GS 425 Pistons Ruined....Options

[gearhead13]

Have the 850 pistons been compared to the orginal ones? Compression height and skirt length the same?

[GregT]

You would have major cooling problems

Not unless he lives in the desert, he won't - the compression won't go up enough to worry anything and the cylinder walls will still be thick enough not to distort...

[BrianH]

Alright So I'm ready to order the pistons and go ahead and get the cylinder bored and.

Im going to use the GS850 69MM piston and step up the bore to 69MM as well.

The 425 had a 67MM bore and pistons.

Besides ordering the pistons rings and pins. And getting the cylinder bored are there any other factors to look at before dropping the cash.

How about modifying the head gasket? Machine shop does that too?

I want to make sure the engine will run properly before spending the cash.

I saw overheating mentioned, and cylinder skirt and compression height mentioned?

If you guys could elaborate on these ideas it would be great and reassuring.

[GregT]

put the 67 and 69 mm pistons side by side on the same gudgeon pin and check that the height of the top edge of the 2 pistons is the same.
look also to see if the overall height to the top of the crown is the same.

If they're different by a significant amount - and the 69mm is the taller - there wil be some machining required.
Piston skirt length can be compared too - again I'd be surprised if they're significantly different.

I wouldn't worry about enlarging the head gasket - they're usually larger than the nominal bore anyway and so long as the new pistons don't come closer to the top of the bore than about .040in you won't have a problem.

I can see no reason why overheating should be a problem - anyone who thinks otherwise please state specifics....

[Nessism]

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.

[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.