GS 425 Pistons Ruined....Options

[TeamDar]

The GS400/425 and GS750/850 pistons are all the same height from the wrist pin to the piston top (compression height). The GS850 pistons will work fine in the GS425. They all use the same valves also. I have used GS425 pistons first over (.50mm) in a GS750 simply because a dealer had a full set w/rings that he almost gave me and it worked great.

 

[GregT]

My main concern, other than possible piston differences as GregT references, is the liner thickness. If Suzuki used over bored 400 liners in the 425 there might not be enough thickness to bore them 2mm more. I'd measure the liner thickness on the bottom to be sure you have enough meat before spending a bunch of money on parts.

Agreed measure first - but I'm accustomed to seeing British twins with 3mm thick bore extensions protruding....I expect the Suzuki liners to finish up much thicker than this. Remember there's only 1mm of thickness coming out when it's bored.

 

[BrianH]

I'll go ahead and take some measurements.

How much cylinder material would be considered sufficient for the 1mm bore?

 

[GregT]

If you finish up with liners at least 4mm thick you won't have any problems IMO. When you have it bored ask the guy doing it to give you a good lead - in chamfer at the bottom of the cylinders - a lot of shops forget to do this.

 

[deyo27]

After all this will the carbs need rejeting?

 

[BrianH]

Measured today with some calipers. I've got about 6MM of material to work with. This should be sufficent if im boring out 2MM of material.

Greg- I was curious why you said that they'll only be removing 1MM of material when they do the bore. In order to fit a 69MM piston i'll need two more millimeters of clearance so they would have to bore out 2MM to get from 67 to 69.

"When you have it bored ask the guy doing it to give you a good lead - in chamfer at the bottom of the cylinders - a lot of shops forget to do this. "

When you mention this i'm assuing youre talking about the taper at the bottom of the cylinder?


As far as re-jetting, yeah I figure i'll have to tune around the displacement difference. I'm already going to be running an aftermarket air filter and exhaust so it would be imperative either way.



Thanks a lot for helping me through this.

 

[TeamDar]

You will be removing 1mm from the sleeve thickness. You will remove 1mm from each side of the bore for a total of 2mm.

 

[GregT]

"When you have it bored ask the guy doing it to give you a good lead - in chamfer at the bottom of the cylinders - a lot of shops forget to do this. "

When you mention this i'm assuing youre talking about the taper at the bottom of the cylinder?

Correct - having a good taper here makes reinstalling the barrel much easier.

tuning - you may be surprised at how little you need to change things...

 

[BrianH]

I'm getting the cylinders bored. I've got a set of pistons and pins on the way off a GS750(same piston as an 850 69MM).

What kind of rings should I run on these? The stock size rings or the 1MM larger size?

 

[tkent02]

750 and 850 are not the same diameter.

 

[SqDancerLynn1]

He was talking about a 71mm bore at one point, that's why i mentioned cooling

 

[Nessism]

The 400 and 750 used the same pistons.

 

[BrianH]

Oh crap I'll have to cancel that order then.

 

[droyce]

I have a gs400 and from what I'm getting out of this is, I can bore my 65mm out to a 69mm and slap some gs850 pistons(69mm) in it? and bingo I go from 398cc to 448cc?

What about pins/clips?

 

[Guest]

This may be of some assistance, there is allot to read but there is lots of information as well.

http://www.thegsresources.com/_forum/showthread.php?t=170287

and this

http://www.thegsresources.com/_forum/showthread.php?t=172422http://www.thegsresources.com/_forum/showthread.php?t=172422

 

[droyce]

That's definitely an interesting building you have there Mekanix. In one of those threads it says you can slap any GS4xx-5xx cylinder/head on to a GS4xx base? Wouldn't the difference in stoke cause a problem with this?

And I never saw an answer to my GS850 piston question, did I miss it?

 

[Guest]

Depends on the year. Anything after 1980 400 to 500 have a 56.6mm stroke. (that I've been able to find) In the 1970's they had a 60mm stroke and also had a roller bearing crankshaft. After 80 they had plain bearings.

What year is yours?

Is it the 2 or 4 valve per cylinder model?

 

[droyce]

1977 GS400, 2valves per cylinder 65mm bore, 60mm stroke

 

[Guest]

Ok if you find a piston with a bore that you can comfortably bore to. Like a 67mm or 69mm and still have enough liner left then its just a matter of using a few gaskets to move your cylinder and head higher. Cometic can make you a base gasket in what ever thickness you want.

you need to know what the wrist pin diameter is for yours.

Hopefully its 18mm, there are several engines using the 18mm pin and a few using a 20mm pin.

after that you need to find out what the deck height is and compare it to another piston your interested in.

 

[Guest]

We have our 69mm GS400 heads gaskets done, and will now be offering a 69mm kit for these motors.

Jay