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AC Capacitor

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Raphael Golez26/03/2019 15:25:23
101 forum posts
55 photos

Thanks for the support gents. Im in the UK if anybody is wondering with regards to the power supply. Its great to have a very helpful and informative suggestions and opinion here.

Robert, the piston is not seized. I have taken the cap off and cranked the top bearing with a 4mm steel bar that fits in the slot and its easily turned by hand, I must admit I need to change the oil as its never been change since I bought it second hand (no idea if it was changed previously). In terms of oil, 1 liter of SJ-27 (recommended oil for JUN-AIR) is around 30 GBP!!! and I think my compressor need at least 1.5L. Does anybody here use equivalent oil rating in their compressor other than the brand recommends? Splashing out 60 GBP for the oil is not easy.

John, yes it has start relay and got the 3 pins (i will counter check when I go home) to make sure it has 3 pins. If the capacitor does not work then I will look into it. Getting the right part will be challenging as my model is dated 1980, I could not see any motor (orange) model (I guess its the older version with 15 L tank coloured green). I tried the website but I guess they are all the updated models although there might be a possibility that it share a common internal and electrical parts. Just getting the right part for the right motor size would be the way to go i think.

Clive Foster26/03/2019 15:59:50
2136 forum posts
73 photos


I believe that SB42 is specified for older JunAir compressors so maybe this :-**LINK** will do for yours.

Not sure what older means in this context. Ours had a sticker on the side giving oil grade but what it was I don't know as its over 20 years since I last saw it, probably new around mid 1980's.

Nothing super magic about the oil. Its just a long life synthetic. Remember many of the JunAir range are intended to be on all day everyday with a pretty high duty cycle. If you're on occasional home shop uses slightly less than ideal oil probably won't matter as the compressor just won't clock up the hours.


PS According to the JUnAir MDS data sheet it looks like S27 viscosity is 20 cSt nominally equivalent to ISO 20 at 40°C and 95 SUS at 100C.  Quite thin.

Edited By Clive Foster on 26/03/2019 16:17:30

Raphael Golez26/03/2019 19:00:04
101 forum posts
55 photos

Thanks Clive. I don't want to sound stupid but can I use a motor oil? I have search online for equivalent SAE viscosity for S27 but i could not find any. I don't no if anybody here have any knowledge with regards to this. Im just curious as to why its very expensive for 1L of S27 oil. If I can substitute it to a more affordable equivalent then it would be a lot of cost savings. I might be cutting cost but in the long run damage the mechanical part and pay the price. It would be good to hear all of your opinion here. Thanks again for everyone contributing to this discussion.



Edited By RAPHAEL VAL GOLEZ 1 on 26/03/2019 19:01:19

Simon Williams 326/03/2019 19:24:30
491 forum posts
78 photos

Hello Raphael, good evening again.

As I said before I don't know this particular machine, but I'd be leery of using motor oil in any compressor because of the detergent content. Compressors suffer from condensation by virtue of the water in the compressed air coalescing on the internals; this water gets mixed into the oil and emulsifies if the oil has a detergent content. You actually want an oil which does the opposite and is hydrophobic, i.e. it rejects the water, not dissolves it. Of course - just like a car engine - if you get water mixed in the oil it not only goes a nasty grey sludgy treacle but it also ruins its lubricity properties.

I spent a lot of time in a previous life playing with Atlas Copco industrial compressors, mostly reciprocating, some screw compressors. The reciprocating compressors normally used an oil called POA which stands for poly alpha olefin, and which has the long life lubricating and water rejection characteristics. Others will hopefully tell us if this is a suitable lubricant for your application, if so it's available in the after market quite readily. It's not cheap but it certainly isn't as expensive as the prices you mentioned above.

We used to take this oil and discard it, which sounds a bit barmy but we used the compressors in the food industry, injecting compressed air directly into the food process. This meant that the compressor oil had to be food safe, so we actually used an oil made by Rocol called FoodLube. Again I don't feel confident to say definitively that this is suitable for your compressor, but I'd be happy to give it a go if it were mine.

Hope this helps, I'm a bit reluctant to recommend an oil, but I'm pretty certain straight motor oil isn't compatible with your compressor.

Best rgds Simon

Clive Foster26/03/2019 21:13:48
2136 forum posts
73 photos


Thanks for chiming in on this oil problem. Given that the JunAir is an oil free compressor, unless something is worn, there should be no intermixing between the air end and the crankcase / oil reservoir. If this is the case surely the oil should stay essentially dry save for any general moisture in the air that gets in via the breather system.

I know what you mean about emulsification. Way back Duckhams 25/50 was notorious for leaving sludgy deposits around oil filler caps and other cooler places of an engine. With motor oils the emulsification is reversed and any entrained water driven out via the breather system once the engine gets hot enough. The problem with using motor oils in cool running systems was always said to be that the oil never gets hot enough to release the entrained water so sludge got left everywhere. Seems to me that modern oils have got much better at releasing the water into the breathed air at lower temperatures. Certainly I have run compressors on home shop duties using good quality motor oils and not seen any emulsion or sludge issues. The Atlas Copco KE2 I had for years prior to going hydrovane seemed quite happy. No deposits onto dipstick and the oil came out quite clean at change intervals. Crankcase got hot enough during use that you didn't want to touch it for any length of time. Not burn hot but definitely "Ooo, thats really worm.".


I don't pretend to understand oil viscosity classifications and comparison between the various units and ways of measuring it. Hard to think they could have made things more complex if they tried. Fundamentally its all about oil getting thinner as it gets hotter. Basically it needs too be thin enough when cold to get in where it needs to go and thick enough when hot so as not to escape through seals or past the piston rings.

The charts and write-ups I have suggest that 5W is pretty much equivalent to the 20 cSt (=ISO 20) quoted on the SD27 data sheet at 40°C. SAE 50 quoted as being 20 cSt at 100°C (actually measured at 99°  so a 5W-50 synthetic will be pretty much constant viscosity over any reasonable compressor temperature range. SAE 30 is quoted as 11 cSt at 100°C and SAE 20 6 cST at 100°C so it does get rather thinner when really hot. Seems to be a 10 to 1 drop off between 40°C and 100°C.

As I recall it the case on our JunAir got up to around "really hot radiator" temperatures when running for 5 or 10 minutes in room temperature stabilised at 20°C ambient. Odds are SAE 20 would still be around 20 cSt at maximum running temperatures. I'd be very tempted to try a good quality 5W-20 fully synthetic oil and drain it out for a look see after a few hours running.


Edited By Clive Foster on 26/03/2019 21:15:12

Edited By Clive Foster on 26/03/2019 21:15:52

Raphael Golez05/04/2019 12:53:12
101 forum posts
55 photos

Just an updates:

Compressor is working fine with capacitor change. Settled for the specific compressor oil as per JUN-AIR recommendation. Thanks for all your help.


Brian Sweeting05/04/2019 16:18:42
413 forum posts
1 photos

Thank you for the update, always good to know when a problem is resolved.

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