GS 750 Crankshaft bearings matching

[Nicolas1982]

Hi guys,

I'm rebuilding 1981 gs 750 16 valve engine, and I'd like to ensure, that i'm doing it properly
Original motor was wasted as You can on the picture bellow. Crankshaft was damaged and bearing pins went straight to garbage.


I manage to get another engine, with a nice crank, pistons, rods etc. and decided to swap all the "insides" and good stuff into my old crankcase (engine numbers must match the frame numbers). According to service manual:

I have to check letters on the "old" crankcase and compare it with the letters on the "new" crankshaft.
Here is the crankcase:


And here's the crankshaft:


SO, crankcase is B B B B B B, while the crankshaft is B B B B B O(???). It seems i have to use "brown" crankshaft bearings 12229 45400 030 all the way, but what's with the "O" at the end of crank?? Also "brown" bearing? What are Your thoughts? Maybe check it with the plastigauge??

Another thing is the rod bearings issue. They seem to be in quite good shape, and they were orignally matched with the crankshaft shown earlier, so no problems at all. Anyway, should i replace them with the new ones,or leave it as it is?
Here they are:








Thanks a lot for any tips or advices.

Nicolas

 

[Guest]

I have no idea what the difference between the crank case and crank number would be. Since the rods are off, change all the bearings and Plastigauge everything. Hmmm, if the rods came off the crank you showed first how are the bearings in good shape compared to what the crank journals look like? Anyways, as being a trained car mechanic and a self trained bike one now lol, never ever reuse used friction bearings, just asking for a heart ache of trouble down the road. During reassembly make sure you use lots and lots of assembly lube.

 

[Nessism]

I have no problem reusing the old bearings as long as they plastigauge out in-spec. The bearings don't actually touch anything when the engine is running, so no problem with them being "used" as long as the clearance is good.

Those "B" and "O" marks are obviously size classifications. Sorry, but I'm not familiar with how Suzuki graded the fit, but regardless of whatever you decide, I'd check the fit using plastigauge before buttoning up the engine.

Lastly, the second generation 750 engines had oil system problems. Shimming the oil pressure relief spring was promoted by Suzuki as a fix so you might want to research this countermeasaure before buttoning up your engine.

Good luck

 

[Nicolas1982]

Tomorrow i'm gonna plastigauge everything. I got few "used" sets of bearings, but i think they are useless regarding to new-old crankcase. As far as i'm concerned crankcase/crankshaft bearing selection is crucial for this kind of job. The question is, how to check it?
Let's say i'm gonna use original bearing set and crankshaft. It worked just fine in original crankcase (A A A A A A check the pic below) replaced now by B B B B B B (shiny and cleaned one shown before).
Service limit for the width of compressed plastigauge is 0,080mm (0,0031 in).
If the clearence exceeded the limit i should replace bearing according to this spec:

Green one - thickness 1,484 - 1,488mm (0,0584-0,0586in)
Black one - thickness 1,488 - 1,492mm (0,0586-0,0587in)
Brown one - thickness 1,492 - 1,496mm (0,0587-0,0589in)
Yellow one - thickness 1,496 - 1,500mm (0,0589-0,0591in)

And it's all perfect when You know exactly the original bearing set up. Let's say i was on Green one - width is more than 0,080mm i switch for the Black one, correct??

BTW it's quite hard to check the bearing color while it's rusty and 30 years of age...

mrbill5491 and Nessism thanks a lot for the tips. It seems You both represent different schools of motor assembly One says - leave the bearings, the other one - replace them One thing common though - plastigauge, and I'm gonna go for it.
Let's not forget it's an old bike,and it's not so easy to get every bearing size. Even on ebay etc. they're quite rare.
What's also quite interesting - there's a lot of projects and rebuilds of this bike,and noone did it before? (or at least i can't find it).

Anyway, just for the record, markings on the old crankase:

 

[bbjumper]

Good Luck Nicolas,

Personally I'm with Mr. Bill on this one, the photos of your rod bearings are showing metal to metal contact already. The bearing material is VERY soft, you can actually scratch it with your finger nail. Even under load and at high rpm there should always be a thin film of oil protecting the surface material. when you have metal to metal contact you end up moving material around which tightens up the clearance on the bearing surface and makes more contact almost a certainty and eventual bearing failure, not good.

With all the effort and cash your investing in this one I wouldn't want to have to go back in there later to fix it again in short order.

Just my opinion.

Cheers

 

[rapidray]

Those rod bearings are NOT good anymore. Some might think so but THAT motor has a HISTORY of eating rod bearings (I KNOW this as I used to race them!) & I couldn't be PAID to re-use those. Ray.

 

[Nessism]

Boulevard Suzuki lists the various rod bearings as available for $10.46 each. For that price I'd replace them as suggested by the others. http://www.boulevardsuzuki.com/fich...y=Motorcycles&make=SUZUKI&year=1980&fveh=2136

 

[gearhead13]

I rebuilt my '82 750, new rod bearings, and an 816cc wiseco kit. It had 60k km on it at that time.
The mains were like new, so I left them. I was able to find out the correct rod bearings I needed by the codes and was able to barely make out the color of the old bearings. I was able to get the rod bearings for it, but I think they are getting scarce. I plastic gauged all the rods before assembly to make sure.
I never shimmed the pump relief valve and beat the crap out of it and never had a problem
It ran one summer at between 7-9k rpm for several hundred miles with the oil temp gauge pegged
I also buried the tach a few times on it.
I did do oil changes often, used dino bike oil.

 

[gearhead13]

Here is my thread on the build, sadly I lost most all the pictures
http://www.thegsresources.com/_forum/showthread.php?t=135313

 

[john82q]

So to try and answer Nicolas question.

So to try and answer Nicolas question.

"how to pick the correct bearing shell, when changing the cases?"

So the crankcase your going to use is B and the crank is B (I think you might find the O is meant to be a B regardless your going to check the clearance, arnt you)

So from the table in the service manual you need to buy "Brown" bearings, put them in and plastigauge them, if the resulting clearence is out of spec, then you have little choice but to buy another set.

Suck it and see does indeed suck, when things are hard to find, but I cant see any other way forward, as measuring worn bearings is meaningless. Therorectically the crank wont have worn much if any, but it might have.


if you want it to last dont try and reuse the bearings, either cranks or big ends , you need new.

 

[Nicolas1982]

Thanks again guys,

I was hoping to plastigauge everything yesterday, but my local shop doesn't have one on the stock, so I have to wait till tuesday. Motor is in pieces anyway, so i decided (it's the majority opinion anyway ) to put new crank and rod bearings. In the meantime I'll check crankshaft runout to be absolutely sure that the whole work makes sense.
What sucks really bad (and john82q already said it) is the fact, that i have to buy new "brown" bearings, which are not so easy to get, plastigauge them and pray for right results. If the measurements won't be right I'm gonna lost like 150-200 bucks (shipping to europe etc.) + cost of new - this time "yellow" bearings, so fingers crossed Who said buying 30 year old bike, can cause any problems at all,right?

I'll inform You about the progress. Thanks a lot people!!