GS750 Opinions: NOS 798cc 10.35:1 big bore kit vs used GS850 pistons/new rings?

[Chuck78]

I'm looking to build 2 performance street engines as an upgrade for 2 friends' 77/78 GS750 2v's. I'm trying to decide what to push these guys toward. Im potentially offering to buy the parts in trade for remodeling labor assistance from them, so budget is a factor. My options are:

-ebay used GS850 OEM pistons and new OEM rings bored into the 750 cylinder same overbore as the popular Wiseco kit ($225-275 parts)

vs.

-NOS 67mm MTC 798cc 10.35:1 forged big bore pistons ($265).

-Wiseco 10.25:1 844cc kit ($390 (($440 w/head gasket actually)) )
Both will require $140+ boring & maybe cylinder & head decking if needed for cleanup or if deck height has the piston too far down in the hole. I'll push for more decking with 850 pistons to get stock 8.7:1 compression above 9.2:1 if there is anything wrong with the surface at all. So with the 798 big bores, I might save some cash from being less inclined to deck the head and block.



What do you all think on this from a performance standpoint? Which one will make better power? Is it just not worth it to tear down a decent running engine to gain 96cc's at stock compression ratio or 50cc's with a huge bump in compression ratio from 8.8 stock to 10.35 & deeper valve cuts for bigger cams?
The Wiseco 844 10.25:1 kit will basically cost only $125 more.

I'm tempted to build one at 844 and one at 798cc high compression and compare the performance. 850 will make more torque surely. The 798 will probably make close to that torque but may do better up high especially with bigger cam capabilities down the road...

[Chuck78]

No comments?
So far 2 votes to 1 the Wiseco 844cc setup wins...

Let me remind you that I'm offering to buy two sets of pistons for 2 of my best buds who also work in the construction industry, in trade for them helpin me do a quickie rehab on a $16,000 house I just bought that sits on an island of land surrounded by alleys on all sides behind my house...

So the cheaper options were the intent. Keep that in mind if you think it's not worth it to bore them out for stock compression +96cc gain or only 50cc more but a big bump in compression.
If you still vote Wiseco, I will take that as a strong opinion of "dude...not even worth it to tear apart a decent running engine for any less gains."


HERE'S A BETTER COST COMPARISON:
Wiseco is $440 but includes head gasket, so figure:
$390 844cc 10.25:1
$265 798cc 10.35:1
$225 844cc 9:1

[Ace07]

I chose the smaller bore and lager compression, because it is cheaper. Also having a higher compression ratio normally makes the engine run more efficiently, as long as you don't go above a 10.5 and have to run non-pump gas.

[Chuck78]

The 798cc will likely run cooler & last longer since it will have thicker cylinder walls for better cooling and more block rigidity....

I'm tempted to build one of each, head milled a good bit with GS850 pistons, & 798cc zero decked with head just cleaned up with a light milling. If one is substantially faster than the other, I might have 1 friend not as pleased with me!!! Haha

[Chuck78]

Well I bid up to $70 on a decent average used set of GS850 pistons, and got outbid, they went for $71! I was contemplating doing one build of decked head GS850 oem pistons, one with the 798cc 10.35:1 pistons...

[tkent02]

Pretty sure I have some 850 pistons around here, not going to use them because I always put 1000 engines in my 750s.

[Chuck78]

Well I'm definitely interested in those then...

[gmansyz]

Why not just get the 850 pistons and the matching cylinder? Hone lightly and use it. Way cheap and not too much compression.
Otherwise, put 1000 sleeves in the 850 cylinder and bore those to 72mm.
G

[Chuck78]

1000 sleeves are too long, they need shortened substantially.

For my bike, I have bored gs850 cylinders to their max limit of +3mm (1.5mm taken off the cyl wall) & am running custom 72mm 920cc pistons.

For my 2 project bikes, I am going to have GS450 or shortened GS1000 sleeves installed in a GS850 & a GS425 cylinder block for 73mm 944cc/502cc pistons.

