Cylinder / piston / rod Configurations

[250R_SV]

Can anyone comment over the difference between the different setups for a 88 250R and what difference it makes in the performance:

-88 cylinder, long rod, 86 piston +5mm spacer
-88 cylinder, long rod, 89 piston

I have a ported cylinder I will install so Ill need a new piston, Got fatty pipe, fmf powercore2,36mm carb, rad valve w/o spacer.

[Honda 250r 001]

I dont think there is any change in performance, the only thing is the 86 style piston has more over bore sizes.

[matt250r21]

The 86 piston is a little better design in my opinion. Less blow by were the wrist pin is. The 87-89 pistons had more material removed in this area then the 86 style piston. 86 pistons also have more bore sizes. The spacer plate gives you more crankcase volume over stock, not sure if that is a performance advantage or not though, just a fact of useing the 86 piston in a long rod engine.

[86 Quad R]

i've noticed a lil difference in how the torque is delivered when using the spacer plate with the smaller bores of the oem cylinder. it seems to take away a fair amount of the crankcase pressure of which imo is really what makes the r engine what it is. this is also affected by using reed spacers.

it doesnt seem to be as noticable when this setup is used on the larger aftermarket cylinders. i personally am more partial to the short rod 85/86 setup without a spacer plate.

[Honda 250r 001]

Originally posted by 86 Quad R
i've noticed a lil difference in how the torque is delivered when using the spacer plate with the smaller bores of the oem cylinder. it seems to take away a fair amount of the crankcase pressure of which imo is really what makes the r engine what it is. this is also affected by using reed spacers.

it doesnt seem to be as noticable when this setup is used on the larger aftermarket cylinders. i personally am more partial to the short rod 85/86 setup without a spacer plate.

I agree with that, im running a 310 bigbore with a short rod crank, and a 86 style piston with no spacer and it seems to run great. the spacer just doenst seem like a good idea to me. Or it really need to be port matched to the bottom of the cylinder and the top of the cases otherwise it would great wind drag...

[C-LEIGH RACING]

Think of it like this when the engines running.

When the engines turning high rpms, every function inside the engine needs to work fast, like the reeds need to open & close fast, the fresh charge in the crankcase needs to fly up through & out of the transfer ports real fast & the pipe needs to be a build that would make the wave traveling inside of it fast.

If a high rpm type of engine is what you like & can use, then maybe the cylinder spacer plates not the best for you to use, because by adding that little extra space to the crackcase it will slow the charge up through the ports just a little, not much but just a little.

When your building the engine & it has to have a spacer plate, you got to compensate & figure in how your building it & in one way or the other to get that charge speed through the ports back up.
Now it could be, you could compensate by over porting the cylinder just a bit, like cut the exhaust to far past a top end port job & then the slower moving charge up through the ports would take out some of the wild light switch like hit when the engine jumps up on the pipe, if you can understand what I'm saying.

Think of a spacer plate to be used on a engine you want to pull good down low rpms, a real high compression type engine, what I call a grunt engine.

Most any type of build on a 250R engine, going to be a spacer plate type of engine because the short rod is getting harder to get & just a mater of compensating in the build getting all those functions matched back up together.
Neil

[250rPRIDE]

i thought the spacer was only 3mm and 2mm for the yz piston?? is that right niel?

[C-LEIGH RACING]

3mm for the old 85/86 piston & long rod crank & 2.5mm for the YZ250 piston & long rod.
The Pro-x & ESR cylinders use a 5mm plate because thats how the cylinders are machined to use one that thick.

Depending on if the stock cylinders been machined on any, it could take either two or three base gaskets with the 3mm or the 2.5.
Neil

[All250R]

The problem with a spacer plate if you're experiencing one isn't a matter of increased distance up the transfer ports to travel. The pressure void will be filled in every area where the pressure is low and turbulence isn't impeding...

The problem arises potentially because air is an elastic fluid. When the depression in pressure begins, the more elastic fluid the signal has to travel through, the slower that signal is felt by the intake tract. If the engine is running at a higher rate of speed, that reduced pressure equalization time may become a sore spot for your engine.

I'm not convinced that adding crankcase volume via a reed or cylinder spacer universally adds bottom end. If it adds power somewhere it's because some other tuned variable wasn't configured correctly that just so happens to be alleviated by reducing cc compression ratio...

[All250R]

Another interesting point for you guys that want an OEM short rod. If you have good journals, you can install a CR rod. I forget the exact year, but Honda or dealer can easily find you the part number that is still available. From what i hear, the ProX rod is a quality piece from Japan as well. Hire quality workmanship to install the rod on your OEM bottom and you're still in the short rod build.