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Wiseco high comp piston questions. 1981 GS850G.

  • Thread starter Thread starter Colt45
  • Start date Start date
C

Colt45

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Up for discussion I want more power out of my 850. I've been trying to find a set of high compression pistons for my 850 and there are none out there. But I did find high compression for a 79 750 2 valve. The bore is under 1 mm larger. Which I believe would be perfect to send my jugs out to get bored out.


Second option would be can you fit 1000 jugs between a 850 head and crankcase? There's Lot more options for a 1000. And I happen to have a 1000 sitting as a parts bike.
 
Is your 1000 an E or a G? If it's a G, just do an engine swap.

Just swapping jugs is not easy. Yes, the spacing is correct, but you will need custom pistons, as the wrist pins are different diameters.

Unfortunately, the 850 was not built for speed. It was designed as a very adequate work horse, a role it fills very nicely. :encouragement:

.
 
the 77-79 750 pistons are 844cc 69mm - exact same as the stock 850, but much nicer build and higher compression.

You can bolt in a GS1100G engine to that and make it a frighteningly fast beast as well. Lots of piston options for that! You can go 1134cc I believe with a kit from APE intended for the 1100G. I happen to know where there is an 1100G engine or two sitting (besides mine, I may likely fit the top end to a GS1000 chain drive). I know of a guy locally who travels around the US buying bikes and parts, and may even be able to deliver an engine to you if his routes worked themselves out your way. Of you could have an engine strapped to a pallet and shipped via DHL or other carriers, or find your own shipping via youship.com or is it u-ship.com or something?


Wiseco pistons into good 850 cylinder bores with Megacycle cams will be the best off the shelf you can do for an 850. Custom pistons up to 920cc can be made to order if you talk to RapidRay on here. He can get you some real nice CP's or JE Pistons. a cammed 920cc is a great street engine... I can attest first hand!
 
1000 parts will not bolt to an 850, either. cam tunnel and cam chain areas are different.
 
I found a set of 69mm wiseco pistons for a great price. Don't want to do an engine swap yet. Maybe in the near future. I just want to make sure the pistons will work with my case and head. Like wrist pin placement not to low or high. And will I have to get the jugs bored or just clean them up with a cylinder hone and slap them in. And cams will be something to do this winder when I have the time to rebuild the head.

I have a 1980 GS1000G as a parts bike. I just don't want to swap it. There's a lot of guys who built 1000's and 1100's. I want a 850 that hauls ass. And my 850 has turned a lot of heads. I want it to turn more.
 
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The Wiseco gs750 pistons are the exact same diameter as 850 stock bore pistons, 69mm. No boring required. A glaze breaker hone job will be necessary however.and measuring the bores to make sure they are perfectly good and useable.

When you have the head off, swap valve springs and do a diy valve lapping job. Put in valve springs recommended by megacycle cams so you can run their .380" lift cams. this will haul @$$ as long as your stock bores are tight enough for the wiseco pistons. If not, you could find a used 750 cylinder and bore it out to 69mm for a very fresh clean and lightweight cylinder!

The 1100G swap would be the best bang for the buck& cheapest
 
i found a set of 480 springs that i was thinking about running but will it be to much for the stock valves? i ordered the piston kit this morning. i got it for an unbelievable price on amazon. so if they will not work ill be putting them up for sale some were on here. itll probably be a little while until i start the build.. and with them being the same bore i wont have a stroke issue with hitting the valves revving it to 9500-10500 rpms? and will it be a true 10.25.1 compression ratio?
 
If you mill the block deck surface down approximately .020" or 0.5mm (wherever it takes to get your quench height / squish band to around 1mm or .040" i.e. compressed head gasket thickness + piston depth down in cylinder at top dead center, or minus piston height at TDC sticking out above deck surface if thicker head gasket is used or block decked significantly), you will be pretty close to 10.25.

Those pistons are designed for that head and that engine, same as 750 head, stroke, rod. No issues other than you won't have the tight clearances Wiseco calls for since it is a used cylinder not being over bored. Wiseco probably calls for a 0.0025" piston to wall clearance.
 
Colt45 said:
i found a set of 480 springs that i was thinking about running but will it be to much for the stock valves? i ordered the piston kit this morning. i got it for an unbelievable price on amazon. so if they will not work ill be putting them up for sale some were on here. itll probably be a little while until i start the build.. and with them being the same bore i wont have a stroke issue with hitting the valves revving it to 9500-10500 rpms? and will it be a true 10.25.1 compression ratio?
Click to expand...
Some times the unbelievable price is for a single piston with wrist pin and rings. I hope that's not what you got. Dar
 
I hope so as well. The shipping weight is the same for multiple sites so we'll have to see. As far a valve springs go can I use 1980gs1000g performance valve springs? I'm having a hard time finding 850 valve springs.

thanks.
 
Looks like I got pretty lucky. The kit came in today and it's all the right stuff. Thanks to amazon I got the whole kit for under 130.00 with 2 day shipping. Now to figure out valve springs😫
 
Got lucky. Not a single piston.
 

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It was the last one. The next one is 420 plus shipping.
 

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Where should I get valve springs? And how much do they cost? And what size?
 
SpecialK said:
I can't comment on that one but I am skeptical because he the item weight is 0.7 pounds (11 ounces) whereas the 844cc kit is 2.3lbs
Click to expand...

pic on Amazon is of just 1 piston, vs 4 pistons on the GS750 kit pic.
You are probably right.
Price of 4 GS750 pistons is amazing, hope i get lucky too :D
 
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