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How to, BSA Stroke Increase Mod

Includes instructions to adjust the stroke on the later type of BSA Airsporter or Mercury that have a buffer washer, (the same principle can also be done with the Meteor and Scorpion Pistol), first check the gap between the rear of the cocking arm and slot in the piston. After the mod you need about a 1 mm gap or clearance to make sure that the rear of the piston does not strike the cocking arm when the gun is fired. It involves gaining the stroke by removing the buffer washer which is nearly 5 mm.
If you have a 6 mm gap then the most you can increase the stroke is 5 mm to leave a minimum of 1 mm gap or clearance. This is very rarely a problem with BSAs. This is quite a large stroke increase so always check the power after any mods to make sure the power is under the limit of your country.
If you want to increase the stroke by 5 mm then you just file off that amount from the front spigot of the piston. This will allow the the piston head to slide over the spigot nearly an extra 5 mm. The head is then turned 90 degrees so that a new cross pin can be drilled (1/8th) using the new head as a guide to make sure is in alignment.
This can be done to hold the head on firmly. If you want a free floating head then you can use a smaller pin or drill the piston spigot hole a little larger. But doing it this way will lose a little bit of the stroke, because the head will stay forward in the hole as the gun is cocked.

If you want to use a 2 mm buffer washer then put that buffer washer over the spigot before you cross drill etc. I would increase the stroke to the maximum and if the power is too high then reduce the spring power by cutting a coil off.

Below shows the Airsporter/Mercury type piston modified to increase the stroke by 4.5 mm and the new hole drilled at 90 degrees to the original hole. We normally use a roll pin .125" (1/8th) so drill the hole accordingly.
Check the power after running in to make sure it does not exceed the legal limit.