GS 425 Pistons Ruined....Options

[BrianH]

Pulled the head and the cylinder apart to investigate a strange clicking noise coming from the engine. The separation revealed a chipped up and overly worn left piston. There is also some damage to the inside of the cylinder.

I have been searching high and low for a replacement piston. The OEM piston is out of production. The piston off a GS850 is the incorrect diameter. I cant find any aftermarket options either.

What are my options besides waiting until a used one comes around on ebay.

I was thinking of putting a GS450 head/cylinder/piston combo on the GS425 lower end. Or a GS400 head/cylinder/piston.

Any one know the compatibility of either of these options?

Thanks
I want to get this thing back on the road.

 

[cowboyup3371]

Can you not find oversized ones and maybe get the head redone?

 

[Nessism]

I don't think 450 parts will fit since the engine is quite different. The 400 parts should fit though. The cylinder head gasket is the same between the 400 and 425, so I suspect the head is interchangeable between the two engines. The cranks are the same part number.

I'd look for a nice 400 cylinder. I'm selling a pair of NOS 400 pistons and rings so let me know if interested.

Edit: the 425 has a 67mm bore and the 850 uses 69mm. My guess is you can bore out your old cylinder.

The 400 and 750 share pistons, and the 850 pistons can be used on a 750 so that means the piston pin is the same.

Boring out your cylinder won't be cheap, and not sure about the head gasket fit so you might want to measure your old one before spending a bunch of money going down this path.

 

[GregT]

You don't say if your original head has any damage.
If it does not have any other than a mark or so which could be expected, I'd clean up this head, stem seals,valve grind etc - all of which you can do yourself.
850 pistons seem fairly common and boring the cylinders is not a big deal - most car engine reconditioners can do it or check your local yellow pages.
IMO this is the cheapest/easiest way out which will give you a motor you know is good for some time.

 

[BrianH]

The head is in great shape, just had a valve job done on it like 10k miles ago.

I'd prefer the lager displacement. How much would bore job cost?(apx)

Would the crank be able to handle the additional size and weight of the 69mm pistons?

 

[tkent02]

The head is in great shape, just had a valve job done on it like 10k miles ago.

I'd prefer the lager displacement. How much would bore job cost?(apx)

Would the crank be able to handle the additional size and weight of the 69mm pistons?

Boring is fairly cheap. With a good scale and a little time and effort, you could make the 69mm pistons weigh exactly what the 67mm ones weighed.

 

[Nessism]

Every shop is different but bore and hone for a twin from Ape is $145. http://aperaceparts.com/machining.html

 

[7981GS]

You might try a machine shop that is off of the beaten path and/or doesn't do a lot of work this time of year.
I found a boat and marine machine shop that bored/honed and faced the top of the cylinder on a 1000 for $140.00 OTD.
So about $70.00 for the work plus the pistons/gaskets would make the 448cc 850 pistons sound pretty good to me.

Daniel

 

[BrianH]

I plan on using the same machine shop that did my vlavles and head work. I would be ok with paying around $60 per cylinder in order to get the extra displacement.

If I am going to bore the thing out why not just step up the size of the piston to an even larger size?

The gs450 pistons are 71mm I believe. Is there some kind of rule of thumb to how large you can go?

Also is it imperative to weight match the replacement pistons to the stock pistons?

Im excited to do some serious engine modification.

 

[Nessism]

How large you can bore the liner depends on how thick the liner is. The 1000cc engines can be bored out 3mm safely, 3.5mm if longevity is not a primer factor. Since the 400 was already bored out you might have to do some study to figure out how large you can go.

 

[BrianH]

The sleeve looks like its maybe 8 or 9 mm thick. I wouldn't be worried about boring 2mm deeper to get the 69mm piston. Larger than that would be pushing it.

It's tempting to go big, but i'd prefer to not have the cylinder fail while i'm on the bike.

How crucial is the piston weight matching?

I'll try to get some pics of the piston carnage tomorrow.

 

[Nessism]

It's the cylinder thickness on the bottom of the sleeve that's critical, not on top.

I wouldn't worry about piston weight. There may be a small about of extra vibration but not enough to justify grinding on the pistons.

 

[tkent02]

I wouldn't worry about piston weight. There may be a small about of extra vibration but not enough to justify grinding on the pistons.

From a perfectionist like yourself, that surprises me. A few minutes with a hand file is all it would take.

 

[GregT]

It'd take a bit more than that to match the piston weights...

i wouldn't even worry - the twins have a balance shaft which removes most of the 180 deg crank tingles, and all twin balance is a compromise anyway.

i've never been sure quite why Suzuki went to the forged plain bearing crank for the 450 - looking at how the fours last I doubt if strength was a factor - more likely the one piece crank is cheaper to make.
Anyhow I'd think the roller crank is strong enough to accept any overbore which could be done.

 

[SqDancerLynn1]

You would have major cooling problems

 

[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.