How much blowby is normal in an air compressor?

[taem]

Bought a used 60 gal. Central Pneumatic (Harbor Freight) upright air compressor. Single stage, 3 pistons, 5 HP motor. Cleaned tank, new pressure switch, runs OK, builds pressure though a little slowly. Has quite a lot of blowby from oil fill hole, though no visible oil and no oil loss. Measures 18-20 psi while motor running filling tank to 123 psi. Is this too much blow by? Or should I just use it and see what happens? Haven't changed oil yet, but doing that now.
Bonus question: HF doesn't list this item any more. Anyone know where to find a manual?

 

[BrandonV]

Here is the manual.

https://manuals.harborfreight.com/manuals/93000-93999/93274.pdf

 

[taem]

Here is the manual.

https://manuals.harborfreight.com/manuals/93000-93999/93274.pdf

Thank you. That is close but not identical: mine has 3 pistons about 40 degrees apart, all the same size and plumbed in parallel. I called HF and they don't bring anything up with the product number.

 

[BrandonV]

Thank you. That is close but not identical: mine has 3 pistons about 40 degrees apart, all the same size and plumbed in parallel. I called HF and they don't bring anything up with the product number.

What is the Harbor Freight product number?

 

[vwpieces]

Changing oil may help if proper oil was not used in it.

 

[taem]

62299 which their tech help people claimed wasn't on file

 

[BrandonV]

62299 which their tech help people claimed wasn't on file

Here you go.

 

[MacMcMacmac]

If you are blocking the crankcase vent to measure blowby, then I wouldn't be surprised at that figure. I plugged a vent on an old CH compressor once trying to homebrew a new vent breather and it launched with a mighty blast. If there is no oil coming out the breather I wouldn't worry about it.

It sounds like a FuSheng TA 80 or equivalent UnoAir or Dayleap.






DayLeap

 

[taem]

Here you go.

Wow. Thank you!

 

[American Locomotive]

It's probably not actual blow-by. Whenever you have a compressor (or engine) with an odd amount of pistons, it's not really possible to have equal and opposite piston movement. The same actually goes for V-twin compressors too. For every rotation of the crankshaft, you're going to run into a situation where there is more piston down vs. piston up (or vice versa).

This is going to create strong pressure pulses in the crankcase, which can appear as blowby (but actually isn't). Most compressor breathers have a check valve that's designed to let air out, but not in. This creates a partial vacuum, reducing the "pumping" going on. When you take the breather off, it can often appear that the compressor has severe blowby because now the pistons can pump free air on the bottom side.

Unless it's pumping an oil mist out the breather or pushing oil out the crank seal - don't worry about it.

 

[Jswain]

X2 on what's been already posted. If you are concerned with possible blow by I would first make sure you have no leaks ANYWHERE especially on the discharge tube and then measure CFM on the compressor from 0psi-cutout.

As long as that is good and you aren't losing oil into the tank(or elsewhere) then you should be fine. Also, as mentioned using the correct oil will help keep it in the crankcase as well.