Compressor quandry

[John Rudd]

I was gifted a single phase 14cfm air compressor recently.
First issue was the motor, the start cap had failed....so motor no worky....
Got a new cap from the usual market place for under a tenner......
Next problem, the pressure/ stop-start switch didnt feel very well, the on off action was a bit suspect, so a new one acquired from Machine Mart.....another 20 quid.....
So now I got a compressor that runs but aint pumping well.....
So I lifted the head.....oh dear, a broken exhaust valve petal.....so stripped the top end and removed the cylinder block too.....oh my, the piston doesnt look well....so I removed the piston from its rod....gudgeon pin badly scored too....in fact it wasnt it had picked up on the little end eye of the con rod.
Well after cleaning up the piston, signs of it nipping up above the top ring, polishing the gudgeon pin, I put it all back together....had to wait for a gasket set and replacement valve petals got it running....
Now its knocking from the top end, I think its due to the excessive clearances of the gudgeon pin in the little end eye and the piston bosses....
Now a new piston/ pin/ conrod is going to run to £50.....
I'm guessing that considering the amount of oil in the crankcase, all these failures are due to a lack of it.....

I know this is a freebie, but any thoughts in going any further with it?
Its turning into a money pit.....

[Joules]

You now have a tank you can use with another compressor   

[AdeV]

Would now be a good time to try your hand at turning your own piston?

Or, if you don't fancy going that far, why not turn up a bush for the little end pin to run in? If you can make it a press-fit, then ream it to size in situ, so long as you get it dead nuts it should run better than it does now; and if it doesn't, it's not like you're going to harm it any worse than it already is...

[John Rudd]

Guys thnx for the replies....
I had considered another pump...£89 from fleabay, two things jump out at me.....one is the bolt hole mount detail, not really an issue....the second is the flywheel size....again not insurmountable but its faffing about with it....

I had considered reaming the conrod and fitting a bush, but it would need some careful planning and good machining to get it right....a new piston eh? Hmmm doable but I aint got ang ally bar big enough at the moment, piston is 65 mm, not exactly small....
So what about the gudgeon pin? Would EN8 be any good?

[Spurry]

Well...if it was mine...I would complete the strip-down and see EXACTLY which parts would be needed to make a decently operating machine. Then do the sums to see if it's worthwhile.

FWIW
Pete

[John Rudd]

Pete,
For me it is becoming a money magnet, but on the whole it could be worth it.....it should easily run my shot blasting kit ( pot and cabinet ) where as my other compressor is a shade too small...

The remaining parts of the compressor are in good nick, the other piston assy is ok, as is the crankshaft/bearings and seals.....there isnt too much left.....
BTW, a replacement pump from MachineMart is £180..... Ouch!
I think once this is fettled it will be a good compressor worth £300, they are over £550 new.....

[AdeV]

So what about the gudgeon pin? Would EN8 be any good?

I imagine it would want a heat treatment and grind to final size after machining, but there's no reason a replacement gudgeon pin wouldn't work; except, aren't you going to have to machine it over-size to take the wear out of the small end? In which case you're going to end up having to very carefully ream the piston to take the larger gudgeon pin... which sounds like more work than the bush...

[John Rudd]

I can bush the con rod little end......bore out to clean it up, machine a bit of pb bar, drill out and ream to size.....
Piston could be machined from solid, just found a billet of 75mm ally bar...the bosses for the gp could be reamed to size....
The gp made from EN8 machined to fit the little end and piston bosses....

Does this sound doable?

Else I could buy a new piston and gp and just fettle the con rod....

[AdeV]

Is the gudgeon pin damaged?

If not, I'd be tempted to bore the conrod, make an interference-fit phos-bronze bush, freeze the bush & warm the conrod in an oven (maybe to 150 degrees C? Perhaps some material experts could give you a safe temperature to heat it to which won't adversely affect any heat treatment it's had); install the cold bush in the hot conrod (it should just drop in, if you got your sizes right); when it's all returned to normal temperature, trim the bush and ream to the exact right size (I guess a tight sliding fit on the gudgeon pin? Or maybe it's a press-fit, depends if the little-end rotates on the pin or if the pin is designed to rotate in the piston...). Either way, the bush should be very tightly held by the conrod, and being phos-bronze it will be fine to let the pin rotate in it.

If the pin is damaged, but the little end can be rescued; then depending on how much meat is available on the piston pin bosses, I'd be looking to turn a slightly over-sized pin, clean up the small end, and bore out the piston pin bosses to take the larger pin.

Only if the whole lot is knackered, ovalled, etc, would I write off the piston & the pin.

[chipenter]

EN8 is good can be machined fully hard but the incerts don't last very long , get it neer to size before hardening .

[John Rudd]

 

Right, its all nicely stripped cleaned ready for re-assembly one way or another.....

The gudgeon pin is completely useless, it has worn eccentrically on the ends .....
The piston is worse for wear, the gudgon pin bosses are not in good shape and will need reaming oversize.

