Has anyone done 69mm Wiseco GS750 10.25:1 844cc pistons in a GS400/425?

[pete]

It all sounds like a load of fun

[Chuck78]

If you wanted to go all out, you could most likely very easily bore the gs400/425 cylinder block to accept GS850 sleeves boeed out to the max at 72mm, and get 72mm pistons made to give you 489cc's. I happen to have a set of such pistons that MTC Engineering used to manufacture for the GS750 and 850, 920cc 10.5:1. Winter project for my '77 GS750B to make it the largest reasonably attainable displacement in a first generation kickstart GS engine.

[pete]

Ok now my head's spinning

Will definitely give this stuff serious thought when the time is right!

[Mekanix]

4 valve head can flow up to 90 cfm with porting and cam's
stock is around 60 IIRC.

The pre 1980 rollers had 16mm pins
after that they're all 18mm.

You have lots of options for base and head gaskets between the gs400-gs500 from fiber to metal in different thickness. 0.010, to 0.043. And then there is cometic that can make anything.

The GS450/GS500 head and pistons are not cross compatible.

The 450 has a half hemisphere head and a piston that fits into that dome.
The GS500 piston would fit into the 450 head but compression might be a little less but that would be made up by the increase in bore size.

The GS450 "type" of piston doesn't fit into the gs500 head when you get up to 74mm +.

The 450 head with a set of 500 cams and 500 pistons and porting seem to be the best combination that I can think of.

If you stick with the type of dome on the 450 and use a big bore kit for the 750 it should work just fine but that piston wouldn't work with the gs500 head.

http://www.thegsresources.com/_forum/showthread.php?p=1832392#post1832392

If you go with the 4 valve design, anything larger than 67mm will require head work to open it up a bit. Otherwise compression goes up really quickly.
It already starts out at 10.5!

A set of stock gs750 pistons are the same part number as the gs400's (post 1980)
The 1100 pistons will not work the way they are. The valve reliefs are too far out and need to be brought in to match the valves again.

The GS500 block is useful for this mod. It starts off at 74mm and the GS1100 big bore kit for the 1177 or 1260 would drop in with minimal boring.

[bogmonster]

Almost done with mine

I have a 77 gs400 that i bord out to accept the Wiseco pistons, cometic made me a head gasket for it. I added some Mikuni vm 32 carbs, fresh valve job and swapped in a electronic ignition from a later year gs425. Just need some final tuning and it will be road test time

[pete]

Be keen to see how that goes!

Any idea of the cc's after boring it out?

[Chuck78]

449cc's & 10.25:1 c.r.

[bogmonster]

Runs strong

got out for a few short shakedown rides, so far impressed with the power. I never rode the bike before the motor work so i cannot comapre to stock performance. I will say a buddy was on his xs 650 ,and he was having trouble keeping up even though i was being easy on it to break in the rings.

Plan on getting it on a dyno this winter for the final tune. More pics to come

[Chuck78]

No but I still plan to in the next year or so - but I accidentally happened upon enough spare parts to build my spare bike as a '77 or '78 gs750 instead of a gs twin. Definitely wanting to build up a stripped down hot rodded gs twin 449cc 10.25:1 compression in the future. It should be no different than a standard rebuild other than tuning & jetting, & I think the 10.25:1 pistons with the 400 heads will yield a little more than 10.25:1 that is advertised with using the 750 head. The 750 advertised 8.8:1 compression, the 400's advertised 9.0:1 stock.

[Chuck78]

Just go for the wiseco's and a custom bore cometic head gasket if a gs425 gasket doesnt work. The extra compression ratio will give you more performance as if you added more cc's

[Chuck78]

yes, the compression ratio will be great with the wiseco's (greater than 10.25:1), and will increase the performance a whole lot over 850 pistons. The 850 piston's will probably yield 8.7:1.

[Chuck78]

back from the archives, I just picked up a spare seized GS 425 engine, as the engine in the 425 has great compression and I don't want to tear it down!

I will probably do a 4 millimeter overbore, same as the Wiseco kit in the GS750 844cc's, assuming that the cylinder wall thickness on the 425 wasstill the same as the cylinder wall thickness in the 400 &750.
using the same pistons that will take an 850 cylinder in a 750 up to 894cc @ 11:1, the GS425 would knock down 475cc & 11.4:1. not sure if this is too high for 91 octane, but I would be running upgraded cams with bigger duration than stock, so it may be completely streetable. I'll have to ask around to some of the more experienced engine builders here. at that compression ratio, I would probably want to fit an oil cooler.

if I end up going 798 cc 11:1 with some new old stock pistons I found, and not custom JE Pistons 894's, then I may look into having rapid ray resleeve this spare 425 cylinder block to go 502cc or 516cc with gs1000, 1100, or 450 sleeves, and the ultimate - Carillo's CP Pistons custom for this bike. I was going to try to get slightly better pricing than $680-ish for 4 custom JE Pistons by ordering 6, same for the Rickman GS750 & the GS425, or 8, or 10... if I find anyone else looking to do the same on a 750, 850, or GS425. that would drop the price per piston down a bit.

Ray - I plan on sending you at least one or two heads this winter for some work! Need to get your phone number again and give you a call sometime early winter to discuss.