Shaving Wiseco Big Bore Pistons

bighairyralph

bighairyralph

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I've got a set of Wiseco big bore pistons but I'm not good with the high compression they give. Can anybody give me some instruction on shaving the tops of the pistons to drop the compression to something reasonable (say, 9.5:1)?
 
Hey, Ralph. The best way to go about this is to CC the combustion chambers and the piston crowns, figure-in headgasket thickness, and all that racer stuff that uses exotic tools, burettes/pipettes, and such.

Then there's the best-guess estimate system. Warning: Math coming...

Using:
Vc as the compressed chamber volume
Vs as the 'stroked' volume
Vt as the Total Volume
CR as the Compression Ratio

Then:
Vt = Vs + Vc
CR = Vt / Vc

Since your issue is to change compression ratio by shaving the piston, we want to isolate and solve for a change to Vc, since Vs will remain the same, and CR will change
CR = (Vs + Vc) / Vc
CR = (Vs / Vc) + (Vc / Vc)
CR = (Vs / Vc) + 1

Now, isolate Vc:
CR - 1 = Vs / Vc
Vc * (CR - 1) = Vs
Vc = Vs / (CR - 1)

Knowing the Stroked Volume and the initial (advertised) Compression Ratio, we can 'reverse-engineer' the configuration and derive the Compressed Chamber Volume. For example, suppose this is an 80mm piston on a 74mm stroke, and the advertised Compression Ratio is 10.5:1 (which is simply 10.5). Then (computing in CC's):
Vs = π/4 * 80 * 80 * 74 / 1000
Vs = 371.965 cc
Vc = 371.965 / (10.5 - 1)
Vc = 371.965 / 9.5
Vc = 39.154cc

Now, do the same 'solve for Vc' using CR = 9.5
Vc = 371.965 / (9.5 - 1)
Vc = 371.965 / 8.5
Vc = 43.761cc

Find the difference:
43.761 - 39.154 = 4.607cc

So, in this example, you would want to shave off 4.607cc worth of aluminum from the piston. If working with a true spherical dome, you calculate the 'chord of cut' to remove that amount. Or, you could carefully file/grind off the top, save the filings, and precisely weigh the filings (knowing the specific gravity or density of the alloy) until the proper amount has been removed. Or, you could use the Archimedes principle to measure the before and after volume of the piston.

If you want to also throw variances in head and base gasket thicknesses into this mix, you can, but it involves more math.

This should give folks a better appreciation of why good race engine builders are hard to find...
 
To Many - you should consider changing your tag to "To Much" :) That was very helpful. How ever, on the practical side, how thin can a guy cut the piston crown before weakening it?

Jay - I emailed Smed last night about having that done. I'm really hoping to cut them myself (I'm well equipped) and don't feel right asking a guy "how you do dat" when it's how he makes his bread.

Gary - while a thicker gasket would take care of the compression, it would eliminate any possibility of setting squish. From what I have read, getting good squish is difficult with these chambers, I'd like to at least have a chance. Thanks for the link, I've saved it for later.

I believe Bob Bertaut (?) also does this, but I can't find anything on his site.
 
Just a couple pics of a wisco and cruzin image 2nd over (basically a stock piston) side by side

pistons 003.jpg pistons 004.jpg

wisco box label I think the wisco is recent production.
pistons 001.JPG.

Doing a search on that Wiseco # I find no compression info????\

I'm stump dumb, but round the valve pocket edges as a
start?
 
Thanks Gary, but I expect I'd have to take off more material that that. The pistons I've seen in the past had the top of the pistol shaved off - if you look at the piston on Smeds site (http://www.smedspeed.co.uk/750cc.html) you'll see what I'm talking about.

I'm looking for more specific information on how much material to be removed, or how much thickness of the crown must be maintained.
 
bighairyralph said:
I've got a set of Wiseco big bore pistons but I'm not good with the high compression they give. Can anybody give me some instruction on shaving the tops of the pistons to drop the compression to something reasonable (say, 9.5:1)?
Click to expand...

Wiseco XS bigbore 750 pistons are like the KZ900/1000 pistons in that they're quite heavy with the mass being in the dome. The KZ 900 10.5 have a middle done thickness of.350,meaning they can be literally mill flat while still providing a acceptable dome thickness but I doubt I'd push it that far. Over at www.650motrcycles.com you'll have to dig deep in the archives where Bob Bertaut milled a set of Wiseco's rehased pistons down to 9.5. depending on bore size,generally the pistons will have a -cc of 20 to bring combustion chamber volume down to 43cc - or + for the 8.5 to 9 compression. Look for that photo,most likely Bob milled .050 to .075 off that piston maybe more, best to start in small increments till the desired compression is a achieved. The stock C/Cv is 63ccs -+ a few and you need a minimum dome thickness of.0160 to be safe.
 
