I've helicoiled them before with no problems. Making cylinder studs is a bit tricky, the threads need to be rolled rather than cut to avoid stress risers. Even Ducati have failed at making studs, the 900 engine from the early nineties had studs that were too hard and snapped like a carrot, drilling those out when they've snapped is a pain
I have used Helicoils in the past and as a Mechanical Engineer by trade have to point out that the weak point in the repair is not the insert but the remaining aluminium it is anchored in to. In theory, a fitted helicoil is stronger than the original machined thread in the casing - in fact, I use them on occasions to reinforce magnesium casing threads. Once the helicoil is fitted, inserting and tightening the stud will force the coil outwards and in hard contact with the recut aluminium thread (due to the 60 degree angle on the contact faces between helicoil and aluminium). Using a bush is easier to install and can be more robust as it allows for a larger tapped thread in the damaged casing and for even a coarse pitch thread, but it sounds like you don't have enough meat in this area. In summary:- 1.Check that the drilled tapped helicoil thread in the ali is accurate for diameter, if it is oversized it will result in a slack insert. Better to use a pillar drill or similar than do it by hand. 2. Use the longest helicoil you can and trim the excess off rather than using one that is too short. This is a high stress area and Engineers use a rule of thumb of 2 x the thread diameter as a design thread depth with more not being unheard of in alloy. I think that Helicoils only come in a maximum of 2D lengths from memory... if the original thread is longer than this, I would be wary of Helicoils as Ducati have done the sums..... 3. Rely on Loctite only to prevent the helicoil from unwinding should you remove the stud at some point, it wont prevent aluminium failure in the tapped thread.
Steve, Many thanks for reply. I have chosen the "time sert" bushes. I believe the helicoil and time-serts bushes are the same thread on the OD - M12 x 1.5. The original thread is M10 x 1.5 so the outer and inner thread pitches correspond to each other and somewhat overlap as they go down the insert. There are larger dia bushes at I think M14 but that does not leave much wall. Keenserts quote 1.5D boss diameter from memory, I have 20mm effective boss dia. See below reply from timesert. ----------------------------------------------------------------------------------------------------------- Hello, Sorry for any delay. You should be able to install the insert in a wall thickness as thin as 4mm. We would not know exactly want force is exerted on the casting when the insert is installed. We need to cold roll the last 3 threads to lock the insert in place but would not know exactly what the force would be. Just make sure you use oil on the install driver tool like the instuctions. Best Regards, TIME-SERT -------------------------------------------------------------------------------------------------------------- Actually I have done a test piece, the "wire insert tap" for a helicoil matched my M12 x 1.5 tap, and the insert fits well into this, I also need a counterbore to sink insert into casing. I have made with the help of someone at work, OK someone at work made it, a jig so I can bolt this to the casing and insert a guide bush to drill out the hole squarely. So, Iam just waiting for him to finish that. I went for the bush as they come in 24mm long which is the original thread length and the bush fits nicely on the end of the stud. Yes approx 2D is generally used to make sure fastener fails in shear before threads pull out. Obviously the M12 can take more force to pull out than a M10. Rightly or wrongly I don't plan to use the time-sert locking mechanism. I have tapped out the extra material in the bottom of the inserts used to push the insert into the housing. On installation I plan to locktite the bush to the stud and use it as if it were a normal stud with larger thread on the end anyway we shall see. Will post some pics when done. Cheers Gaz.
Well finally reached a milestone - inserts done. As I mentioned before I had a jig made. I located the jig and guides with the 8.5mm tapping drill and bolted it down. I then drilled out the threads, I did this in stages 9/9.5/10 and finally the 13/32nd 10.32mm this is the size from the Timesert web site. It was a bit grabby but seemes to look OK. Possibly could have done with 9.25 ans 9.75 drill, I then swapped the guide to tap the threads by hand. The tapped hole needed a counterbore in the top. Timesert say 1/2" and probably abour 4-5mm deep to get the studs to the original depth. I chose to grind the shoulder on the timesert down to just under 12mm, and drilled a 12mm dia x about 3mm counter bore. I did this because I was close to breaking out. I worked ont the principle that if you got the drill more or less square then once it was drilling it would follow the hole. Time sert with shoulder ground down and fitted to old stud (silver). New stud (black) at top. Jig and Guides M12 x 1.5 counter bored hole for Timesert Stud and Timesert in place. Stud not as low as originals but feel there is enough thread on the other end to be OK. Timesert and casing covered with clear tape. Original M10 hole I used grease on the drills and tap to capture the swarf and cleaned the holes out regularly as I went, especially tapping the blind hole as swarf gets compacted at the bottom. We shall see how square they are when I put the cylinder head back on. Cheers Gaz
