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TWO STROKE PISTON FITTING INSTRUCTIONS

Mitaka pistons and rings are made from the same material as O.E. and so fitting instructions are the same as the manufacturers.

All O.E. Two Stroke pistons are "cast" and not "forged". As a consequence, Mitaka pistons do not need the increased clearance of a forged piston.

If you do not have the manufacturers fitting instructions, the following can be used as a general guide.

Select the correct "Grade" of piston. Mitaka pistons are supplied in 3 or 4 (or even 5) grades or sizes. These vary in increments of 0.010mm. For a new or re-plated cylinder an A or B grade will generally be suitable. For a worn cylinder you may need a C or D grade piston. Do not use C or D grade pistons in a new or re-plated cylinder without checking the clearance carefully. Do not assume that because you have taken a B grade piston out, you should replace it with a B grade piston. If the piston has been in for just a few hours, a B grade piston should be correct, but if the piston has been in for a long time, then a larger grade piston will almost certainly be necessary. Some manufacturers mark new cylinders with an A or B to indicate the grade of piston needed. This will no longer apply as the cylinder wears with age!

Clearance or Fit :-  In general 125 - 250 pistons will need a clearance of 0.040 - 0.050mm. 60s and 80s, a little less.

So a bore measuring 53.99 will need a piston size 53.94/5.



This will usually be an A or B grade piston. A bore measuring 54.01 will need a C or D. You will not be able to measure the cylinder bore sufficiently accurately with a digital caliper. It needs to be done with an internal or bore micrometer. But you can check the piston to bore clearance quite easily yourself ! You just need a set of "feeler gauges". Place a 0.040mm feeler gauge in the bore at the front or back and carefully slide the piston in without forcing it. If the clearance is correct, the feeler gauge should be lightly gripped by the piston. Remember that the piston is oval front to back and tapered (smaller at the top).

If there is any damage to the bore or the plating, do not fit a new piston, it will fail quickly, often causing expensive damage to other parts of the engine. If the piston you take out shows signs of excessive wear, or it has broken, then the bore is almost certainly worn beyond its service limits and needs rectification by re-plating or sleeveing before further use. (Details of liners and fitting can be found by clicking this link :-
Click to go to sleeves. If the piston you take out has failed, it is important to establish why. This web site home page gives reasons for piston failure.

Running a piston in a bore with excessive clearance will not only result in a loss of power, but will ultimately cause the piston to break, usually at the back.

Pistons will generally be marked with an arrow to the front (exhaust side) with ring peg(s) at the back. It is common for ring pegs to be in the centre of the piston at the back. In such a position, they will run across the small back transfer port. This is normal modern 2 stroke practice.

When fitting the piston up the bore of a cylinder with a reed valve on the back of the cylinder, it is important that the reed valve is taken off so that the ring can be pushed in (use an ice lolly stick !) as it passes over the inlet port. If you don't do this, there is a danger of the ring riding over the ring peg and jamming in an incorrect position. This will damage the bore as soon as the engine is started. In service, the piston does not descend so far down the bore, so it is only a risk during assembly.

Unlike 4 strokes, 2 stroke engines are very easy to work on. Just coat everything with 2 stroke oil as you are assembling the motor, follow these simple instructions and all will be well.