Long Rod - How Long to Dwell

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. Many people use the longer studs from late model CR250's. This seems to be a popular conversion especially among those who are looking a wider power spread. When used with substancially increased displacement top ends (Pro-X and 265 cc to 275 cc modified stock TRX parts), the long rod may be favorable especially when used with the better breathing early model cylinder and piston.

It should be noted that though there is some benefit to this combination the longer rod also creates a longer dwell at TDC & BDC which decreases piston acceleration somewhat. Please click here to read more about it. The piston acceleration (answer taken from software) is 114864 ft/sec^2 with its 72 mm stroke using a 125.3 mm rod at 8300 RPM. If the rod is 130.3 mm instead, the number moves to 113880 ft/sec^2. When I did the calculation manually (ugh) I came up with slightly different numbers. I got 114623 ft/sec^2 for the 125.3 mm rod and 113669 ft/sec^2 for the 130.3 rod. It is interesting to note that even the latest model CR250 uses the same short rod as the early ATC & TRX 250R's.

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. Unfortunately, the intake recieved a case of the "restrictions" and the piston design changed to reflect its newly designed "improvement." Because of this new piston/cylinder design, wear on the intake side (thrust) of the barrel is reduced and piston life is extended though the barrels ability to breathe is somewhat impared. The long rod version has a wider, less "peaky" power delivery.

Though the overall power was about the same between the 2 versions, the short rod version revs more freely/quickly but it gives up some power spread. It may be of interest that most current 250cc motorcross engines use a rod length of 125mm (like the short rod TRX at 125.3). A longer connecting rod may be used to tune out some unfavorable characteristic of the engines power delivery. Perhaps where tuning by conventional means produces no favorable results - such as increased port timing - a longer rod may tame a peaky power delivery. Modifying the long rod version TRX cylinder/head to match the power output of the short rod version (using its pieces) proved to be difficult, time consuming and (nearly) impossible.

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.

Rick

Driving Impressions

I am fortunate enough to be able to sample both versions of the TRX motor almost every time I go riding. When my TRX was only a couple of years old (1988), its modifications were a 36 mm PJ carb, FMF "rev" pipe, cleaned up porting and stock 13/39 gearing. I performed identical modifications to a friends newer TRX cylinder (1988 model year - long rod). He had the same (model) pipe but did not have the larger carb. It was running its stock gearing (13/38). We both had the same Ohtsu radials.

While drag racing on a dry lake bed outside of San Filepe, Mexico it became clear that my TRX was the faster of the two. In fact time after time I would win by several bike lengths. We had another friend signal our start by using his arm in a rhythmic motion (and a 1, and a 2, and a 3...). We were both starting out in 2nd from a stop and would race until we reached maximum RPM in top gear (as fast as they would go). Traction was not a problem.

When we switched bikes and raced some more, again I won. Time after time by several bike lengths - about the same distance as when I was riding my bike. Our body weight was not much different. It may have been that I could feel the traction better. I would not rev "over" a gear or let it fall too far behind the gear when upshifting. The overall difference is marginal when drag racing.

Rick

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