I've had a sneaking suspicion for a while, and tonight I've proved myself right. (Damn).
Those of you who spend too much time here already know that I'm following the lead of a few other folks on this board and putting 78mm Wiseco pistons into my bike. This needs the cylinders bored out, but I've been told it needs nothing else...
...except, it didn't work for me. :dunno_white:
I eyeballed the Wiseco pistons and it's easy to see how much taller the crown / dome of the piston is, compared to stock. Also the dome extends farther toward the edge of the piston than does stock. Let's call this "squish area" for convenience. The stock piston has a wider squish band around the top and the Wiseco is quite narrow. When I held the stock piston flat against the underside of the head, the stock piston dome fit nicely into the compression pocket / dome in the head. Yeah, I know, I'm ignoring the valves for the moment. However, when I hold the Wiseco against the bottom of the head, it does not snug into the compression pocket. The crown or dome of the piston is wider than the relief in the head, so the piston rests against the head partway along the angled dome of the piston, and the 'squish' band of the piston is at least an 1/8" below the head when I free fit like this.
Okay, so fast forward. I put the pistons on the conrods and seat the cylinders, then place the head on top and (without bothering to fit cams or operate valves) I rotate the crank to see if the pistons contact the head. Oh man. Do they contact the head! At TDC, they want to lift the head right off the cylinder by at least 5/32".
Now call me crazy, but several members here told me they're running this piston with no other modifications to the head. I've heard one report of a problem after the head was decked, minor interference that was solved by adding a 2nd base gasket. No way another base gasket will lift my head 5/32" though!
So, my first thought is I made a mistake and ordered the wrong pistons. Did I? I got these:
From the Wiseco catalog, I ordered the 78mm pistons that fit: KZ1100, GPZ1100, GPZX1100, ZX1100 1983-84 (18mm pin)
Piston: 4304M07800 Rings: 3071XG Piston Pins: S555 Retainer Clips: CW18
Options:
a) If I find I've got the wrong pistons in there, sell them to some unsuspecting fool and buy the right ones.
b) Investigate removing a WHOLE LOT of metal from the combustion chamber in the head. I have ZERO experience with this.
c) Get Buddha to fix it.
d) Pack bags and slip out the back door before my wife finds out how much money I just sunk into this project, and no, honey it's not ready to ride yet. Can you drive me to work in the morning?
Seriously though, anyone have helpful advice for me here? Where did I go wrong and what do I do next?
I don't have good pictures of the interference. I'll disassemble later in the week and try to illustrate. I'll put some pictures of the piston here though.
You can get an idea of the width of the stock squish area here:
(http://i665.photobucket.com/albums/vv13/arosberg/Engine%20disassembly/6debc20d.jpg)
And the corresponding wide squish band on the piston here:
(http://i665.photobucket.com/albums/vv13/arosberg/Engine%20disassembly/8df3a7a4.jpg)
In reality, the skirt on the Wiseco is shorter, making it look short sitting on the table like this. The distance from the wrist pin to the flat top of the piston is about the same on both pistons, then the dome of the piston rises above that. The dome of the Wiseco is wider, and taller. That's why it interferes with my cylinder head.
(http://i665.photobucket.com/albums/vv13/arosberg/Engine%20disassembly/64ea9418.jpg)
(http://i665.photobucket.com/albums/vv13/arosberg/Engine%20disassembly/126caee8.jpg)
I think the piston 'flat' will be almost level with the deck once I torque everything down. Maybe it'll be 1/32" below? I don't know how to measure it anyway.
(http://i665.photobucket.com/albums/vv13/arosberg/Engine%20disassembly/DSC_6014.jpg)
(http://i665.photobucket.com/albums/vv13/arosberg/Engine%20disassembly/DSC_6013.jpg)
(http://i665.photobucket.com/albums/vv13/arosberg/Engine%20disassembly/DSC_6015.jpg)
They must have changed the dome profile. The 78mm pistons I have look the one below:
(http://img.photobucket.com/albums/v64/dgyver/GSmods/GSpistons02.jpg)
Yikes. Hmm. I guess they did! The dome on mine is much wider, as you can see. Do you have experience with reprofiling the combustion chamber on a GS? Know anyone that has done it? I've written to Wiseco to get them to explain the difference in the 2nd set of numbers printed on the piston. Your old post says your pistons are labled '4304PS' and mine read '1638F' so I want to know if that is related to the change of shape. Still, I then have to figure out what to do about it!
Does anyone have experience with removing material from the head / reshaping the combustion chamber? :dunno_white:
I wouldnt remove material from the head myself, Id pay a shop. The only thing I would do myself is polish the head, and thats because your not removing much metal, When you port, polish, or remover material from the head you start to alter the way the head flows, and if you do each one just a little bit different you may end up making one flow better and the other worse. I mean thats almost like leaving a stock head on one side of the engine and a performance head on the other side, but not quite. one sides gonna work harder. Not good.
