HELI-COILING
Stan Stephens sorts out an RG500 with issues.
In June 2016’s issue of CMM, I covered the stripping of an RG500 engine and someone had bodged all of the head studs in the barrels. Some of the stud threads were just stripped and needed Heli-coiling, others had Heli-coils that had pulled out and some were inserts that had also pulled out. There are 24 head studs in RG500 barrels and some looked okay. Now this applies to all engines, if you have some head bolts that have stripped threads don’t assume that the ones that aren’t stripped are okay. It’s a pain to Heli-coil or repair some of the threads and then find when building the engine that as you torque the head nuts down some more of them won’t torque down and start stripping. Remember, the same person that stripped some of the threads also tightened down the rest of them! What I do, and I have tried to show in a photo, is to put a spacer over a stud with a washer and nut on and torque it down. If it starts to pull through then that’s another one for repair. It’s tedious but do that on all the studs – its better to find out then rather than later. I am sure CMM has covered fitting Heli-coils in the past, so I’ll just run through it briefly. If it’s an 8mm stud, drill
out the stud-hole to 8mm and a little deeper than the original hole. It’s best to use a pillar drill to make sure that the thread-hole is vertical. If you have heads with 12 stud-holes and the studs aren’t upright, you will have a problem. The Heli-coil kit comes with two thread taps, a tapered one that you run through first and the other one that you cut the final thread with. When fitting the Heli-coil, do it carefully, put a little grease on the coil, if you think the coil has not gone in perfectly, pull it back out and fit another one. Just before the coil is fully home, with about one thread to go, cut off the remainder of the coil with a pair of side-cutters so that the coil will finish just below the surface. There are various thread inserts available but I always use Heli-coils for no other reason than I always have done! (Editor’s note, as Pip said last month, he prefers Time-serts, rather than
helicoiling them – Heli-coil is the trade name BTW.) On this RG500 engine there were many head studs that needed Heli-coiling and all the repairs went okay but the ones that were going to give a problem were the ones that had inserts fitted that had pulled out leaving a 10mm hole and some of them had stripped 12mm holes! What has to be done here is to make some really large inserts. There are a few ways to make them, some quite easy that the home mechanic can do. One way is to use a sleeve-head nut, as found on many engines, the old LC is a good example, which has an 8mm internal thread and a 12mm outside diameter. Use a 12mm thread die to put a thread on the outside of the sleeve nut, then drill and tap a 12mm thread in the barrel. Measure the depth of the thread in the hole you have just tapped and cut off the
sleeve nut so that it is very slightly shorter, this is so that when the insert is screwed in it will be just below the surface. To fit the insert (which now doesn’t have a hexagon on it) screw an 8mm nut and bolt into it and lock the nut down onto the insert, put a little Loctite on the thread of the insert and screw it in, then remove the nut and bolt. An alternative is to use a piece of 12mm stud or a 12mm bolt and drill and tap an 8mm hole down the centre of it. The problem I had with some of the RG500 threads were that the holes were already 12mm and stripped. I had to make some special inserts on the lathe with a 14mm outside thread and an 8mm internal thread. In fact for two of them I had to make an insert with a 16mm outside thread. When fitting the heads always tighten them to a low torque first, say 14ft/lb and then re-torque them to 18ft/lb. Then you’re done!