+4 mil stroke with OEM cylinder questions

[Bowtie316]

I am contemplating putting an 87 cylinder and piston I already have on my 4 mil bottom end, I know I will have to figure out the cylinder spacing and dome, as well as adjusting the ports for the additional stroke.  I have read that this type of combo can run very well.  I'm curious if anyone has any feedback on what the low-end and midrange will be like if I keep port timing as low as I can.  

I have read several posts on other sites that the spacer should be 3/16" (0.1875"), that just doesn't add up to me.  Seems to me that using a 2mm (.0787") spacer and and 2 base gaskets, the piston would be just a little (.020") short of where it was at TDC without the stroker or spacer.  

Now if I just use a 1mm (.039") spacer with 2 base gaskets, this would bring the piston up about .0194" higher in the cylinder than before which should bring the squish down closer to an acceptable range (was around .085").  Does this sound right? Doing it this way would help keep the port timing lower correct?

I am going to use some shims to try to space it out and see if it will work, I just wanted to see if anyone had any experience.

[Pumashine]

I believe the 4 mil only gives a need for 2 mil worth of spacer. Base gasket is .030. 2mil = .080. So all you need is a .080-.030= .050 spacer. I can make you one if you want to try and see where the top of the piston ends up.

We also use a 2mm spacer when using the KTM rod on the BB.

[Bowtie316]

I believe the 4 mil only gives a need for 2 mil worth of spacer. Base gasket is .030. 2mil = .080. So all you need is a .080-.030= .050 spacer. I can make you one if you want to try and see where the top of the piston ends up.

We also use a 2mm spacer when using the KTM rod on the BB.

Okay, good info, I thought the compressed thickness on the gasket was .020", that will make a difference.  That is good to know that you can make me a spacer, let me get it mocked up and see what I find.  Thanks again.

[Pumashine]

Okay, good info, I thought the compressed thickness on the gasket was .020", that will make a difference.

I am wrong you are right. Base gaskets are .020" So its .080-.020 = .060" So you need a 1.5mm spacer. I only stock 1mm and 2mm spacer plates. But I am happy to cut you a 1.5mm

[jadleybray]

^does great work, cut custom spacer plate for me too.  5mm with ktm rod

[Bowtie316]

Okay, here's what I found out.

With no gasket, cylinder directly on the case, piston comes out of the hole 1.5mm and I came up with 178 degrees on the exhaust and 123 on the transfer ports.

With a single .020" gasket, the piston comes out of the hole 1mm and I get 180 degrees on the exhaust and 124 on the transfer ports. The stock head gasket is 1.4mm and the dome part of the head is recessed a little bit so it should work as far as squish goes.  Not sure what uccr would be.

At this cylinder height, the bottom of the exhaust and transfers need to be lowered about 2mm.  I was thinking about taking the top of the exhaust port up to about 184 degrees, cleaning up the ports a bit and trying it out.  Any thoughts on how it will run at 184/124 port timing?  Basically all it will cost me at this time is a base gasket, and possibly a different dome for the cool head to get the compression ratio in check.

I'll go back out and see what the squish would be.

[Bowtie316]

Squish with a stock 87 head is right at .054".  I am not sure what it will be with the cool head dome I have. Probably tighter but I could have someone cut a dome for me I suppose.

Based on some crude comparisons I did with past UCCR checks on stock motors and stock with CR head gasket, I think it will end up in the 14 to 15 range on compression ratio. I plan to use sonoco 110.

[udontknowme]

put it together and see what happens. if your not strapped for time then it should be easy to take it apart and make changes if need be.

[udontknowme]

did you happen to notice if the boost port opens a couple degrees before the transfers ?

[Bowtie316]

did you happen to notice if the boost port opens a couple degrees before the transfers ?

No it opens about 2 degrees later, so about 120 degrees open on the boost port.

[udontknowme]

ok it must be a cr500 thing then. these cylinders arent very consistant. last one i had i used .060 gasket which put the boost at 130 then i brought up the transfers to match. got another cylinder and with .060 gasket it puts boost at 132. guess ill probly try a thinner gasket. what upward angle does your cylinder have on the front and rear transfer ?  i got fairly steep on the rear and more flat on the front . trying to raise the jug with gaskets so i can change them angles

[rsss396]

I have not measured the stock 250r cylinder in a long time but your numbers seem about what a stock cylinder would have.  When you add 4mm of stroke the transfers will increase allot and the the exhaust will increase about half that amount.

[Bowtie316]

I have not measured the stock 250r cylinder in a long time but your numbers seem about what a stock cylinder would have.  When you add 4mm of stroke the transfers will increase allot and the the exhaust will increase about half that amount.

It already has a 4mil stroker crank in the bottom end I am checking it on.  I confirmed this twice with depth gauge measuring piston travel.  I think stock tranfers should be about 120, I got 124 on the +4 bottom.  I think using a spacer at the bottom of the cylinder is what brings the timing up so much, thats why I'm trying to keep it down.  

I ordered up the Macdizzy M.U.L.E. build specs, I'll bet thats what he did.

[Pumashine]

It already has a 4mil stroker crank in the bottom end

I apologize I did not see this in your first post. All cylinder 85-89 are the same dimensionally. The pin height changed from 86 to 87. So it comes down to which piston you are using not the difference in a long or short rod. My builder Pete at Hybrid said he commonly runs the piston out the top and cuts the head to correct the squish.

[Bowtie316]

I apologize I did not see this in your first post. All cylinder 85-89 are the same dimensionally. The pin height changed from 86 to 87. So it comes down to which piston you are using not the difference in a long or short rod. My builder Pete at Hybrid said he commonly runs the piston out the top and cuts the head to correct the squish.

Yeah I guess I didn't specify which piston, it's the long rod piston that I will be using. I am just surprised that it looks like it will work without a spacer.