So life intervened for a long while, but I want to tackle this problem again. I'm asking for ideas about where to find experts who can help me learn how to carve some bigger combustion chambers in the head and/or machine down the dome on these pistons. Anyone know an expert? By the way, I'm in Ottawa, Canada if anyone is local. What's needed is an idea of whether there is enough metal in the head that we can remove some. Like, how much is too much?
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I would try posting in the racing part those guy might know something about it, sorry i cant be more help.
i used to race 2 strokes years ago and you couldnt get pistons with the right piston head /head squish clearance so what i did was weld up the head ((i know you cant do much as this is a 4 stroke))and to get the right head profile to machine the head shape i used one of those profile tools ,they look like a flat steel comb with the teeth all moved together .Push it against your piston then match the oppisite side against the head and you will see where you can remove metal on the piston.Other than that you can put a thin layer of plasticne over the piston and turn engine up till it contacts the head this will show you where metal has to be removed ,may have to do it a couple of times.Make sure you give at least .060"clearance between head and piston when you finaly assemble your engine incase you slip out of gear at high revs
Quote from: twinrat on May 02, 2012, 12:56:35 AM
i used one of those profile tools ,they look like a flat steel comb with the teeth all moved together .Push it against your piston then match the oppisite side against the head and you will see where you can remove metal on the piston.
So it's like these things!
(http://www.crazysales.com.au/products_pictures/52/5277_29539_F.jpg)
its like this (http://i1186.photobucket.com/albums/z380/twinrat/IMG.jpg) You can also carve off the corners of the piston dome to make it fit also i would remove metal from the piston squish(http://i1186.photobucket.com/albums/z380/twinrat/126caee8.jpg)you can also remove metal from the head in area marked (http://i1186.photobucket.com/albums/z380/twinrat/6debc20d.jpg) to get piston to fit ,you can remove metal on piston upto what ever the valve pocket thickness is make sure you leave no sharp edges .these pistons look to have higher domes whick will obstruct the flame front make a grove across the top of the piston oppisite the sparkplug as i have roughly show on piston, use a round file for this .you may have to advance your igntion timing a couple of degrees .Just remember not to have any head gasket material protruding into combustion chamber this can be a preignition point
Good advice that previous post had. I would locate a performance motorcycle shop in your area. It is not uncommon for the racer guys to turn a new dome profile into the top of their pistons for special race applications. Happens all the time in the custom racer world. But you've got to have the right volume of free combustion area in the chamber because you don't want to end up with more than 10 or 10.5 to one compression for street use and pump gas. Even that is pushing it.
Also, the weight difference between the old pistons, wrist pins, and clips, versus the new pistons, pins, and clips, will cause vibration, from mild to extreme, depending on the amount of difference. The factory equips the bike with a counter balance shaft. It's needed because the power strokes are only 90 degrees apart. The counter balance shaft is calibrated to the so called stock components. A good engine balancing shop will be able to correctly balance themotor by adding or removing metal to the balancers or crank shaft. The risk of not doing the balance is i think, comfort. The four main bearings of the gs lower end will probably stand the uneven forces for quite a while. The real question is.... will you? Good luck!!
any of these would best serve?
http://www.ebay.com/itm/1987-93-Kawasaki-650-SX-650SX-78mm-Wiseco-Piston-/130550060964?pt=Motorcycles_Parts_Accessories&hash=item1e6563d7a4&vxp=mtr
http://www.ebay.com/itm/CJ750-High-Speed-pistons-rings-32P-OHV-M1S-78mm-/360272783554?pt=Motorcycles_Parts_Accessories&hash=item53e1ee68c2&vxp=mtr
Both are good suggestions and I did some research. The Kawasaki 650SX is a two-stroke twin cylinder watercraft. The pistons have the right wrist pin diameter (18mm) but the piston height is wrong and there would be no pockets on the crown for overhead valves.
The CJ750 is a Chinese copy of a Soviet copy of a 1930's BMW horozontally opposed (boxer) twin. The cylinders are the right diameter, but I know nothing else about them (wrist pin diameter, deck height, dome diameter, dome height, valve pocket size and location). I also don't know about quality (at $45 for two pistons I am circumspect). So in short, I will leave it to someone else to experiment with those.
For me, I'm still considering milling down the pistons a little and grinding the head to make this work. Unfortunately it will be a total shot in the dark to try to make the bike useable.
For anyone out there reading this thread, the answer is that the OLD Wiseco pistons which are stamped '4304PS' were a drop-in fit. The replacement for that old design does NOT fit.
At this point, all other designs appear to be discontinued. Anyone wishing a 78mm piston is going to need to have a custom design built to order (both Wiseco and J&E will do this for you, and do it well -- for a price)!
I know I am bringing up an old topic, but I have been reading and looking around and came across an over bore for a Yamaha XS650. It is 80mm the wrist pin is smaller from what I can tell(12mm) so you would need a bushing made. It is domed and has valve reliefs.
WISECO 8635P80 PISTON
Link with picture:
http://www.mikesxs.net/product/05-0752.html
I did a search on gstwin to see if that part number had been referenced anywhere and no results were found....
Just an idea that might be worth looking into.
Is this still active?
Think about fitting half a gsxr head?