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They were very popular some years ago and fine if you pre drill a guide hole for the screw to fit into. Make sure the guide hole is wide enough to accommodate the center part of the screw so that the threaded part can cut into the mounting material. 

The downsideof using ST screws is that unless the guide hole is not central to the engine mounting hole then there is a real risk of breaking the engine mount lug.

Provide you get the ST screws central to the engine mounting holes then they will perfectly fine and in my experience have never come loose .

Personally I now drill and tap a thread in a mount or use cap screws and bolts but that's just my choice. 

 

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I've found that even M4 threaded holes in a plastic engine mount will strip after a few tightenings. Self tappers have a much coarser thread which suits softer materials better.

If you use machine screws then you would be advised to use a nut, the problem is that the underside of most plastic engine mounts are tapered which doesn't allow it to be tightened properly without first creating a flat.

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Depending on the mount, I think self tappers are fine, especially into the glass filled nylon mounts with the wedge shaped arms. The only proviso is that I try not to remove said screws once they are in. If I need to adjust anything significantly, I'd rather spend a few quid on a new mount than risk redrilling etc. 

 

Always machine screws and tee nuts or nylocs for fixing the mount to the firewall however! 

 

 

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IMO it's the fit in the engine mounting lugs that is important (3mm for an OS 46LA?), a ST is not necessarily going to be the correct diameter plus the surface of the ST in contact with the lugs is going to be less than a machine screw (if it's a fully threaded one).

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I hadn't thought about it in that way Ron. Interesting. Maybe tapping a thread into the nylon mount would work, otherwise on some mounts, fitting a nut isn't very straightforward, at least to me, as the underside of the mount arms isn't flat. Perhaps I'm being dense. 

 

 

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@Matt Carltonyou can always put the machine screw in from the bottom of the mount with the nylocs washered against the engine lugs. If you can’t get at the heads then use slotted heads and solder a wire between 2 of the bolts.

 

@Nigel Rthread contact is relevant, a bolt that is too small will allow movement which will cause elongation of the lug holes.

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The clamping force should allow no movement, as i understand it.

 

You're saying the bolt is not clamping, or only partially clamping,  and is only restricting movement because the lug contacts the thread?

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loads of Irvine 53's (I miss them ?) over 10+ years, all ST using 3.5m OD with a pilot hole and a washer under the head = never had one work loose or move.

When I have used cap heads with nuts on a tapered underside, I have just used a drum sanding disc to create a flat and radius to prevent a stress point. ( as JS1 mentioned)

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