Dumb question - bolt size engine case ground

Rick_A · #1 02-05-2008, 11:47 AM

I feel really dumb but I can't seem to figure out what size bolt I need. There is a threaded hole on the rear of the engine case that I want to use for the ground connection. It's sorta behind the Dip stick tube near the prop oil line. It's an ECI O-360 case.

It's appears to be a 5/16th bolt and I've tried both 18 & 24 tpi bolts and can't get either one started.

Anyone know what size bolt to use?

TSwezey · #2 02-05-2008, 05:12 PM

Did you try a metric size? They have them in Home Depot in the specialty section.

szicree · #3 02-05-2008, 05:18 PM

Why in the world would one solitary hole on the entire engine be tapped metric?

TSwezey · #4 02-05-2008, 05:59 PM

Beats me? I was just offering a suggestion.

szicree · #5 02-05-2008, 06:02 PM

I hear ya Tom, but these engine cases are pretty pricey (and soft). I'd be cautious about threading any old thing into em.

Mel · #6 02-05-2008, 06:26 PM

Most all holes tapped into a casting are coarse thread. You should be able to tell by looking whether it's fine or coarse thread.

Radomir · #7 02-05-2008, 06:29 PM

Do you have a pic of that? I can't remember anything in that area.. except the 1/4-20 hole used for the baffle.. which is directly behind the oil filler tube (it's kinda hidden by the tube)..

Rick_A · #8 02-05-2008, 07:31 PM

Here's a picture of the location but you can't see the hole very well because it facing aft. It the one right above were the dipstick tube is safety wired.

Here's a picture of the actual hole taken using a mirror. It can be seen between the engine mount and the oil line.

szicree · #9 02-05-2008, 07:58 PM

I do seem to remember that hole being a baffle mounting. Speaking of that, I get the impression from the pic that you are doing wires and plumbing before baffles. If so, you may want to reconsider. The baffles have a way of interfering with wires, hoses, etc.

lostpilot28 · #10 02-05-2008, 07:58 PM

I would recommend running a tap into the hole and back out to clean it. Use the correct size, of course. When I built my engine with an A&P I/A, I had the task of putting the valve covers on and managed to have one that wouldn't let the bolt start threading in. Simply cleaning up the threads with a tap did the trick.

I'm guessing you messed the threads up a bit by using the wrong bolt (it probably is 5/16th, but course or fine is anyone's guess). Clean it up and you should be good to go.