Technical 5/16-18 thread repair in aluminum

I have an aluminum timing cover with 3/8"-18 threaded holes for the fuel pump mount. The holes were 5/16"-16 from the factory, one was mostly stripped out, the other already tapped out to 3/8". Fuel pump has 5/16" holes, not wanting to drill the pump, I would like to reduce the 3/8" holes back to 5/16". Will a Helicoil kit get me there?

I've been reading the charts and the 5/16" coil has a .380-.400" outside diameter. 3/8" tap has nominal diameter of .375"

Just not sure if there is going to be enough material for the coil to hold on to. Holes are 3/4 deep, open on both ends.
Engine is together, removing and welding would be the next step, I don't think I want to drill out to 7/16" for a reducing insert.
I was going off Ames Performance parts site for Pontiac's, they said fuel pump mounting could be 3/8 or 5/16 and sold both sizes of bolts, I went with 3/8". I don't believe that was right, as all fuel pumps for this engine have 5/16" holes.

 

Maybe. But because of the thread pitch difference, the heli-coil probably won't have full perch.
A "Time-Sert" might work. Something like this....
https://www.mechanicstoolsandbits.c...l-inserts/time-sert-inch-steel-insert-5-16-18

Just have to find out what outer diameters are available. But because it is solid, you could maybe just epoxy to thread it in to seal it, and fill any gaps.
They are commonly used in VW cases for the head studs. 1/2" outer thread with a 10mm thread for the stud

 

If you want to avoid welding (and it’s distortion) you might consider a larger outside diameter Keensert to repair the holes. If going low buck, I’ve used 1/4” pipe plugs to fill holes then drill and tap what is need

 

Timeserts offer an oversize insert kit, but you are in the $120 range https://www.bing.com/aclick?ld=e8Al...=25f2c7320e0a176f82b6a3736793ab93&ntb=1&ntb=1

 

This may sound like "Snake Oil", but take a look. I used Loc***e Thread Repair to repair a stripped thermostat housing bolt hole in the intake manifold of my Corvette about 35 years ago. I followed the instructions closely and it must have worked, because when completed, the bolt took and held the specified torque when I re-installed the thermostat housing. I still have the car, and that repair has held through a valve job, a total engine rebuild, and at least two thermostat changes. The repair was quite similar, being a through hole using a 5/16" course bolt in an aluminum manifold.

 

If you have the room, I would suggest you drill and tap the cover to 7/16" and use this style of insert.



https://www.mcmaster.com/products/t...steel-easy-to-install-thread-locking-inserts/

 

+1 these are a great solution, i use them on castings where the hole is not salvagable by helicoils.

 

This seems like the simplest solution for your problem. No drilling or tapping or metal chips to deal with.

 

This product works IME

 

I've done this with success multiple times. The 5/16 hole will be no problem, I would Helicoil that one for the better strength. The 3/8 hole is a bit iffier, but if it isn't stripped it will work. The tapping bit diameter on a 3/8-16 bolt is .3125", well under the tapping bit size for the Helicoil of .328. Yes, because of the different thread pitch you won't have the best thread engagement possible and I would recommend using stacked inserts for maximum thread depth along with some red loc***e on the inserts and use bolts, or better yet, studs that fully engage all the threads. Don't get carried away when tightening the bolt or nut and you shouldn't have any issues. I'd just run the Helicoil tap right into the 3/8" hole, no drilling needed, but go slow. That will give you the best possible thread engagement.

 

I should add the one fly in the ointment... If you don't pull the cover off, you're going to lose the Helicoil drive tangs to inside the motor. If you have absolute faith that your oil pick-up screen is good, no harm, no foul. But if one got into your oil pump it won't be good....

 

How rare is that timing cover?

 

Have a shop make a pressed fit plug wit a flange on one end. Have them make the stripped hole the appropriate size. Have the plug drilled and tapped for 5/16 ths and press in from the inside.
I like spending others $$

 

Nutserts. Much better than a helicoil in my opinion. Drill it oversize, tap it, loc***e the nutsert, and BAM!

 

Not sure how a nutserts would work in this particular case, but work great for floor pans, body mounts etc. bought a 32 frame that was boxed with no body mounting nuts welded in before boxing, & nutserts came to the rescue.

 

https://www.amazon.com/***ortment-G...jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==If the cover is off and you can install from the back side these would not require as large of hole as the threaded inserts.

 

3/8 to 5/16 adapter studs. Simple and done.

 

One thermostat housing bolt on my 3x2 intake had ben enlarged to 7/16 and then re-******ed. Drilled and tapped it for a br*** pipe thread plug. Ran it in nice and snug, cut it off flush and finished it with a flat file. Drilled it 3/8-16 and used a stud. Goes great with the 5 heli-coils

 

https://www.mcmaster.com/91349A150/ Turn or grind the hex off

 

Thanks for the advise. The cover is not that rare so I may just hunt down a second one. I was sure hoping the Helicoil would clean out the other thread. I am also looking at the inserts. The studs would be an easy way to go, but this is a restoration engine, so it needs the correct bolts, nuts wouldn't be right, and neither are the 3/8 bolts.

 

A machinist could take the head off a 3/8 bolt then put it in a lathe..Machine the bolt to 5/16 so it could be threaded to be an adapter stud. Stainless would be more compatible. My brother-in-law had a Ford 3/4 ton with aluminum heads that had a spark plug strip out. The garage filled it with weld, drilled and tapped with new threads. If you are going to look for another one then that may be an option to try as there is nothing to lose.

 

They make a product called a "twinsert" which is designed for holes which can't be repaired by a standard size heli coil.

 

I once repaired a stripped thread on a timing cover using that Alumiloy soldering stick. It worked pretty well, but it takes a little practice to get the heat just right.

 

I'm digging up a piece of test aluminum right know, going to give it a try and see just how much torque I put on it.