I've been off and on with XS, I just don't have enough time. But, time is here for me to do some work on my poor ignored project.

Swain looks good, but kind of spendy. If I have the pistons coated, I'll probably do it myself and use Cerracote. End cost is about the same, but I get to have the fun of doing it and will have enough material for doing the combustion chambers and valves too. As thin as that coating is, I could probably do a few projects with just the one batch.

Thanks for the words and link, Jack!
 
I have shaved pistons down on race motors in the past and I have had good luck and bad. If you have to shave half the material out you may have a problem with Heat. Years ago I did a set of KZ1000 pistons and they did not last long before they melted a hole threw the piston. You can experiment but is it worth it to have a piece come off and take out a good head. I would double up the base gasket and get the fattest head gasket I could get and call it a day. If you do it I would have them dipped. If you are looking for less weight you can cut down the skirts.
 
This is for a street bike, that's why I want to lower the compression. Weight does not overly concern me.

Again, opening the combustion chamber by using additional gaskets will eliminate any chance of setting proper squish. That is not an acceptable solution.
 
Maybe I'm missing something. It's a street bike. You are lowering compression so ultimate performance isn't the goal, as long as the squish band isn't too tight it will act just like the rest of the combustion chamber. Spot on jetting, ideal exhaust for the power curve you want, strong perfectly timed ignition, indexed cam with a proper grind, will all have more effect than squish band??? XSjohn RIP had a lot to say about lowering compression on the XS650. This isn't a big "shelf squish band" like some harley heads or 2 strokes either. It's only the 3mm difference from the stock bore to the 750 bore or less than 10% of the area. Use a 75mm bore copper gasket and you can maintain the squish band if you check clearance!
Let the debate rage!
 
Me too?????
I read the post and he was shaving the pistons and changing the design of the flow for MAX performance. If this is just a street bike there are ways like others said plus me to lower the compression. They make a plate for TURBO bikes to add to change compression. So if you are just doing it to see how it works GOOD for you. I thought this was to get max power and the motor was more than a street bike. Never checked on a XS650 but I would measure the piston to valve clearance when you start and finish so that you can see the total combustion pocket size. More gas to explode more heat in pocket. Like I said before I have shaved pistons on Drag Bike Motors and had some good luck and took out some motors too. I am very interested on how this works in the end and the final compression. I am working on a motor now that I changed compression from 11 to 1 to 12.3 to one won't restart on starter when hot but push it 5 feet jump on pop in gear fires right up . Crank motor by hand doesn't feel weird hot or cold. So compression can be tricky.
Please post your results
Rich
 
Squish zones are another black art. They do interesting things in the final stages of compression, and more interesting things in the initial stages after TDC. Ever seen the unusual and unexpected trick of 'grooving' the squish plate on flatheads? Interesting.

Could also try lopping off just a little from the top, maybe a CC or more, do the rest with gasket thicknesses. That'll give you some wiggle room to re-tweak...
 
bighairyralph said:
This is for a street bike, that's why I want to lower the compression. Weight does not overly concern me.

Again, opening the combustion chamber by using additional gaskets will eliminate any chance of setting proper squish. That is not an acceptable solution.
Click to expand...

In all honesty,the xs never had a quench area that amounted to improving the combustion efficiency and the only true quench chamber would be the Lillie Head with it's matching pistons,all these other custom piston are a joke using that small little pad in the chamber for a quench area,then shrouding the squish effect with an upward dome,you'd need a larger area of volume to have any direct impact to enhance C/C turbulence. I'd mill the piston .100 and coat it and be done and focus more attention at improving port flow.
 
TwoManyXS1Bs said:
Squish zones are another black art. They do interesting things in the final stages of compression, and more interesting things in the initial stages after TDC. Ever seen the unusual and unexpected trick of 'grooving' the squish plate on flatheads? Interesting.

Could also try lopping off just a little from the top, maybe a CC or more, do the rest with gasket thicknesses. That'll give you some wiggle room to re-tweak...
Click to expand...

With XS,you'd probably have more of an impact "Singh" grooving the dome with a couple deep channels that influences the flow towards the plug.
 

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