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Spacer Plates

The spacer plates I use for long rod/short rod combinations use are typically .188" (4.775 mm's) to .190" (4.826 mm's) thick. For those of us splitting hairs they're not 5 mm's thick - though generically referred to at that thickness. A plate of that thickness matched with an additional .020" base gasket makes the long rod wrist pin height off set correction factor between .208" (5.283 mm's) to .210" (5.334 mm's). At any rate what's important here when building the motor is that the actual base plate spacer and gasket thickness are used during the mock up session to determine actual port timing, deck height and squish thickness.

If I remember correctly initially when the "5 mm" plates came about it was to allow the 1987-1989 long rod engines the broader range of bore sizes available to them by using the 1986 piston (up to .140" over). When the Pro-X barrels came about these plates helped the later model machines by letting them run the "already obtainable" 86 style piston.

Since that time there have been tons of spacer plate combinations with engines using different brands of pistons (YZ, CR etc) having all kinds of different displacements and wrist pin/port timing edge heights. Combine this variable with other variables like trying to set the deck height or squish thickness right with little other than a spacer plate, its gaskets and the regular combination of parts and it becomes more confusing still.

The machining operations I perform during an engine build include decking the top of the barrel to make the deck height as close to "0" (zero) as possible. This allows me to put the squish thickness in the dome. Since the dome is all aluminum its heat transfer path to the coolant within the cooling jacket is quicker and shorter. Combinations that include leaving the piston at or about -.040" below the deck give up some performance due to the "tougher" route combustion heat has to take as it passes through a ring of iron, then aluminum before it can start to cool.

The ideal squish thickness for the TRX250R motor is perhaps yet to be determined though I can say that I have had the most success using a thickness of at least .034" and no more than .045". This includes versions of this engine from 250 cc's to 392 cc's. In all cases it is much more important to design the combustion chamber squish velocity correctly than to try and gain a small bit of performance by tightening up the squish thickness by a couple of thousandths. Chamber shape is still perhaps the most important factor overall.

The issue of the change in crankcase volume (and primary compression ratio) due to building an engine using a spacer plate is a complex one. Considering all the different bore and stroke combinations it is almost impossible to cover this topic without targeting a single type of build. Having said that I can offer up some things I've noticed. Engines (when changed to larger crankcase volume) tend to have a softer power hit and often produce greater peak power numbers when fitted with a very efficient pipe. When fitted with an "ordinary" pipe the soft hit remains but increases in peak power may not. Some of these affects can be countered by altering other engine parameters but usually not enough to change this basic tendency.

I am of the opinion that it is always best to build an engine using the same parts during mock-up as the final product. This means that it is best to always have the right bottom end for the top end being built. Again, this is probably best but not always practical. I have had very favorable experiences with the TRX250R using my set of mock-up motors (complete bottom ends) which currently consists of a short rod version (125.3 mm rod length), a long rod version (130.3 mm rod length), a + 3 mm stroker version (75 mm stoke w/130.3 mm rod length) and a +4 mm stroker version (76 mm stroke w/130.3 mm rod length). When building an engine of a different stroke or same stroke crankshaft built using a different rod length than what I have ready I install the finished crankshaft in a set of bare cases and take my readings from it. The TRX250R engines (bottom ends and connecting rod lengths primarily) are built to a quality control that is consistent enough to allow this to be done.

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