southwick_mx

Member
Jan 16, 2015
4
0
Hi! New on this forum, delighted to find it!

I have a Q for the really experienced 2-t engine guys:

In Bell's book on two stroke tuning, in the chapter about the bottom end of a 2-t engine, he claims that the crankshaft end float (side to side movement of a healthy crankshaft in a freshly rebuilt bottom end, new main bearings) should be 0.2-0.3 mm. Now this was written quite a few years back, and this measurement is not something our service manuals ever mention, so my uestion is how does this apply to the modern 1 cyl 2-t engine?

Any thoughts on this?

Cheers/

Lasse Svedberg
 

southwick_mx

Member
Jan 16, 2015
4
0
The rod small end side to side movement has got nothing to do with my question here, I am speaking of the correct method of crankshaft/main bearing installation, in order to have a solid bottom end and optimum service life of the main bearings. To obtain this, the bearing inner rings must be perfectly axially aligned to the outer races, thus creating a suitable crankshaft end float, i.e the distance the crankshaft can be pushed from one side to the other in the completed bottom end.

The small end movement is a function of the tolerances and condition of the big end components.

/Lasse
 

jaguar

~SPONSOR~
Jul 29, 2000
1,466
79
South America
You seem pretty smart so this info is probably already obvious to you. The side play tolerance serves two purposes, to maintain the center of the connecting rod under the piston, and to be small enough to prevent any clatter due to irregular side forces and to side pull due to the primary gear teeth not being " straight" (but rather angled to lessen gear noise but that exists more on street bikes). But it´s not something that is "adjustable" since it is determined by any slop in the main bearings. Too much slop indicates it´s time to replace them. I wouldn´t go by any generic figure as to what is too much, but by what the manufacturer says. If the play is a whole lot then it could be there is not a snug fit of the bearing in the crankcase half. I once had that happen in my Husqvarna 125 and it ruined the inside surface of the crankcase where the bearing "presses" into it. Luckily I found a shop that welded it and resurfaced it.
 

southwick_mx

Member
Jan 16, 2015
4
0
In every two stroke engine I have ever rebuilt, the small end rod to piston side to side clearance has been way larger than the big end side to side clearance (which is always close to 0,5mm).
This means the position of the rod is not determined by the small end.
Of course I want the crankshaft/rod absolutely centralized in relation to the cylinder bore, but my question was about the correct method of assembling the crankshaft in its main bearings in the cases in such a manner that both bearing inner and outer races are axially parallell.
Of course I want the crankshaft to be dead center in the cylinder, but I don't think this alignment is as important for engine longevity as being off by a few hundreds of a millimeter in the main bearing arrangement would be.
 
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