Crankcase/Crankshaft

The crankshaft was only ever sold by Honda as a complete unit: Crankshaft, Conrods, Main Bearings (and some seals), Big End Bearings, Thrust Washers.

Because it is not always possible (30 years later) to buy a new replacement crankshaft, NSR250 enthusiasts often need to repair crankshafts themselves.

Repairing the crankshaft requires a little bit of knowledge that the author will now share.

First of all, the attached 4 page diagram is useful. Again, see the attached pdf at the bottom of this page.

However, the document does not discuss the tolerances involved in matching up the conrod big end bore, the big end pin, and the needle rollers in the big end bearing.

The exact dimensions of these three components does differ from crankshaft to crankshaft.

Tolerances between Conrod, Big End Pin and Big End Bearing Needle Rollers

Typically, according to the experience of the author:

1) the conrod big end bore is typically 32.010mm in diameter. This can vary between 32.005mm and 32.020mm

2) the diameter of the big end pin is typically 24.995mm. This can vary from 24.985 to 25.005

3) the diameter of the needle rollers in the big end bearing are typically 3.495mm. Sometimes this is 3.490mm

What is important here is the CLEARANCE between these three components.

A clearance of between 0.025 and 0.035mm is required. This is a critical requirement when assembling crankshaft components.

A clearance (at room temperature) of less than 0.020mm may well result in severe damage and/or the conrod seizing onto the big end pin when running at the extreme end of the machines performance and corresponding operating temperatures.

To calculate the clearance simply add up twice the big end bearing needle roller diameter, and the big end pin diameter and subtract this total from the conrod big end bore diameter.

Typically this would be 32.010 - ( [2 x 3.495] + 24.995) = 0.025mm

The exact numbers that any given crankshaft actually measures up as, can vary, but as long as the clearance is between 0.025 and 0.035mm then it will be OK.

Big End Bearings

The standard Honda big end bearings have a cage that does not hold the needle rollers in place once the conrod or big end pin is removed.

So when pressing apart the crankshaft, expect the big end bearing needle rollers to fall out once they are not supported.

Replacement big end bearings are available from Honda. They are specified as 25mmID x 32mmOD x 15.8mm wide. Be careful to get the bearing 15.8mm wide and NOT 16.0mm wide. The conrods are also 15.8mm wide.

The cage is silver coated. This helps reduce friction with the conrod big end bore.

The bearings are the same as for the 2002-2008 CR250R and the 2007 - 2013 CRF150

Note that it is typically the cage that wears, not the needle rollers.

Unfortunately, it seems impossible to be able to specify the needle roller diameter when ordering big end bearings.

Other sources do sell these big end bearings.

Pro-X part number 22.253216F

You must take care to accurately measure the big end bearing needle roller diameter before fitment to check that the resulting tolerances are within the limits mentioned above.

When measuring to within 0.001mm, make sure that your hands do not heat the parts to be measured, or measuring equipment, too much. Repeat measurements a few times to get a handle on the real measuring accuracy.

The author has found that almost all big end bearings with silver coated cages have needle rollers with 3.495mm diameter.

Some "M-cage" bearings do come with 3.490mm diameter needle rollers. It is possible to carefully push the retained rollers out of the M-cage and into the silver coated cage if need be.