750/850 hybrid pistons search

[petrolhead]

I am building a Suzuki GS750E with GS850G cilinder block and head.
The cylinders fit the casings after some persuasion.
I expected to buy some nice forged pistons but all i can find are the 69mm Wiseco´s for the 750 block.
Then I saw a set of 71 mm GS1000 MTC Pistons at SPS. (no affiliations)
It looks the same and the wrist pin should be 18mm for both.
My biggest concern is if the compression ratio and the squish zone would be okay.
I am shure someone has tried fitting 1000 pistons in a 850.
Any good experiences ?

[zed1015]

I'd ask Nick at SPS first as he should know what fits off the shelf.
Regardless of that though if the pistons fit the rods and the liners will bore to take the 71mm pistons the rest can be sorted using block spacers or a skim depending which way the pin to crown height differs, if at all ..
Squish needs to be no less than 1mm.
All relatively basic stuff when putting together a build motor..

[petrolhead]

Thanks Zed,

Just what i needed to know !
The bore is no issue, the liners can easily take 2mm. more. (69mm original)
I will contact Nick when he´s back.

[TeamDar]

77-79 GS75 and all GS850 uses 16mm wrist pins and the GS1000 uses 18mm wrist pins. The 69mm Wiseco pistons should drop into the GS 850 block with just a light hone job.

[petrolhead]

Ah, my mistake; thanks for the warning.
I have 2 cilinder blocks with the first overbore (0.50) so no 69mm Wiseco´s for me...

[Chuck78]

He is absolutely correct, the pins are larger on the 1000.
You need what I have 7 of, old MTC 72 mm GS850 pistons... Mine are not for sale, however! I thought I had bought the last set. Then due to a machine shop screwing up my valve job, I had severe detonation on one or two cylinders and destroyed one piston. Then another guy had come along and after reading of my pistons, contacted MTC and they ended up digging up another set. He held on to them and did nothing with them and then sold them to me after he lost interest...

The best I could do for you is send you the blown up piston if you were interested in having JE Pistons or Carillo Rods / CP Pistons copy it and make you a custom batch.

I'll eventually be building up another 920cc GS750 engine with this other set, and using the best 2 of 3 remaining pistons from the 1st set to build a 489cc GS400 using GS450, 850, or 1000 sleeves in GS425 cylinders and a ported GS425 head...

I f'd up my shift forks in my current GS750 engine slightly, but it runs AWESOME with 850 cylinders pistons and cams, but if it gets harder and harder to downshift into 2nd & 1st (&3rd?), I may throw together the 920cc ported head Yoshimura Stage 1 cams VM29 smoothbore carbs top end... it'll once again likely be the fastest GS750 around..
I already give my Ninja 750 / ZRX1100 / ZRX1200 buddies a real run for it with them trying to catch up to me in the twisties of Appalachian Southeastern Ohio and West Virginia on my old air cooled "dinosaur," and its plenty fast with this setup... Honestly I scratched my head when these guys on newer more advanced and larger displacement bikes shake their head when looking at my bike while commenting on how fast it is...lol! Maybe it's because I'm not afraid to rev it to the moon? I'm also severely addicted to high speed cornering on really twisty pavement in the remote Appalachian hills... Focus & practice goes a long way with that kind of drive

Despite the current setup's pleasing performance, I have the Yosh cams, VM29 smoothbores, ported head, and 920 pistons/cylinder ready to go...just need to get Pearson Racing to weld up a crankshaft for me for extra strength & insurance against blowing it up...and I'll have the ultimate 8V GS750!

[petrolhead]

Hi Chuck,

Thanks for the offer.
I am gonna pass cause i don´t want to go over 71mm.
Maybe i just use my old 750 bank and go for the Wiseco´s.
I have a 850 cv head and vm 28mm. carbs so that should give it some flow.
First thing will be welding the crank and put the cases together with new seals and cam chain.
Sounds great driving around the twisting mountain roads.
in Holland we use the twisting dike roads following the rivers and fields, no mountains in the low lands ;-)

[Chuck78]

You can get Cruizin Image pistons for 1mm GS850 oversize, they are basically a near exact OEM piston copy, 870cc.

Why don't you want to go over 71mm? I successfully ran 72mm in 850 cylinders for years until a bad valve job (car machine shop inexperience) wrecked one cylinder. Thicker walls left on 72mm vs Wiseco 60mm bored into 750 sleeves...
850 +1.0mm pistons = 870cc FYI, nice grunt added.

[petrolhead]

Well, Cruisin image sells a set for 150 $ compared to 440 $ for a original 1mm oversize set.
Not that the original set is available but the huge difference made me suspicious.
There are no long term reviews, it seems to good to be true and cheap is often expensive.
I might be wrong but i don't want to take any chances.

[Chuck78]

Aside from the alleged GS650 740cc big bore pistons, the other pistons that Cruizin Image sells as big bore kits are hugely popular amongst the Honda crowd, with greater reviews for reliability, quality, etc
I have a set of 70 mm GS 850 Pistons from them new in box still, and they appear identical to OEM, which goes along with what a lot of other people across the internet in various makes of vintage Japanese bike forums have said as well.

[Chuck78]

And if you search the Honda CB forums, you will find lots of long-term reviews of those pistons in the CB big bore variety.

This GS650 big bore kit however is a completely different issue, very careless and irresponsible on their part to sell such a thing that won't even fit the head, they probably never tested it or had a head in hand or any data on what the combustion chamber shape or size was.

[Chuck78]

A member named Ian / 49er custom designed a set of 71mm stock compression ratio pistons several years ago & had JE Pistons manufacture them for him. I think he also did a big big bore setup as well, but didn't really end up doing a lot with that engine as he was very pleased with the 890-ish cc (894cc?) incarnation if my memory serves me correctly. I discussed these engines with him at great length. I'm sure you could have JE Pistons, Carillo/CP, or other manufacturers make the same for you if you sent them a mold of the combustion chamber and a stock piston

[petrolhead]

I did not know they made reliable Honda pistons (i never look at their forum )
For me this is a challenge building a block from the ground up.
And this is the first time i am looking for upgraded pistons.
The idea of forged pistons and higher compression seems nice for tuning..
The different carburation, clearences etc will test my knowledge and the server from gsresources
But making moulds of the combustion chamber, sending them to the USA and order custom pistons seems too complicated for my ´project´
If there would be pistons available off the shelf i would go for it.
Since the Wiseco set has good reviews with the 70 block i think i better let go of the 850 cilinder block and focus on the CV 850 head and the 28mm carbs.
But thanks for the input !!!

[Chuck78]

Also, if you wanted 71mm GS850 big bore pistons for 900cc (894cc actually?), if you found 2 other interested buyers with 750's + 850 cylinders, or just a GS850, Wiseco would make you a minimum batch of 12 custom pistons (3 sets) for about the standard price, and you'd be able to use the Multi-Layer Steel (MLS) GS1100G OEM head gasket. They'd be able to pretty easily re-draw their K844 piston fpr larger bore and smaller dome to have these CNC manufactured. They could easily do 72mm as I have as well,for 920cc even.

Just food for thought.
You'd be happy with stock 850 piatons honestly, but biggercams and higher compression of the Wisecos is even better,especially if it's 50cc or 76cc larger displacement!