Do I need the tach drive?

Sid Lambert · #1 12-20-2013, 06:52 AM

I'm in the process of rebuilding my O-320 and have a question for you guys..

Why do I need to put the tach drive back in the engine? I have 1 EI and 1 mag but the RPM is read from the P-lead on the mag. It's always just had a cover over it. Seems like just something else that can break.

lr172 · #2 12-20-2013, 08:22 AM

I am new to aircraft engines, but have automotive experience. There appears to be no reason why the tach drive be installed, assuming you safety the cap. Without the tach drive and it's seal, you could theoretically lose oil quickly from that opening. With the tach drive and seal installed, a missing cap would cause a slow leak.

I will be installing it on my O-320 rebuild just in case I need it in the future or possibly sell the engine for an upgrade.

Larry

Mel · #3 12-20-2013, 08:41 AM

Quote:

Originally Posted by lr172

With the tach drive and seal installed, a missing cap would cause a slow leak.

Actually the cap is only a dust cover. With the tach drive and seal installed, there should be no leaks at all.

N941WR · #4 12-20-2013, 02:40 PM

As usual, Mel is correct!

You need both the tack drive AND the cap installed.

A friend left out the drive and just put the cap on and it turns out the Lycomings have an oil squirter that lubricates the tach drive. Without that drive in place, it just squirts oil out the breather tube.

nekit · #5 12-21-2013, 08:02 PM

I've looked and looked for a oil passage or oil squirter on the tach drive shaft and can't find one on my O-320. Anyone know where it is?
Thanks

lr172 · #6 12-21-2013, 08:09 PM

I am rebuilding an O-320 right now. There is definitely no squirter on the accy case. My block just came back from the shop yesterday. Next week I'll look and tell you if there is one. However my tach shaft if shielded to avoid liquid oil getting to the breather (i.e. getting splashed from the gears.

Larry

cajunwings · #7 12-21-2013, 08:35 PM

I sent a pm to one our very own & well known forum engine experts and he said there is no squirter unless the case had been modified in some way. He has seen the oil separator flange left out and that caused oil loss out the breather. He also has seen problems caused by running the engine with the tach drive installed but without a tach cable being connected. I did not install the tach drive. I cut a round gasket from some red silicone baffle seal material and covered the drive with the store bought metal cap.

Don B
RV 9 Rebuild in Progress.

N941WR · #8 12-21-2013, 08:40 PM

"Squirter" may be the wrong word. Oil may be splashing off one of the gears in there and deflected off the tach drive shaft. Either way, you want that shaft in there on the O-360's. The O-320's may be different.

If you leave it out and it becomes an issue, your spotters will see it just about the time you rotate. You can always add it later, but it is a pain to do then because you have to drop the sump to get the accessory case off.

As Mel mentioned, regardless of what you do, you will want to install a cover over the tach drive.

N941WR · #9 12-22-2013, 11:34 AM

Quote:

Originally Posted by cajunwings

I sent a pm to one our very own & well known forum engine experts ... He has seen the oil separator flange left out and that caused oil loss out the breather...

Don B
RV 9 Rebuild in Progress.

I checked with my friend who had this problem and just like my engine, the oil separator flange was pressed into the accessory case prior to assembly. So yes, it was in place, along with the tach cover. The only part he left out was the tach drive shaft.

If you have a forward breather tube like my O-290 had, then this isn't an issue.

lr172 · #10 12-22-2013, 12:09 PM

Quote:

Originally Posted by cajunwings

He also has seen problems caused by running the engine with the tach drive installed but without a tach cable being connected.

Did he mention what kinds of problems? I also will not be installing a tach cable and wonder if this is just oil leaking without a cap or possibly the shaft having runout due to no support from the tach cable.

Quote:

Originally Posted by cajunwings

...and covered the drive with the store bought metal cap.

Where did you find one? I have struggled to find anything in 7/8-18 thread. I was thinking of a silicone rubber cap if I installed the tach drive with seal, but would want something threaded and safety'ed if I left the tach drive out.

Thanks,

Larry