Diamond Geezer Posted June 24, 2022 Share Posted June 24, 2022 Hi guys and ladies just got an engine mount from spec come with self tappers never used self tappers before are they ok for a os 46la Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted June 24, 2022 Share Posted June 24, 2022 Personally I would use 3mm mounting bolts either tapped into the mount or through bolted with nyloc nuts. Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted June 24, 2022 Share Posted June 24, 2022 hello DG....self tappers rule,i've used them on an OS 91/4st...no problems...you'll get different opinions.... ken anderson...ne...1..self tapper dept. Quote Link to comment Share on other sites More sharing options...
Engine Doctor Posted June 24, 2022 Share Posted June 24, 2022 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. Quote Link to comment Share on other sites More sharing options...
Andy Stephenson Posted June 24, 2022 Share Posted June 24, 2022 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. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 24, 2022 Share Posted June 24, 2022 I use self tappers every time into nylon mounts. Take care drilling the pilot hole. Must be vertical, must be central to the lug. 46LA probably needs 3.5mm OD screws? Quote Link to comment Share on other sites More sharing options...
John Lee Posted June 24, 2022 Share Posted June 24, 2022 Modelfixings’ cap headed self tappers are excellent in this application. I always used them as my default when I flew IC & I still have a couple of packets of No6 (3.6mm) in my screws draw. https://www.modelfixings.co.uk/cap_head_woodscrews.htm Quote Link to comment Share on other sites More sharing options...
Jon H Posted June 24, 2022 Share Posted June 24, 2022 I always vote for machine screws and nyloc nuts. I think M3.5 for an OS 46 Quote Link to comment Share on other sites More sharing options...
Matt Carlton Posted June 24, 2022 Share Posted June 24, 2022 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! Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted June 24, 2022 Share Posted June 24, 2022 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). Quote Link to comment Share on other sites More sharing options...
Matt Carlton Posted June 24, 2022 Share Posted June 24, 2022 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. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 24, 2022 Share Posted June 24, 2022 The head of the screw clamps on top of the lug. Thread contact is not relevant - I think. Happy to be corrected. Quote Link to comment Share on other sites More sharing options...
john stones 1 - Moderator Posted June 24, 2022 Share Posted June 24, 2022 2 hours ago, Matt Carlton said: 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. Make it flat. Quote Link to comment Share on other sites More sharing options...
Ron Gray Posted June 24, 2022 Share Posted June 24, 2022 @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. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted June 24, 2022 Share Posted June 24, 2022 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? Quote Link to comment Share on other sites More sharing options...
David Ovenden Posted June 24, 2022 Share Posted June 24, 2022 Belt and braces myself. I drill and tap GF/plastic engine mounts and use cap head hex socket screws into the tapped holes with nyloc nuts underneath as well. Then the bolts never come loose! Quote Link to comment Share on other sites More sharing options...
Ace Posted June 24, 2022 Share Posted June 24, 2022 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) Quote Link to comment Share on other sites More sharing options...
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