- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- Rick
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- Driving Impressions
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- 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.
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- 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.
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- 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.
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- Rick
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