Recommended rpms for an older Speedaire 3z172 compressor?

by Bud Norwood
(Dunnellon, Fl)

I recently aquired an older Speedaire 3Z172 compressor. It looks like the old “filling station compressor” mounted on about an 80 gallon tank.

Original Dayton motor had been replaced with a 5 hp 1750 rpm 184 frame GE motor at some point.

The “new” motor was lacking a start capacitor and the local motor shop determined that the start winding was bad. That motor has been scrapped. This unit is probably around 40+ years old.

I'm planning to replace it with a 5 hp Marathon 3450 rpm motor with a 5/8” shaft that I can get for a good price. I'll be using an H-bushing double groove pulley.

I'm a retired millwright/welder/machinist who grew up as a carpenter so I have a variety of air tools but I'm usually working alone in my home shop so maximum CFM won't be an issue.

The oil the compressor appeared to be clean when I got the compressor but I changed it out with new Mobil Rarus 427 compressor oil per advice from a turbine engineer I worked with.

I'm planning to run that a few hours for cleaning, drain it and replace it with Amsol synthetic compressor oil.

Finally, the question – is there a recommended RPM for this compressor? Since I'm buying a pulley for it anyway, I'd like to get the proper size pulley to avoid overworking an older compressor.

Another question just came to mind, too. This tank has no condensate drain. Should I install one? I was a pipe welder at one point in my life so I can weld a coupling to the bottom of the tank and then drill through it and install a 1/4” drain line and isolation valve.

Thanks for any info!

Comments for Recommended rpms for an older Speedaire 3z172 compressor?

Click here to add your own comments

Jan 22, 2018
by: Doug in

Amsoil has its adherents, but some people like snakes.

Why not just use Champlub (P13929A) synthetic? About 30-40 bucks a quart.

Or, Rorus 827, the synthetic version Rorus 427, assuming you want to go synthetic.

Maybe ask the turbine guy, although I'm not at all sure turbine lubes have much relation to reciprocating compressors.

Jan 21, 2018
Finally got it all together - I hope.
by: Bud

Been a while but I had to regroup and change motors. A friend who's supposed to know about compressors assured me that a 5hp 56 frame motor would handle a Speedaire 3Z172. It won't!
It ran for a while but went out on high temp. Rather than kill that motor I sold it and replaced it with a 5 HP 184T TEFC motor. Runs good now!

The same guy who advised me on the motor also said I *really* need to switch from Mobil Rarus 427 to Amsoil for compressors. The guy who recommended Rarus 427 is a turbine engineer I used to work with who specialized in lube oil & oil flushes.

Is the Amsoil worth the investment?

Also, how do I add a pic here? Thanks!

(To add photos, start a new thread in this same forum, use the same title and say they are for an existing post with the same same title, and you can then upload up to four new photos. They will be moved here - Moderator)

Jun 20, 2017
Almost ready
by: Doug in

Hey Bud -

Send us some pix before you put the shed up, huh?

You can start a new thread, send pix, reference here, and Bill can combine them.


Jun 20, 2017
Almost in service
by: Bud Norwood

Got a slab poured last week, got the new motor from eBay for $209 with free shipping, H bushing and pulley from Surplus Center for $20.
Ran it up to 110 psi earlier today and all seems fine. Gotta get it mounted on the slab and the shed built over it this week and it'll be good to go!
Thanks for the assistance in figuring out what to do with it!

Jun 12, 2017
I-R 588 wrench
by: Doug in

Yeah, that'd do it.

But I think $20k and 50K ft-lbs is a bit overkill...Unless you have one laying around.

Jun 11, 2017
BIG impact needed - we used 2-1/2" drive
by: Anonymous

We had one we used on steam turbines - Ingersoll Rand 588. The "588" indicated the weight. It was a 2-1/2" drive. If it didn't bring something loose we'd just get the air-arc and cutting torches going!

Jun 10, 2017
Tank coat
by: Doug in

There is, but I don't recall the name.

I used some to seal an old brass gas gauge float with pinholes. I think it was intended for gasoline tanks, but no doubt it would be fine with water.

And again, impact wrench.

Jun 10, 2017
Yep - 1/2" NPT plug
by: Anonymous

It was a hex socket plug so it was well hidden. I might have to see about getting an inspection camera to check it out. I usually avoid Harbor Freight and the *new* Craftsman stuff but if it's cheap enough, it works once or twice.

I have an old can of "Chisel" gasket remover that will eat the paint off so some PB Blaster can soak the threads. Got some Kroil to try, too.

If I can get it open and clean, is there any product that's made for coating the inside of a tank like this?

Jun 09, 2017
Speedaire 3z172 drain
by: Doug in

So the drain was just plugged? Weird.

FWIW, I've found an impact wrench and some patience will usually get those plugs out. Getting a socket that fits is the only potential problem.

Then again, you could use that half inch port to inspect with right camera...

Anyway, thanks for the update.

Jun 08, 2017
Found the drain plug
by: Anonymous

Underneath the tank (18" d x 48" l) was a 1/2" npt plug. Got it out.....the 2" plugs on each end didn't move even with a 5 ft cheater on a 36" pipe wrench so I guess I'm not going to inspect the interior. The stuff that came out wasn't as bad as I had expected.
Already got a 1/2" street elbow to a nipple to get the drain out from under the tank with a ball valve on it to keep critters out.
I'll route the pipe to have the valve where it'll be easy to drain/vent when I get this unit on line.
I'll get my motor & pulley ordered next week when I have money and it should be in operation soon.

Jun 07, 2017
3z172 rpms
by: Doug in

You're welcome.

Keep us posted, OK?

Jun 07, 2017
Thanks for the info!
by: Bud

Thanks for the info! Hadn't thought about the drain from the top side! I'll check it out in the morning. Looks like a stock 3.55" pulley will give me 758 RPM on the compressor.

Considering what a new compressor runs I think this was worth trading a table saw and a 48" mowing deck for - even with it being older.
Thanks again!

I found a useful calculator while hunting info -

Jun 07, 2017
3z172 rpms
by: Doug in

788 with 5 HP

specs here:

About the tank: Take another look - some tanks have a drain near the top connected to an internal tube to the bottom of the tank.

If it really doesn't have one, then it's up to you whether your skills are adequate to the task of adding one.

Good luck and have fun.

Click here to add your own comments

Add your own question or comment. It's easy to do. How? Simply click here to return to Speedaire Compressor Issues.