Any update on this?  I'm hoping to get pistons this winter when I rebuild my motor.....

can any one confirm that these pistons will not fit the gs500?
I am extremely interested in buying a set although they went up in price but not if they don't fit

I believe the last I heard wiesco wasnt making these any more but someone found a replacement from another bike that could be used in place.

Quote from: Chanse on August 30, 2009, 04:58:04 AM
I believe the last I heard wiesco wasnt making these any more but someone found a replacement from another bike that could be used in place.

Not correct. Wiseco stopped making the 79mm pistons for V&H years ago (1992 maybe?), which were originally design for the GS450. These pistons are made for the FJ1200.

Apparently Wiseco had changed the dome profile by increasing it between the time I acquired a set and now. Note the previous post by 5thAve reguarding part numbers of the pistons he has that did not fit. Modifications to the piston dome (if there is enough material to be removed) or head clearance would be needed. A thicker head gasket may work, depending on the amount of interference.

Well after doing the research on the pistons and things like this yes I know its old and I myself have not posted in a while. The piston listed from the start of the post is for a GPZ1100  set up for 2 valve which is good.  78mm good. pin size 18mm good. 66.2 stroke.  Bad um I know you can shift stroke like one or maybe 2 mm but the stock stroke for the little pup is 56.6mm and this is almost a 10mm or to put it in terms of inches about 3/8ths that is a lot. That means this piston had a stroke of 66.2mm in mind when created.  Now if the pin location is shifted this would be or could be why it does not fit. We have to be a two valve piston face in the 78mm range bore with a stroke around 55-58mm. I haven't said it won't work but I wouldn't throw the dough out there for that to not work  and at the current list prices of 155 per piston. I would find that maybe if there is enough people intrested in doing a bored over motor then we could get a group buy going but i can't see that with me right now as the econ.  I bookmarked this one in hopes of getting around to it and doing this  one myself figured I would let any of those out there know.  As for why it worked for Dgyver I have no clue.  Generally with manufacturing you never change a part number to a complete new profile.

Hey,

I was just looking through this tread and decided to check out Wiseco myself. Would the 1995 Yamaha GPZ 1100 Pistons work. I randomly looked at them and noticed they have a 58mm stroke as opposed to the 1983's 66.2 mm stroke. Going off what the previous poster said, would this make the difference?

Here is the 1983 Pistons:
http://www.wiseco.com/ProductDetail.aspx?ItemID=4304M07800&ModelID=175&ModelYear=1983&AppID=7880

Here is the 1995 Pistons:
http://www.wiseco.com/ProductDetail.aspx?ItemID=4578M07800&ModelID=175&ModelYear=1995&AppID=7648

The pins also have the same inner and outer diameter.

Jay

Stroke is determined by the crankshaft, not the pistons or the connecting rods. So, the difference in stroke won't matter. There is something that could be a concern; I recall reading in another thread that WISECO changed the shape of their 78mm pistons, increasing the size of the piston crown. From what I recall, there may be clearance issues with the stock GS500 heads. Some investigation would be valuable.

For my part, I'm racing the GS, and have no plans to go larger than 76mm.

FYI: http://gstwins.com/gsboard/index.php?topic=48095.0

Make sure you get the right 78mm pistons...

Also, WISECO part #4578M07800 is wrong for your engine. It's for a ewer GPZ engine, which is 4 valves per cylinder. You're going to need a pair of the older GPZ1100 pistons, for a ~1982 model year bike.

Ahhh ok, I was hoping it was not 4 valve. I was just going off what the previous poster said becasue it sounded like he was thinking that the pin on the pistons designed with a longer stroke may have been placed farther away from the top of the piston thus making the total length of the rod+piston a bit longer(but keeping the stroke the same) so therefore making it hit the head

Jay

I would think that that would also bump up the compression as well though and that would not make to much sense as they are meant to be replacement pistons I think.

