase for the cylinders to go all the way down. I gave the head a bit of an overhaul by lapping the valve and putting in new seals.

Anyway I got it all together on Saturday and it fired right up. It is -11 C here and lots of ice and loose gravel on the road but I took it around the block anyway. Just about froze but I thought I imagined it having more power. Will have to wait till spring now to really test it out.

BTW I used GS850 cams.

Brian

[maxfielding]

850 Jugs on GS750

Hi All,
I have a 1976 GS750 wire wheel and removed the head and cylinders to replace leaking gaskets, hone cylinders and lap valves. I am reassembling at the moment but noticed a mismatch between cylinder liners (72mm OD) and crankcase openings (78mm ID). As the cylinders are dowelled adequately to the crankcase they will position accurately but apart from the usual paper gasket at the joint there are four O ring seals around the liners. The O rings are approx. 2.5mm cross section and fit in their own grooves at the base of the cylinders but with the sizes above will not bear on anything and will do a poor job of preventing oil leaking up outside the liners.

My dilemma is whether to fit them as the shop manual suggests or leave them out in case they break and get into the oilways. The only explanation I can think of is that the crankcase is made to also take GS850 cylinders for commonality of parts between models. Any ideas please?
Cheers, Max

PS. The crankcase is stamped GS750 and the cylinders 748CC so they match and the bore is 65.5mm dia (first oversize).

[Steve]

I doubt that they designed the 750 to accept the 850 upper end, as the 850 did not come out until three years after the 750.

If you are looking for a performance upgrade while you have your engine apart, the pistons and barrels and head from a '79 850 will bolt right on. You can use newer barrels, but will have to either use your stock head or get a set of CV carbs that will match the '80-and-up 850 head.

.

[maxfielding]

I doubt that they designed the 750 to accept the 850 upper end, as the 850 did not come out until three years after the 750.

If you are looking for a performance upgrade while you have your engine apart, the pistons and barrels and head from a '79 850 will bolt right on. You can use newer barrels, but will have to either use your stock head or get a set of CV carbs that will match the '80-and-up 850 head.

.

It's an amazing coincidence the crankcase opening is perfect for the GS850 cylinders don't you agree Steve? The O rings would actually seal then. I will probably find 850 cylinders and pistons after I have closed it up! Thanks for the suggestion.

[maxfielding]

I doubt that they designed the 750 to accept the 850 upper end, as the 850 did not come out until three years after the 750.

If you are looking for a performance upgrade while you have your engine apart, the pistons and barrels and head from a '79 850 will bolt right on. You can use newer barrels, but will have to either use your stock head or get a set of CV carbs that will match the '80-and-up 850 head.

.

I saw a Suzuki GS850 disassembled today Steve and it has the same problem. The O rings are only held by the gasket with the crankcase openings irregularly shaped, and open between the cylinders, They would provide little or no support to the O rings.

I think I will just silicon them in the grooves and hope they stay there. My gasket is quality Japanese after market, not genuine, but will provide no support to the O rings as it is cut to the case opening.
Regards, Max

[Thunderpipes]

Hi,I also have a gs750 1977 with low compression on one cylinder. The valve clearances are good,so I presume that I have a burnt out exhaust valve or broken rings? The thought of installing gs850 barrel & pistons sounds like an exciting upgrade! Good luck and let us know how you get on.