Long rod VS Short rod

[BVCRacing.com]

[BATMAN258]

This looks to be what i have on my 86. What is the gain from this for a trail machine?

[C-Leigh Racing]

That view is showing that there is 5mm difference in height between the ol 85/86 model piston & the 87/88/89, but its realy not.
It is for sure 5mm difference between the old model rod & the new model center to center.
The pistons on the other hand, its not quite 5mm if you measure from the center of the wrist pin hole to the top timming edge of the old model piston.

If you take an old model piston & mount it on the new model long rod & then sit the stock 250R cylinder up on the cases & bolt it down with no gasket or spacer plate, when you rotate the crank to TDC, the piston will be less than 5mm sticking out of the top of the cylinder.

Normal thinking, one would add a spacer plate under the cylinder to make up for the 5mm longer rod so the piston wouldnt be sticking out of the cylinder, but only thing a 4.5mm spacer plate is what most speed shops supply. A 4.5mm plate & 2 base gaskets would = up to the 5mm, but you dont need that much.

Using the old model 85/86 piston on the new model long rod crank, you need a 3.5mm spacer plate & 2 base gasket for it to work right, that is if nobody has machined the deck of the cylinder. Set up like this & if no one has machined the deck of the cylinder, the piston at TDC will be around 10 to 12 thou down in the cylinder. If you could only use one base gasket the piston would be just a bit sticking out of the top of the cylinder, which would realy be good if you were using the old thick OEM gead gasket.
If you have the long rod crank & going to set the engine up with the ol 86 YZ250 Yamaha piston, then you will only need a 3mm spacer plate & 2 base gaskets to make it work.
The 4.5mm spacer plates, is a plate made & set up the right thickness for the Pro-x or ESR aftermarket 250R cylinders.
Lot of those 4.5mm plates, are sold to unknowing people, that want to get some more bores out of their new model cylinders by using the old model pistons, only to find out once together the piston at TDC is down in the bore around 2.5mm.
3.5mm plates, are not easy to come by because builders dont normaly stock them to sell to the public.
Its a deal where you cant readly get the plates, so if you want an engine set up like that you got to go to a shop that has them or can build the engine like that for you.

But, here the key, I might just know a shop that can get you those 3.5mm plates .
The 3mm thickness plates, there no sheet material you can find to cut those out of unless it could be some sheets special made.
I use 3, 1mm pieces three bond sealant coated together to form a 3mm plate.
Neil

[hartwill]

finally I found this. I am confused, I am using an 86 piston and cylinder with a long rod crank and a 5mm spacer. Does this 3.5mm spacer apply for 86 piston and cylinder too? Or is this just for 86 piston with long rod and 87-89 cylinder? Because regaurdless I am 5mm taller than stock, right? Maybe I'm overlooking something dumb here if so sorry. Thanks again you have helped me alot lately.

[C-Leigh Racing]

New model long rod TRX crank & old model 85/86 TRX piston,,,,3mm spacer plate & 3 base gaskets. If you can get a 3.5mm spacer plate, use that & 2 base gaskets.
It makes no difference, which year OEM TRX cylinder you got or want to use, because they are all machined the same lenght from base to deck.

New model long rod TRX crank & 1986 YZ250 Yamaha piston,,,, 2.5mm spacer plate & 2 base gaskets.

Hows that, Mr I am confused :P .
What if your momma named you Neil like my momma did me.
Never mind, I'm just acting stupid, there was a guy though years ago named Confusus .
Neil

[hartwill]

I still don't get it. Did the piston not come up even with the top of the cylinder from the factory? +5mm(stroke)-5mm(spacer)=back to even right? Ah oh well that's why your the engine builder and not me right? Lol, anyways you say you have these 3.5mm spacers I guess I need one. Can you pm me a price shipped to 44052? Thanks, will

[Pumashine]

Would it not be easier and cheaper to get the standard spacer plates and use non standard base gaskets. With this in mind I ordered the standard .020" gasket along with the .031" and .039" to get the right overall thickness required.

[C-Leigh Racing]

Who you getting the 0.039 gasket from, Cometic. Thats a good idea, less gaskets to seal together.
Neil

[jeffz0rz]

is this to boost compression? more stroke? guess i missed the idea behind the long rod, and the short rod cylinder.

[NormalZ]

Long rod crank setup cuts down the rod angle. The more the rod angle (the shorter the rod), the more pressure you put on the front of the cylinder.