750B Wiseco BB kit questions

[dardoonk]

I have decided not to hone my existing cyliners and use the stock pistons, but am going to spend a few bucks on a Wiseco BB kit.

The problem I am having is deciding what exactly I get with the Wiseco. My stock bore is 65mm & the Wiseco kit says it is 2mm over, bu also lists the piston as 69mm. Obviously this doesn't add up. I have no problem using a 69mm piston, but will require my machine work to leave only a 2mm wall on the sleeves. Will the engine hold up if I machine the sleeves to ~2.00 thickness?

Stock compression ratio is supposedly 8.7:1 and the Wiseco kit lists the CR at 10.25:1. A big jump in compression, but how are they acheiving this? I don't think you can get that big of a jump from thinner gaskets, and the pistons I have are already domed (I assume they are stock).

[Tarbash 27]

yes it will hold up, thats basically what a gs850 is, bored over gs750.

[brveagle]

yes it will hold up, thats basically what a gs850 is, bored over gs750.

Not nessecarily... (sp?)

With that thin of a sleeve diameter you might run into problems. Also, the increased compression will also make for a hotter engine, increasing the chance of overheating. Plus if you mill the head any, you'll be running even hotter.

That being said, a machinst friend of mine said that the mininum thickness he would bore my cylinder's out, is 2mm. In my personal opinion, you'll be okay.. But i'd plan on adding an oil cooler and maybe switching to synthetic. Good luck

[Footy.]

The 69mm bore size is right for the 850 conversion, but i think it works out at something like "840"cc in reality.

You will be OK boring the 750 out to 69mm, i used to do these alot in the early 80's to the GS750 using wiseco or Yoshimura pistons. Many people used to do this conversion for racing GS750's in those days with no reliability issues.

There are a few points to mention.

I have just got hold of some STD GS850 pistons, and i am converting my own 78 GS750 over the winter.

I decided on the STD suzuki pistons for two reasons. First, i don't want the higher compression ratio on my otherwise "standard" bike. Unless you intend tuning (hot cams, exhaust etc), i don't see the point. Also modern pump fuels are not that reliable on detonation resistence, particularly if you run these "old" air cooled engines in hot climates.

Secondly i seem to recall the Wiseco pistons are forged alloy, the higher compression is simply because of a "higher" dome. Forged pistons usually run at a higher clearance than the standard cast high silicon content pistons. This greater clearance means a bit more noise.

It's been said here before that you can fit the 850 barrels to a 750, but you often have to mod the crankcase in places so the liners will slot in. This means the 850 liners must be slightly larger on the OD.

As i said, i have not had any reliability issues boring out these 750's to 850's, but the thinner liner tends to "ring" a bit more, so they are noisier. But this didn't matter for racing.

If you do have your cyl bored out, it needs to be done by someone with experience as you can get distortion problems.

Before boring, the "used" surface of the bores has tended to work harden the cylinder liner, and the hot/cold cycles in use build up stresses in the liner. When this is then bored, the hardend layer is removed and the metal can "relax" and distort over a couple of hours.

This can be a problem where you have a thin unsupported liner section, as the GS's have sticking out under the barrel.

So you can find that after you have bored these out, you can measure the bores after a few hours and find ovality in the unsupported part of the liner. I have measured as much as four thou ( 0.1mm) ovality because of this.

Not having a "very" sharp cutter, particularly for the final cut can also magnify distortion.

I have a spare 750 cylinder that i have already bored out to 68.5mm. I am leaving this now to "normalise" for a while. So when i bore the last half mm out and hone it, i can be sure its dead round.

The problem with a shop doing the job for you is they want to do it all at once. So you should find a shop that is aware of the potential problems with thin bike liners like ours.

Footy.

[Tarbash 27]

the actual size of an 850 is 844cc. the wiseco kit says 844cc for the bb 750.

[dardoonk]

Footy & everyone, thanks, great info.

How long does the shop need to wait before the final bore? A day or so, or do they need to wait a couple of weeks?

[Footy.]

Footy & everyone, thanks, great info.

How long does the shop need to wait before the final bore? A day or so, or do they need to wait a couple of weeks?

It should be OK after a day, this kind of distortion usually shows up after an hour or so. But this is longer than it takes to bore one cylinder 3mm over, so i usually leave the final cut for a couple of hours at least on this type of thin liner.

The average shop with an all purpose boring stand and "Minibar" set up, would probably not want to do it this way because it would mean re-mounting each cylinder again. Thats like doing the job twice, and they are likely to charge more.

The other quicker way would be to bore out each cylinder fully (with a very sharp tool), but leave at least three thou (0.075mm) in for honing to final size.

This lets you do all the boring at once, then the honing would remove any slight ovality after its been left for a while.

I think most good bike cylinder boring shops would probably do it this way, and you should ask for recommendations in your area.

Good luck.

Footy.

[dardoonk]

The machine shop I'll use does a lot of bikes here, so hopefully they're familiar with this issue.

BB kit says it comes with a graphite head gasket. Does anyone know about them? Should I buy a better one? I am not doing this twice for no reason, so I'd prefer to "spend now - save later".

thanks

[Nessism]

Just curious but what do you have to do to get the motor to tolerate the bump in compression from 8.7:1 to 10.25:1? I used to own an 1981 Kawasaki Gpz550 and that darn thing would detonate all the time regardless of what grade of fuel was used. I'm a little leary of high compression on these old motors.

[SqDancerLynn1]

Make sure to add an oil cooler if using the Wisco kit. PREMIUM fuel will be required, run it a tad rich. You may even need to modify the advance curve to slow it down

[dardoonk]

I was figuring in a oil cooler & premium fuel, but hadn't considered changes to the timing that may be needed.

I'll defer that question to the experts on here. Any takers?

[Tarbash 27]

Have you considered just putting stock GS850 pistons in and keep the same compression? Or do you want a higher compression?

[dardoonk]

That's a consideration if I don't work out the loose ends on the Wiseco kit.

[49er]

850 barrells are 75mm OD. STD pistons are 69mm leaving a 3mm wall thickness. I have bored mine to 71mm to maintain 2mm wall thickness. If your oversize requires you to go under this figure you should re-sleeve as well.
If you decide to fit your Wiseco pistons, do as Lynn suggests and also consider fitting spark plugs that are one range colder than standard. Your tuning will need to be spot on, if you run colder plugs.
Ask around, as many members run road bikes with Wiseco pistons at around 10.25-1. See if they have any major hassles apart from fuel quality.

[Pete's GS]

The higher compression won't be a problem as long as you run Premium (91 and up octane). The head gasket is a good piece. I think Wiseco uses Cometic. You'll need a new bottom gasket, though, so you might as well get a complete set and do them all. Bigger cams will probably help the engine digest that higher compression, but may not be necessary. If you go for the cams from Megacycle, go for new valve springs while you're at it. I suspect you'll have to re-jet your carbs, but ask someone else on the site for a suggested jet size and needle setting for your application. It was said the old GS750 was dead in a roll-on, with no midrange, but bumping it up should fill out your midrange nicely. Have fun!