The International Cessna^® 170 Association

First this is a '48 C-145-2 with the 3 hole sump and accessory case. The engine had 5/16 studs at the bottom holding the accessory case and the sump together before I disassembled it for a rebuild and despite the '56 engine parts manual showiing 5/16 course thread bolts at the bottom (this with the 5 hole sump and case) I reassembled it as I found it using the studs.

We just finished its second installation to find an oil leak at the the accessory case and the oil sump. It is the second installation because after the first run up of the first installation there was a big leak on the right side of the accessory case and the sump. That turned out to be a heli coil that pulled out of the sump instead of tightening the sump to the acces. case. We removed and repaired the heli coil in the sump and reinstalled the sump and accessory case. I have looked all over for the correct torque for these studs but the overhaul manual doesn't have any. Continential has a SB with all the torgue values for all their engines but I can't find it there either. There also is no torgue for bolts that i can find if I was using them.

So far I have torqued the nuts (fine thread) on the studs to 120 inch pounds. This is mid range for a torgue spec (100 to 150 in. lbs. or about 8 to 12 ft. lbs.) from the Continental SB for a 5/16-18 "driven stud". Not sure it this woulb be a driven stud or not. This is the closest torque value I could find. 120 inch pounds is also the torque that stripped out the heli coil the first time but the heli coil might have been damaged prior to the first installation. I am reluctent to go with a higher torque because of the first experiance I had but the acces. case/sump is leaking slightly just above both side bolt holes.

I should also tell you that before the second assembly we checked the flatness of the accessory case and found some pulling around the bolt holes which we carefully dressed with a flat file so that there was less than .0015 thousands gap between the bolt holes and the case side when a flat bar spaned te bolt holes.

We used new gaskets of course and lightly coated each side of the gaskets with permatex aviation gasket sealer.

I also have installed a new F&M filter adapter and am wondering if this is some how causeing the oil pressure in this area to be higher than normal causing the leak. My idle oil pressure cold with straight 50 mineral is about 30lbs and rises to just over 60lbs at about 2250 rpm.

So what do you all think. What is the torque value for the accessory case when using 5/16 studs? Are studs the right thing to use for this year engine at the bottom of the sump and accessory case. I am thinking the torque wouldn't be any higher using bolts in this application as the weakest link is the heli coils in the sump. What say you. HELP.

Bruce,
You didn't mention it, so I thought I'd ask. On assembly of sump/accessory case, did you leave the sump nuts loose while you tightened the acc case bolts/nuts first, then tighten sump nuts last?
This allows the back of the sump to be pulled up tight to the accy case.
Max

Max
The second time we assembled the sump/accessory case we did. But of course if I didn't torque the nuts up tight enough on the accessory case then who knows.

Didn't think of it till to late the first time which as it turned out didn't matter.

Bruce,
I am away from the desk but I will check my notes on torque when I get home. Last time I did one of those it came out OK, no leaks. But mine had coarse thread bolts going into the sump. Those mag sumps can't take a lot of torque, even if there are heli-coils.
Max

Thanks Max I'd be curious what your notes say.

I called Continental and Columbia Air services who have been rebuilding Continentals darn near as long as theyv'e been made. Both sources indicated that since there is no specified torque you would use standard torque which for a 5/16-24 stud with a plain nut is at least 175in. lbs. according to Columbia and 180 in. lbs according to Continental. (Haven't looked for myself). When I pointed out that the weakest link was probably not the stud but the helicoil in the mag. sump, Continental said the helicoil would hold if it was installed right. Columbia said the mag sumps are pesky and the helicoil might hold, might not, but you still have to torque to at least the minimum.

So armed with my torque wrench now set for 175 in. lbs. (min according to Columbia) I retorqued the nuts and so far the leaks have dissappeared.

I just checked my C-145 parts book and my overhaul manual--they both indicate 5/16" cap screws ( either 3 or 5-- early or late model engines)into the sump helicoils thru the lower accessory case.
I'm not much of a mechanic,Bruce,but I think you're way better off staying with the studs instead of going to bolts at this location as per the manuals. Instead of turning under tork in the helicoil,with possible resultant damage,you're just turning the nuts. Much less chance of stripping out the helicoils in the sump, or the helicoils themselves, IMHO.
Sounds like they held up to the tork,and you got rid of your leaks. Great!

Eric

Threaded inserts (Helicoil is a brand-name) are stronger than the surrounding case if properly installed (drilled and tapped with the recommended size drills/taps and proper depth.) They actually transfer the fastener's torque to a larger area than non-insert threads and their material is usually superior (especially in aluminum or magnesium.)
Studs are more capable than bolts of accepting high torque without stripping because there is no frictional stress applied to the case/insert threads. Studs installed into thread-inserts are best of all.

Bruce, looks like you have this handled, but for future reference we should all have a copy of Continental's SB96-7B, service bulletin on torque limits. This bulletin has extensive tables on what torques are to be used for what fastener in which engine by specific location.
There are also tables for brass, aluminum and steel fittings, hose fittings, etc.
The 175 in/lb you used for your sump studs looks right, according to this.
Max

Max,
Great advice on the TCM Bulletin SB96-7B, I just went to the TCM WEB site and printed a copy for my files. I found the Bulletin at http://www.tcmlink.com/pdf2/SB96-7B.pdf
Johneb

Max
I had the SB you mention but was unable to find a torque specifically for that application. There are a list of standard torques but again I don't recall a table for studs. There is a tabel for "Driven Studs". I asked the TCM guy what a "Driven Stud" is and he said that would be the torque used to insert the stud. He also told me that the SB should have every torque for every engine that Continental made so if it's not there or in the overhaul manual, he wouldn't have it.

Bruce, I think I went with the torque for "Bolt, oil sump flange", even though it wasn't listed for the C145/O300 specifically. I didn't want to go over 175 in/lb in a magnesium casting.
Max