Would you - or how would you recover this?

[Elmo Putney]

Had a punt on a Jun Air compressor sold on the bay of thieves as for "for parts not working". Its a rocking piston oil-less design, and was pretty high spec as new. On quick inspection both cylinders are foobared with scoring and wear, and the (PTFE?) rings are toast.

It appears that you cant buy a cylinder and ring kit for it - the closest kit includes the rod, bearing piston, heads etc.. and to do both cylinders they want £800 - £1000, which is a non starter for a £20 ebay punt.

So im looking at other homebrew ways to fix it - I have 2 main issues: 1 - recovering the aluminium cylinders and 2 - where to get new PTFE ring/seals from.

1 - the bores/cylinders, fairly simple design, aluminium, symmetrical in all senses, 80mm diameter - don't know if they would have had a surface coating (thinking nikasil (sp?) or similar)
The non running zones look like someone may have honed them lightly - possibly as an attempted fix - I would think such a bore should be a smooth finish with PTFE running surfaces?

Anyone suggest a way of rejuvenating them - I was thinking of someone like Poeton who have various bore coating/repair - often aimed at 2 stroke racers, though no idea on cost or how much material they can build up.

2 - the ring- well it's not really a ring as the piston splits in half via a bolt through the crown so it's more of a disc, any ideas where I can get one - or one made - the piston is a lot smaller than the cylinder so it's not acting like a conventional ring.

From the service kit picture perhaps rings have previously been fitted the wrong way up, torx bolts in the tops of the pistons had definatley been pre-chewed by someone.?:

I like a challenge but this may be a basket case - let me know what you think....

 

[dannyp]

if i was bodgeing it. id get the pot in a lathe and get those bores as smooth and shiney as possible for the ring to run on, looking at it the size isn't over ritical so it then just guestimate a size for the ringy thing buy a block of ptfe if you can and turb one up. well that would be the first battel plan

 

[Fazerruss]

if i was bodgeing it. id get the pot in a lathe and get those bores as smooth and shiney as possible for the ring to run on, looking at it the size isn't over ritical so it then just guestimate a size for the ringy thing buy a block of ptfe if you can and turb one up. well that would be the first battel plan

Yep it can't be that critical as there doesn't appear to be a gudgeon pin on the piston so it must have a fair amount of slack by design.
Just re read the first post saying it's a rocking piston so obviously no pin duh

 

[mike 109444]

You could get it honed out to remove scoring then makeup your rings to match. If you get a sheet of ptfe the same thickness as the ring groove rather than a block you should be able to rough out the discs needed with a big hole saw, then it's just a case of fine trimming (could even mount on a pillar drill and use a course file then fine. I have turned up a number of items in ptfe and nylon that way.
If you did go for a coating I think it's usually chrome in 2 strokes.
Post back how you get on

 

[mike 109444]

Yep it can't be that critical as there doesn't appear to be a gudgeon pin on the piston so it must have a fair amount of slack by design

I think that is why the ring has the angle/chamfer to allow it to pivot a few degrees.

 

[MrFluffy]

Could bore it out well oversize, and push a cast iron liner in and bore that to the original size. Then you just have to make a new ring somehow if the rings aren't seperate parts to purchase. I would think redoing the nikasil wouldn't be economically viable.

 

[the snooper]

Looks like quite a lot missing from the original ptfe disc looks like they are cup shaped with flared edges so the pressure generated seals it against the cylinder walls

 

[rikrobson]

like @dannyp says i'd be looking to get it chucked up in a lathe and get it re bored. It'll probably be only a couple of mm bigger. then make a new ring for the piston out of ptfe or nylon. it looks lke its meant to have the upper disk sit in a recess in the seal and the angle extending from the bottom disc.

I'm guessing the seal has gone at some point and the metal has engaged to cause the damage to the bore.

 

[armalites]

That looks very similar to the insides of the air compressor used on some vehicle suspension. Aluminium piston with a ptfe ring

 

[Parm]

plasma spraying and honing back to original size is another option, but don't know how costly it is. Only seen it done at an aero engine rebuilder just outside Cardiff where they had thier own kit

 

[Chris Stevens]

If the cylinder is nikasil plated it can be redone by Langcourt at about £150, but would it not be better just to buy a used working compressor than expensive repairs on something badly damaged?

 

[500e]

We came by a twin pump version (2 complete units on one tank, motors an integral part of each pump) both were knackered same as yours throw & forget does not warrant time or the angs
Tank was good though

 

[gordon stephenson]

As suggested, skim the bore to a very smooth surface, And turn a new PTFE seal/ring to suit. they just look very similar to a bike pump washer, with the lip facing the pressure side, Your rings were probably fitted the correct way originally but when they got worn and thin the pressure may have forced them to invert.

 

[the snooper]

Can't see this being cost effective to repair at all for what you will spend you could buy a working machine that will outlast a new one of those machines

 

[Chris Stevens]

As suggested, skim the bore to a very smooth surface, And turn a new PTFE seal/ring to suit. they just look very similar to a bike pump washer, with the lip facing the pressure side, Your rings were probably fitted the correct way originally but when they got worn and thin the pressure may have forced them to invert.

Once the Nikasil is gone bore would wear out in maybe 20 minutes, so not that great an idea..................

 

[the snooper]

those cylinders look to have been atacked with some sort of abrasive paper on a drum via a drill its very rough it wouldnt look like that at all done with proper tools, my opinion would be its scrap

 

[Chris Stevens]

Nikasil cylinder isnt scrap when the plating is damaged, but cost of replating and replacement parts is likely to cost substantially more than a good used compressor which is much better quality.

 

[slim_boy_fat]

Nikasil cylinder isnt scrap when the plating is damaged, but cost of replating and replacement parts is likely to cost substantially more

As BMW found out in the '90s.

 

[Chris Stevens]

Nikasil is generally trouble free, but BMW I think tried to plate cast iron cylinder blocks?

 

[Nick67GT]

Don't forget that machining PTFE can be harzardous:


https://www.fluoridealert.org/wp-content/pesticides/teflon.decomposition.prod.htm