MK5 Service Issues

[herman]

I am aware of that but it is for minor nicks, I am thinking more along the lines of larger damage that just polishing the edge can not repair. I am talking about removing a considerable amount of metal then restoring the knife edge.

 

[halocline]

I've worked on some MK10 pistons in kind of a two step process, first lap-sanding them on some micromesh to get the end of the piston nice and flat with no flaws, then using some micromesh wrapped around a cone of some sorts (I think a pencil didn't quite have the right angle) to remove a bit of metal around the inside of the edge until there was a reasonable knife edge again. I think one could come with a system to do something like this with a lot more precision. The only thing is that any length you remove from the piston will result in an increase in IP. I guess you'd only have to remove a very small amount of metal from the end to get it nice and true.

I've also polished the rounded pistons, MK20 in particular, with really good results. Those can be pretty creepy.

 

[redacted]

I've worked on some MK10 pistons in kind of a two step process, first lap-sanding them on some micromesh to get the end of the piston nice and flat with no flaws, then using some micromesh wrapped around a cone of some sorts (I think a pencil didn't quite have the right angle) to remove a bit of metal around the inside of the edge until there was a reasonable knife edge again. I think one could come with a system to do something like this with a lot more precision. The only thing is that any length you remove from the piston will result in an increase in IP. I guess you'd only have to remove a very small amount of metal from the end to get it nice and true.

I've also polished the rounded pistons, MK20 in particular, with really good results. Those can be pretty creepy.

While there are limits as to how much you can remove, the first step is remove all washers (shims). Then you might add a washer thickness shim or two between the HP seat and the retainer. Finally, there appears to be a good amount of sealing surface in the ambient chamber, so a little trimming on the length of the HP spring is probably possible. But too much effort may not be cost effective when there are so many older Mk5s in junk piles here and there.

 

[Luis H]

awap/halocline,

Do either of you know if the piston is stainless or chrome plated brass?
If you have a piston that is beyond repair on its edge, lets talk off line, I am wondering if it can be acceptably repaired. I have an idea on how it might be.

It is stainless steel.

 

[Michael Ward]

For those interested, sent the reg to Bryan at Vintage Double Hose, he polished the knifeedge on the piston just a bit, and IP maintained overnight at 132 psi. Good to go! Thanks for the info and for helping me keep my favorite reg alive