Power Base plate spacers

[Jasle]

Can anyone explain the theory of the spacers that you add under your cylinder on a two stroke? I guess it changes your port timing but how does this affect the power band.
I am specifically asking because my pistion has been back-ordered for a few months while the dealer looks for a new vendor. The replacment which is made for a newer bike is the same dimensions except the wrist pins makes the pistion stick out of the top of the cylinder. The newer cylinder has more area added to the top of the cylinder. I could easily make a base plate spacer the .080 I need to push the cylinder up to accomodate the pistion but how will this affect me? Also I could make the same spacer for the top. The seal is o-ring so I could machine a spacer with an o-ring groove. The top pistion ring is below the top of the cylinder so just the top portion of the piston would protrude in to the fabricated spacer. The cylinder is nik-a-sil coated so I guess I would have to coat the spacer too? The spacer on top of the cylinder doesn't really sound like the way to go but it seems like it would be the easiest way to match the stock bikes new style head.
I know its starting to sound cheasy but I need to make this work until the right pistion comes out unless i want to drop $450 for a new cylinder.
Thanks Jason

 

[David Trustrum]

Um, eek?

You didn’t mention the bike.


Either way you would want the piston to retain the same timing & squish clearance as before when tightened up. The timing is affected by a piston height change so a spacer on top will change the characteristics retarding timing somewhat & pressure leak is more likely & a few other things.

As I am typing I am thinking the piston could be different in many other ways such as ring peg position, bearing retention on wristpin & skirt vs sealing of ports. I would really have to look at it to say go ahead, so is it from a different year same model? Why the change?

More info required.

Um, ok well assuming the piston is the same except for that, then yes you could put a spacer under the barrel to retain status quo. This would mean the spacer (when compressed) was equivalent to the height change of the piston. Use a base gasket either side of the spacer removing 1 base gasket from thickness.

Hmm, surely you can find the right piston?

 

[EricGorr]

Cometic makes custom base plates in a variety of thicknesses for about $20. You use a paper base gasket on each side. Tell me what the year, make, and model of your bike and I'll figure out what Wiseco piston will fit in it.

 

[Jasle]

Well its for a 1999 Cobra CM 50 with a 43mm piston. So its actually a 60. Talking to one of the dealers he said Cobra used to source the pistion from Wiseco but no longer. The old pistion was a single ring one. The 696M which is the only thing I see in their cataqlog is a two ring piston. One of the local guys said they would take the 696M and machine off the top of the pistion about .065" making the top ring groove unusable and bumping up compression but it doesn't look like you could take .065 off the top without really compromising the piston.
I called Cobra and they told me that they don't have anymore of the pistions that were made for the 1999 bike in 43mm. They said maybe in about a month but their not for sure.
I have two of the 1999 cylinders and they are the same and one was new in the box so I am sure they were'nt modified. I borrowed someones 2000 cylinder and you can see that it has extra material up top. the ports seem to be in the same places. I have called and E-mail several of the dealer stores throughout the country seeing if anyone has a piston lying around and basically what I am getting is no we don't have that anymore. Most people don't race the 99 anymore. When we were at the World mini's this was evident. We were the only Aircooled Cobra I saw in the finals.
Sorry about not mentioning the bike but it seems that the software automatically bumps the post to the Mini's section if it contains C.obra?
Thanks.

 

[Jasle]

I tried the base plate spacer. The bike runs decent but its getting way too much fuel. I dropped 3 jet sizes and I am still running rich. Black plug and spooge.
So Eric if you can find a piston throught Wiseco I'll take it. I am kinda hesitant to add the spacer up top. I measured the 2000 cylinder and the 1999 cylinder and the port spacing seems to be right on. the only difference is the 2mm of extra cylinder at the top of the bore.
Thanks
Jason

 

[EricGorr]

As far as I know, the piston that interchanges is a stock KX60 piston. Thats what we used to put in the cobras to make them 60s before Cobra commisioned a custom Wiseco with the Cobra label.
The Wiseco part number is 648M04300
Good luck

 

[Jasle]

I talked to another guy this morning that said the same thing. He said you just have to machine down the skirt so it doesn't hit the case and open up the port window a bit. Thanks!!! I'll order one ASAP.
JAson