I was reading in the back of my dirtwheels magazine about a porting/big bore kit,and it said the kit came with a "cylinder spacer".what does the cylinder spacer do,or what is it they are talking about?

 

The 1987 and later TRX 250R's come equipped with a crankshaft that has a connecting rod that is 5 mm longer (center to center) than their earlier model cousins (1985 & 1986 ATC250R & 1986 TRX250R & the 1984 thru current model CR 250R). The piston of the long rod version engine has its wrist pin located 5 mm higher to offset it. A long rod crank used in combination with the short rod piston will bolt up - along with a base plate spacer if you want to increase the crankcase volume and take advantage of the longer rods leverage. In addition to this - because of the extra thickness the base plate spacer adds, it is necessary to buy longer cylinder base studs and install them.

The long rod crank can be used with the long rod piston, independent of the year model of the cylinder barrel - any year is ok -though there are some differences to later year model porting configurations. Generally the later model barrels have better porting on the exhaust side - the exhaust timing was more advanced and the front transfer ports are aimed better. There are some improvements in the squish thickness and area as well.

When combining the long rod and the 1986 piston the cylinder head-stay will have to be modified or removed since the cylinder will now be raised with the base plate spacer. If you decide to remove the head stay it is probably best to leave the tube connecting the frame rails together so it can still be used to stiffen the backbone of the frame as it was intended.

To reverse this design, use an 86 model year crankshaft with it's shorter rod and the 86 piston. This combination can be also used in any year model barrel. The connecting rods (125.3 mm and 130.3 mm) and main bearings are available individually for replacement and are much cheaper than the purchase of a complete new crankshaft.

The 86 style piston has a large cut out in its intake to allow for a lot of intake timing. There really is no need for an additional hole in it. Compared to the small hole on the later model pistons - the cut out of the 86 is huge. The only reason to use the 86 piston is to make up for the base plate spacer which is used on (later model) long rod motors to increase crankcase volume. The later model year pistons last longer because they have more skirt to them and their barrels have additional intake bridges to support them better.

 

Now THAT is an informative reply.