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I have an application where I am going to bore out stainless steel rod and then internally tap it with an M8 screw. What I am not sure about is what diameter rod I need to use to give the threads adequate strength. Can anybody offer some advice.

Many thanks

That's all going to depend on the application. How much load is anticipated, in what direction relative to the bolt is it going to be applied, and how deep is the threaded hole to be?

Just to chuck some numbers around, an M8 bolt will have a cross sectional area of around 50 sqmm, so to have the same strength in pure tension (same X-sectional area of metal) a rod with an 8mm hole in the middle would need an outside diameter of just under 12mm, call it 13 or 14mm to allow for the threads. However, any off center loading on the bolt will tend to try and buckle the side walls of the tapped hole so I'd be tempted to go for twice the bolt diameter - 16mm - but then I do tend to over-engineer stuff.

I'd be inclined to go for 13+, depends on what its going to be doing...

An m8 nut is generally 13mm, plus the shoulders- remove the shoulders untill it was a round part, ant it would be 13mm diameter.

Many thanks,

The load is along the long axis of the bolt. The thread length will be about 50 mm but the bolt will only go into it about 40mm. The load will not be too high but I am not sure how high until I test. The application is to fit flood gates to doors in our house. The fixing will be used to pull down the gate on to a wall and compress the sealing material and that will constitute the load. Any flood water will just assist the compression and if anything lessen the load on the fixing.

I have looked at the commercially available units and I am not impressed. The worst part about them is they usually have a channel permanently attached to the wall which looks terrible. With this type of fixing the impact will be much less. The plan is to make up a set for one door, test it and if ok fit to two houses.

Next step is to identify a sealing material which is good at forming against slightly rough surfaces. Something like, but much better than, the stick on draft excluded tape you can get for doors.

Thanks again for the help.

Doesn't sound like the load on the bolt will be very high at all, but I'd be interested to know how you're going to test your flood gate to ensure that it will form an adequate seal and will hold back both static and dynamic pressures for what could be a matter of days!