Jay

Yes, if the distance between the pin and crown increase, compression will also increase. Pretty much all of the aftermarket pistons will bump up the compression ratio... Which really is fine, given how low the ratio is on the stock GS.

You may have to run premium after installing new pistons, but you'll get a bump in power just from the increased compression ratio.

Just say no they don't work.... 5thAve provide it. And going back and looking at the post again and seeing the pictures my suspect is they have made a drastic change as Wesico in there manufacturing process I did not catch it before but they did change the manufactured number.. though it may have the same part number 4304M07800 Its not the same Forging #'s ? which would indicate the major change.  Yeah it still doesn't exclude the fact that they redesigned the piston. Kind of looks like to me they went to a full CNC billet as the forged one that Dgyver has looks less CNC milled where as the 5thAve could be fully processed on a CNC machine today. Though if someone really wanted to beat them up on it here is  a nice http://www.wiseco.com/PDFs/DealerNewsArticle.pdf read down and see how they dedicate each Forging for an application. That would kind of indicate that you should be able to order the old profile number according to there own Dealer News as of 2009.

I can see how my statement for the stroke of 66.2 could miss lead. But its more the pin height from crown of piston to the center of the piston pin that matters and the rod info I was referencing . Also the quench area is drastically wider examining the pictures from 5thAve vs Dgyvers pistons.

Yes its old and I still would like to see it happen.  

Jay These are in no way meant for our bike. They are not meant to be replacement pistons for our motorcycles. So your statement about bumping compression would take place is a yes a massive bump. So massive they will smash into the Top head according to 5th Ave 5/32 but I Suspect more.  To the statement of Burning1 make sure you get the right 78mm pistons would be correct too. I went through the whole catalog There is no out of the box design that works for our motors.

Again you can't install these piston unless you can verify they are the Dgyvers profile.

Just to keep this thread up to date, I still have the 4304M07800 pistons and I'm still talking back and forth with Wiseco to figure out why they are a different shape of dome than what others before me apparently received. As of right now, I recommend that these pistons are NOT a drop-in fit. Some machining of the piston dome and/or combustion chamber in the head will be required.

If / when I make any progress on this issue I'll post here. 

Be sure to read this:
http://gstwins.com/gsboard/index.php?topic=48095

Some people are mentioning the 1985 FJ1100 pistons. Note that was a 4-valve per cylinder head design. Even though the pistons are the right diameter and wrist pin size (18mm), I doubt very much they would have proper clearance in our 2-valve per cylinder engine. I think the confusion comes because some of the racers on thegsresources.com have used FJ pistons in their EIGHT VALVE GS450 hybrid racebikes. (Yes there were both 2 and 4-valve per cylinder versions of the GS450).

5thAve: I started some initial grinding work on my GS500 head. I'll probably do enough work to fit the stock GS450 pistons. When I do the full race build, I'll probably have JE make me some custom pistons.

I'd love to see photos of your modified head.

Sent from my T-Mobile G2 using Tapatalk 2

Quote from: 5thAve on June 19, 2012, 09:41:16 AM
I'd love to see photos of your modified head.

Sent from my T-Mobile G2 using Tapatalk 2

I'll take some pictures when it's finished, before installing it on the bike.

Found out a few things from my test grindings...

If you're taking off just a small amount of material, a sanding roll works fine. Seems to produce smoother results than using a burr grinder.

Machinists blue die is pretty handy. Might be able to make a pretty accurate guide line by blueig the head, and marking by running a scribe around the bore of the block.

UPDATE!

Dgyver and I started on a 569cc engine for my bike.Here are some specs.

80mm Wiesco pistons and sleeves-We are unsure what bike they came from,but they test fit perfectly.
Copper head gasket
polished and ported head
Mega Cycle Cams with adjustable cam gears
36mm GSXR carbs-36mm Flat Slides are on hold until break in is accomplished.

Bottom end and case checked out so all new bearings and seals have been installed.We buttoned up the bottom end this evening.I did swap out a bad output shaft for a used one in good condition.

Go big or go home!