Drill and tap question

[AlexGross]

Here's the issue: I was attaching the oil cooler to the block and one of the mounting studs decided to back itself out. I went to remove said stud and it came right out...along with the threads that were once part of the case.

I am just looking for some advice...is this something that I could just retap (to just chase the threads out with a t-handle tap), or am I going to have to get a helicoil put in by a machinist? Just for reference, this is the upper right stud used to mount the Behr oil cooler. The engine is a 1975 911S with a magnesium case.
Thank you in advance,
Alex

[confused]

Helicoils are for aluminum, Timecerts are from magnesium. The threads on your hole are gone and they can not be brought back as they are. You must drill out the hole, tap it with a larger tap, then insert a timecert. There is a handy little kit that has everything you will need, just look up timecert. You can do this yourself if you can drill straight and have some mechanical ability however remember this is a one shot deal and if you drill at an angle or mess up the install of the insert you could ruin the hole forever.

Not nessecarily your fault. Old magnesium does this sometimes, especially a 75-77 case that has had thermal reactors or a 5 blade fan on it some time in the past.

A good rule of thumb before assembling an engine is to put a little added torque on every stud in your case before the engine is assembled. If a stud does not hold 5lbs over the recomended torque then you should install timecerts or you will never get torque and your engine WILL LEAK.

[Flat6pac]

Heli coil wont be the first choice, you need a time cert. The tap kit and insert is available on e bay to do it your self but a machine shop will probably charge less for the one shot.
With the mag case youll need to watch the bell housing studs because they will pull too.
Bruce

[safe]

But this isn't really a highly stressed stud. Heli coil might not be the optimal solution but it will probably work just fine. I would probably use it just because I have a set in the garage.

[pors1968]

I would use time sert . I use wurth time sert .

[Jcslocum]

I would use a heli-coil for this application. Not a lot of on and off of this part and not highly loaded. I have done some research on these in the past and found that Boeing uses over 300,000 helicoils on a 747 in alloys where higher thread strength is required. Put a wee bit of loctite on the case threads and screw in the insert. Clean the internal threads if the coil with a Qtip and lacquer thinner to remove any loctite that might squeeze thru. Let the thread lock dry before installing the new bolt.

Just my .002

[cgarr]

is that 8 or 10mm?

If its 8mm just tap it out to 10mm and use a step stud, 10mm on one end and 8mm on the other..

[AlexGross]

Thanks guys, that is kinda what I was thinking. I've never drilled and tapped anything before, but I've done a fair amount of fabricating and steel work, so I'm thinking that this could be within my reach. That being said, @Bruce: I've noticed that only one of the four bell housing studs has had a time cert inserted...indicating to me that the last time the bell housing studs were torqued down, one of them failed and had to be fixed. It just makes me wonder if I should just bite the bullet and have a machine shop put in certs all around. I think I'll give the bell housing studs a little torque test and see how everything holds up.

Also, I know this would be the MacGyver method, but here's what popped into my head: the stud that unthreaded the case for me actually is mounted into a through hole in the block...that is, the hole that the stud mounts in is open on the back end. Would it be too risky to just clean up the hole, and then pass a bolt through it, with a good locking nut/washer set-up to keep it tight? My concern would be that to generate the necessary strength that one gets from all of the treads of the stud in the case, I would have to torque the bolt down to firmly and could potentially damage the oil cooler mounting flange or the case itself...just thinking out loud.

[Ronnie's.930]

No to the nut & bolt idea above and yes to have the additional bell housing studs removed and the holes Timeserted. It reads as if you are leaning this way, but it would probably be a good idea to take it to a machine shop for this work since you note that you have never drilled & tapped before (as mentioned by others, you don't want to make mistakes with the case).

[keynsham1]

Why is a helicoil not a good option? As far as I understand, they will work in any metal. Timecerts seem to be an American invention. I have never seen them for sale in the UK. I believe that timecerts have a coarse thread on the outside which I suspect is preferable for the magnesium case, but it does require a much bigger hole that using a helicoil, which would reduce further reparability in the future.

[chris_seven]

Wurth UK have been selling Timesert kits for as long as I remember and always have stock of the common sizes.

The drill size for an M8 x 1.25 insert is 8.2mm diameter (Letter P Drill) and 8.3mm for a Helicoil so little practical difference.

I don't see much difference in either approach for this application although fitting the Helicoil is probably easier.

[JJ 911SC]

Originally Posted by AlexGross

...I've never drilled and tapped anything before, but I've done a fair amount of fabricating and steel work, so I'm thinking that this could be within my reach..

Joint the club...

I talked to the Time-Serts technical guys to advised on the length of the insert and got the kit from Amazon.

Easy steps by steps to follow. If you go with Time-Sert, get the optional Tap Guide to get the perfect alignment. I also got 2 of each length as they are only a couple of dollar each for future use (hopefully not) or re-sale.

I also did (my second tap) a larger one last year to fix the 14mm (PO special) back to a 12mm size on the transmission mounting hole. A bit more tricky working on your back...

Time-Serts to the rescue