Pistons for Hirth 300cc Motors

Pistons for our Hirth motors are getting harder to find. With that said, I hope this information can help you find what you are looking for.

Commonly called a “Hirth 300″, that description fits a number of different models made between 1964 and 1969. Hirth models 52R, 53R, 54R, 55R, 190R and 191R are all 300cc motors and all take the same flat top 75mm pistons. Some pistons are advertised as 300cc pistons but have a dome top. I have tried this piston and the motor ran terribly with no power.

The piston design changed in the first few years.  The earlier pistons had 3 rings and the later pistons have 2 rings. I’m told this change was made to reduce the friction in the cylinder. I’ve also been told that if using a new 3 ring piston, you can remove the bottom ring and run with two rings. I have done this without issue.

All original standard cylinders are 75mm. The early pistons are stamped 74.88 on the top of the piston which means the piston diameter is 74.88mm. This gives the piston .12mm of clearance. Later pistons are 74.83mm and this adds a little more clearance. For the same reason as switching from a 3 ring to a 2 ring design, I believe the extra clearance was added to reduce friction in the cylinder.

For those who understand inches better than millimeters,

- A 75 mm cylinder is 2.953″

- A 74.88 mm piston is 2.948″

- A 74.83 mm piston is 2.946″

- Clearance is 0.005 to 0.007″

Oversize pistons are often referred to “Once Over” to “Twice Over”. In the USA, we think of this as “0.020 or 0.040 inch” over. For these pistons that are measured in millimeters, it is actually “0.5mm or 1mm” over. In this case, the measurements are almost the same either way.

Be careful when buying pistons more than 76mm (twice over). Sometimes they are so big, the compression is so high that the manual start models are very difficult to pull start and the electric start model starters won’t turn the motors over. Also, some of the larger pistons may fit in a bored-out cylinder but they won’t fit into the motor case.

Below are part numbers to the best of my knowledge:

- Hirth 3-ring piston – 75mm Std Bore = 0496/9, 75.5mm oversize bore = 0496/19, 76mm oversize bore = 0496/29

- 2mm rings for the above 0496 piston – 75mm = 0496/1, 75.5mm = 0496/11, 76mm = 0496/21

- 2.5mm rings for the above 0496 piston – 75mm = 0250/2, 75.5mm = 0251/2, 76mm = 0252/2

- Hirth 3-ring piston – 75mm = 014.26/9, 75.5mm = 014.26/19, 76mm = 014.26/29

- Rings for the above 014.26 piston – 75mm = 014.26/1, 75.5mm = 014.26/11, 76mm = 014.26/21

- Hirth 2-ring piston – 75mm = 014.27, 75.5mm = 014.27/10, 76mm = 014.27/20

- Rings for the above 014.27 piston – 75mm = 014.26/1, 75.5mm = 014.26/11, 76mm = 014.26/21

- Hirth 2-ring piston – 75mm = 014.32, 75.5mm = 014.32/10, 76mm = 014.32/20

- Wiseco piston – 75mm = 2041PS, 76mm = 2041P4

- Mahle – 2k 21130/15 is an oversize piston

- Hastings – 6626 is a standard piston