This thread was meant to be a budget proposal, that is why I wasn't so much considering the Wiseco k844 kit, but rather boring 750 cylinders to a fresh +2mm or +4mm over for 798cc 10.35:1 $250 pistons/rings/pins/clips OR 844cc 9:1 $210 used pistons and pins and new rings and clips.

I'll be likely doing this to two friends' bikes and footing the bill, as work trade incentive to get them to join me in remodeling a $16k house I just bought in the alley behind me.

I hadn't really considered buying 850 cylinders as unless they are from a knowledgeable seller or member here, I could end up buying four or 5 before I ended up with one that was flawless and would clean up as good as new simply with a fresh hone job...

Although Tom has offered up 2 cylinders and sets of 850 pistons to me in trade... I'll have to deck the cylinder heads a fair amount to bump up from the stock 850 compression ratio of 8.7:1 up to above 9:1, and I'd prefer 9.5:1 or greater but that gets into slotting cam sprockets and degreeing the cams... I suppose that would be best anyway. Degree at 105/107, 106/108, or 107/108 for a boost in low end...

[gmansyz]

Well Chuck, it really looks like you got it all solved. I guess only the dollars will dictate what you will do.
Have you thought about the GS1100G cylinder? Is it taller than the 750/850? I cannot remember. If it is the same, could be a better starting point.
Just my thoughts.
G

[Chuck78]

The 1100G may be an option for MY project builds, but probably not the GS425 cylinders, as the rear middle studs are too close to the sleeve and would require cutting a notch in the top of the sleeve, & may have me running out of aluminum in between the cooling fins...

The 850 cylinders should be able to fit 1100 sleeves if I wanted to go 73mm 944cc on my Rickman (gs1100g is 72mm bore x 66mn stroke, gs850 is 69mm bore x 56.4mm stroke).

I'm still torn between 844cc gs850 pistons snd extra head decking for cr increase, vs 798cc MTC pistons.

Basically it comes down to:
*Thicker 750 cylinder walls @ 798cc vs thin 750 or 850 stock jugs (better cooling with thicker walls vs a fresh bore job +4mm over on 750 jugs)
*10.35:1 compression ratio vs slightly over 9:1
*extra deep valve pockets if my buddies ever want to drop in megacycle or Web Cams (not terribly likely)
*more torque with the 844cc gs850 setup

I only threw the Wiseco k844 kit into the mix as a "hold it dude...if it's apart just go for the best of all (except price and cyl wall thickness w/ fresh +4mm overbore) for only $150 more" option if people think it's ridiculous to save $150-200 and skip the potential the Wiseco kit brings

[gmansyz]

Okay, I know nothing about the 425. But have you thought about offset boring the cylinder for the sleeves?
If I remember correctly, the stock sleeves should not be bored any more than 3mm. The 1100 cyl. should go to 75mm. Replacing the sleeves, it would be safe to 78mm.
An idea. Since the bikes share the same bore centers, along with many other similarities, Do you think a 4-valve GS 1100 crank would fit in the old 750 cases? I have put a shafty 1000 crank into 850 cases.
How big would it be with a 78mm bore and the 56.4mm stroke?
G

[gmansyz]

Oh, never skimp on cams. The stock cams are so small. Degreeing them helps but you are holding back the engine and it's potential.
G

[Chuck78]

The 750 sleeves I believe are about 3.9mm thick, & Wiseco says +4mm overbore is acceptable on those. The GS850 sleeves are only about 3.1mm thick, so +3mm overbore (1.5mm off cylinder walls) is the max there, leaving you with 1.6mm cyl walls. Pretty thin but same as what the Wiseco 844 kit leaves the thicker 750 cyl walls at, & no one ever has problems with those things.

The 67mm leaves a lot more cyl wall thickness snd comes with hard chromed rings, so that setup would run slightly cooler and last a very long time, & accept big lift cams if one desired.

[Chuck78]

The 77-79 cams especially the gs400c/gs550/gs750 are actually no slouches at all, 750 cams in a gs1000 gives you a bit of top end hp gains due to the big duration of those early cams. The later cams had slightly more lift but less duration, better for mild street scenarios.