Stripped thread in cast iron.

[Herbiev]

I'm having a big problem with stripped 3mm x0.5 mm threads in a cast iron cylinder block. Each time the thread strips I drill a bit deeper and retap but it soon strips again. I've tried filling the holes with JB weld and retapping but no joy. Any help greatly appreciated. :wall:

 

[canadianhorsepower]

I'm having a big problem with stripped 3mm x0.5 mm threads in a cast iron cylinder block. Each time the thread strips I drill a bit deeper and retap but it soon strips again. I've tried filling the holes with JB weld and retapping but no joy. Any help greatly appreciated. :wall:

I assume you are using a bolt..
tap it one more time but use a stud
instead and will be the end of your problem

cheers

 

[Draw-Tech]

Hey Herbie

Take the part to a local machine shop and have them insert a helicoil. It would cost about 100.00 to buy the inserts, tap, and insert tool. This will add a hard steel thread. Go to www.mcmaster.com search helicoil

Jack
draw-tech

 

[tornitore45]

I am not saying that tapping 3mm 0.5 in cast iron is impossible but, considering how CI crumble rather than cutting I would have switched to #5-40 or #4-40
0.5 mm is nearly 50 TPI

 

[Swifty]

Everyone has given good feedback, because of the grain structure of cast iron, it does not take kindly to fine pitch threads. You may have the same problem with helicoils, as you will still have to tap a fine pitch thread to accept the insert. Luc's comment about using a stud may be the answer, even if the threads crumble a bit, put the stud in with loctite.

If you have the room, you may be able to drill and tap a much larger hole and put a steel insert in, then just tap the insert M3 in the centre.

Paul.

 

[10K Pete]

Historically, studs were used in iron because of this issue. Studs were
coarse thread in the iron and frequently fine thread on the other end.
Look at older engines where the manifold studs were coarse/fine.

Pete

 

[bazmak]

I have used swifties suggestion in the past. If you have room drill and tap
say M6 as deep as you can,screw in and loctite a setscrew and then drill and tap M3 in the steel screw

 

[Wizard69]

As others have pointed out this is a tough thread for cast iron. You could try Timeserts which are a thread insert that in my opinion is a bit better than Helicoils. Usually in a case like this I might put a tiny amount of bearing mounting compound on the insert to help support it.

The other approach is to drill out the hole and for a reasonably larger Screw that you can drill out and tap. The general problem here is that often there isn't enough clearance. The stud you put in the hole should have a course thread and should be loctited in place. Let the loctite dry 24 hours. Cut the stud off and machine flush (also a potential problem).

You could also try a larger bolt with a coarser thread. Unless the cast iron has specific processing to strengthen it, threads are at best limited use. Even on old Chevy engine blocks I can remember my brother ""stressing"" to be careful with the oil pan screws. I'd suggest going to an auto mechanic but I doubt they have thread inserts that small just hanging around.

 

[Tin Falcon]

If you have room drill and tap
say M6 as deep as you can,screw in and loctite a setscrew and then drill and tap M3 in the steel screw

That would be like making your own slimsert. Again buying the "correct tooling for installation /removal would be expensive. but you should be able to improvise enout tooling for a few. All you ever wanted to know about slimserts and likely more in the below link.
Something I learned about in aircraft training. Also slimserts good fro spark plug hole repair helicoils not as much.

Slimsert Tech sheet
Tin

 

[Herbiev]

Many thanks for some great suggestions. I think I shall go for the stud/loctite option as this seems relatively simple and cheap. The only question is which Loctite
There seem to be millions of them that do similar jobs. 620 was my first thought.

 

[Swifty]

Herbie, 620 is made for loctiting shafts with a maximum of .0015" gap, you may be better off with one made for threads. I use 222 or 243 for threads, I happen to have both as they were leftovers from when I worked.

Paul.

 

[Herbiev]

Herbie, 620 is made for loctiting shafts with a maximum of .0015" gap, you may be better off with one made for threads. I use 222 or 243 for threads, I happen to have both as they were leftovers from when I worked.

Paul.

I looked at 243 Paul but the specs say 6mm to 20mm threads. 3mm may not work too well

 

[doc1955]

I would use one of these http://www.mcmaster.com/#97120a190/=zeya8z I've used other sizes and they work very well.

 

[Herbiev]

Thanks to some great advice from forum members I think the problem is solved. I found some Loctite 263 at the local Bunnings hardware store for $12.00. It is rated as a stud and thread lock and I found it to be exceptionally strong.