Old bsa imp mod d piston fit question

[vbull]

Am I right in thinking that the piston in an old BSA Imp Mod D Light Pattern should not scrape the sides of the cylinder ? It seems the piston on this gun is slightly out of round and the front end of the piston is quite a bit more than an interference fit on one side only, so just thought I'd check before spinning it up in the lathe. It's not tight but all my polishing of the piston front end has been ruined with the scuffing in the cylinder.

[timewell48]

Am I right in thinking that the piston in an old BSA Imp Mod D Light Pattern should not scrape the sides of the cylinder ? It seems the piston on this gun is slightly out of round and the front end of the piston is quite a bit more than an interference fit on one side only, so just thought I'd check before spinning it up in the lathe. It's not tight but all my polishing of the piston front end has been ruined with the scuffing in the cylinder.

The piston is usually a slack fit without the seal, it should drop under its own weight, and the fit is made by the leather seal on the end. If you're getting scuffing I would suggest that either the seal has embedded swarf, or as you say the piston or compression tube are out of round. Suggestions are;

1) If possible remove the leather washer and try the fit of the piston only.
2) put the piston in your lathe and check for concentricity, if OK the cylinder may be slightly oval.

Either way if the scuffing is light it probably won't affect performance badly, after all it is an old gun!
A good clean, polish and light greasing of piston and compression tube should suffice to get a decent seal, and possibly a new leather washer if the existing one looks a bit tired.

Steve

[edbear2]

As Steve says!

(they are a fairly loose fit, maybe 0.010")

ATB, Ed

[vbull]

I had hoped just a good polish would sort this one out but from what's been said above it looks like I'll have to remove a couple of thou off the front of the piston. Gun didn't look like it had been opened so it seems it was put together with a tight piston cylinder fit. I always thought these old BSA's and Webley's were well made examples of good old British enineering so how would this one have passed inspection with such a tight piston ?

[edbear2]

Hi, I have never come across this before in dozens of guns, the pistons are normally accurate to under a thou and very well finished on the bearing surfaces on little used guns I have had apart. Even the piston rods are 100% concentric etc.

I suppose it could happen, I know they had returns as this is mentioned in various books I have read, generally it was tap related or damage caused by bad loading procedure.

Compared to contemporary and previous offerings, at the time these were worlds apart from anything made but you may have a got a rare dodgy one!

It is a very rare occurrence in my experience if it is a factory problem that has not been picked up over the years, I know also that is no consolation

ATB, Ed

[timewell48]

I suppose it is possible someone has had the cylinder in a vice and made it slightly oval? It would take some doing, but it is a consideration. Is the piston tight all the way down or only in one spot? As suggested, check the piston for concentricity and that will tell you if skimming the piston will help.

Steve

[vbull]

The outside of the cylinder shows no damage or signs of crushing in a vice etc but the inside was quite badly gouged which has been improved by polishing so I don't think it needs to be honed. The piston will drop under it's own weight for about half it's travel but as soon as the rear gets passed the cap threads it tightens up. I've checked and the rear of the piston seems to be just clear of the cylinder walls so it is the front end causing the binding. My lathe is not set up at the moment so can't test for concentricity just now but I reckon that is the problem as there is nothing I can see or feel in the cylinder which could account for the scraping.