The conrod is worn to the point that the pin rocks about, in fact you can see daylight between the gap above the pin with it sitting on the bottom of the l.e.e.

So, I could bore out the conrod l.e.e, think I'd have to do that in the mill. The outside is too irregular to fit in the 3 jaw on the lathe....and expect it to run true(-ish...)

Machine up a new gudgeon pin, I cant finish grind to size, so will have to polish with Wet n Dry.....

Bush, got some 3/4" pb, turned down to 16mm to fit into conrod, then ream id to fit pin.
Next up, ream the piston bosses, same diameter as the id of a pb bush....thats doable...

As for reamers, I have a set of adjustable that will accomodate the sizes I require, but are they the right way to go?
I remember doing kingpin bushes with adjustable reamers, so mebbe this the right approach?

Comments?
Btw, new piston/pin/conrod will cost me around £55 from Machinemart....

[hermetic]

Hi John, The thing you will have to be careful of is to get the small end bush reamed at 90 deg to the bore or you will put unequal pressure on the sides of the piston. £55 for new parts sounds almost reasonable, given the amount of work, and the hassle of getting the small end reamed square with an adjustable reamer. On the other hand if this has failed due to a bit of under engineering in the original design, to do what you have suggested will improve the thing no end, if there is enough meat on the small end of the con rod!

[John Rudd]

Hermetic,
I think you could be right...for the money.....£55 works out to be less than two hours work.....Think I'll just order the bits.... I'd like to sort this asap, as I'm going to Oz next Monday for four weeks...I'll have forgotten what I'm doing when I get back...

I think most of the repairs required are down to poor maintenance, low oil level.... Lube is by splash...

Another thought, if I ream the piston bosses, the clips holding the pin in will need new grooves machining for bigger clips, the orig ones may be too small?

[AdeV]

Hi John,

You do have to answer the question to yourself: are you doing this because it's fun? Or because you need a compressor?

If it's the fun/learning and general enjoyment of machining, then I'd say DIY.
If you need the compressor, and you've got more interesting things to machine, then frankly £55 doesn't sound that expensive - and you'll still have the knackered bits if you fancy a go at it later. And you'll have a working compressor...

I'm quite lucky I suppose, I've got 3 compressors (admittedly, one is attached to an ultra-temperamental Villiers petrol engine), a large standalone receiver, etc... so I'm kinda spoilt on the compressor front. All of mine run ridiculously well...

[John Rudd]

Answer: Well its fun machining bits for a repair.....  but at the same time, I do need a bigger machine to power my blasting kit that I have, a pot and a cabinet....my smaller 14cfm compressor only has a 90 l tank, so 150l will last a bit longer but will take that bit extra to fill...

I've ordered the bits to effect a repair, they should arrive Wednesday, so it should all be back together and working same day....I'll have my holiday time to think  about whether to keeep or sell....should fetch £250-300 on ebay if I do.....

[John Rudd]

 
Well the bits I ordered yesterday morning arrived today.....
Great service from Clarke and Royal Mail....

Tomorrow I need to putvthexrunfs on the new piston, bolt the conrod into its big end and complete the re-assembly of the remaining parts of the pump....then we will see if it was money well spent...

[John Rudd]

 
Ok,
So some fotos.....
Here's the gudgeon pin.......the first image shows the worn sections where it rides in the piston bosses, the second image, the pin is rotated 180 degrees.....

[John Rudd]

 
Here's the piston, the marks at the top above the first ring groove...

[John Rudd]

 
Finally,
The conrod, it may not lok obvious but the l.e.e is worn oval
The second image shows the gap above the pin....

[John Rudd]

And finally,
After re-assembly, it runs nice and quiet as it should.....no knocking noises just the sound of the reservoir filling.....but.....

Its losing air from the pressure switch when it reaches its setpoint...Argghhhhhh....
A quick diagnosis, its the nrv.....Stripped and cleaned, its now back together....

So total cost is around £100 for parts....should get me £250 on ebay..

Anyone want a good working compressor? 

Right chill time as I'm off on me hols soon.....

[Spurry]

So total cost is around £100 for parts....should get me £250 on ebay..

John
I thought you 'needed' a larger compressor for your foray into blasting things with your kit??
Pete 

[hermetic]

Yes, looks like low oil, the piston has definitely grabbed and nipped the ring. NEVER seen a gudgeon pin wear like that, run a file across it and see if it is not hardened properly! It is common for the soft to wear the hard, but with not enough oil and splash lube, what can you expect! I have an on-gong problem with my compressor, and my memory, The compressor develops a very worrying knock, so I check the oil, put some more in just in case, overfill it, end up with a pool on the floor, then remember that it is the motor pulley that comes loose, tighten it up, and it purrs like a kitten again. Done it three times now, think it's me age!!

[John Rudd]

A bit of an update...
After replacing all the broken bits and about £100 lighter, I sold the comp for £275......nice

[hermetic]

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