Checking the crank
The crankshaft on a twostroke does not last as long as on a fourstroke. The reason is
mainly that a fourstroke crank bathes in oil (=very good lubrication) while a
twostroke crank has to rely on the lubrication it gets from the gas/oil/air-mixture.
On a stock Yamaha RD 350 the crank will normally last about 20'000 km (this can vary a lot
depending on for example oil-quality, carbjetting, how hard you ride the bike......)
On this page you can read how you check your crank. If you want to renovate your crank, it is usually not to expensive to go to a shop and have it done. Rebuilding the crank takes about two hours for an experienced mechanic.
It is very important that the crank-measurements are in service-limit because a crank failure usually means new crank, cylinder and piston which is very costly.
To fully check the crank you need to take the engineblock apart, but you can get a fairly good tell of the cranks condition by performing the measurements possible after having removed only the cylinders.
Usually the first thing that shows that the crank needs a rebuild is the big-end bearing side clearance being to big. It is measured by using a feeler gauges as shown on the picture below. It should be within 0,25 - 0,75 mm. It is possible to measure this with only the barrels removed.
To measure the rest of the crank the enginecases need to be taken apart. Usually it is only necessary to measure the rest of the crank after a rebuild.
Check that all bearings feel ok, and then measure the crank.
| Maximum deflection (S) : 0,05
mm |
 |
When using a sliding caliper, measure a few times to get a good reading.
Measuring maximum deflection (S) is often only necessary after a rebuild/renovation of the crank.
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E-mail : Rickard