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I have a MAH-112-C-4 relay that is supposed to be capable of working as a relay for fog light, head light, battery control, oil pumb control, fan control, air conditioner, harness.

MAH-112C Relay

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There is a flange on it with a 5.2mm hole but the relay specification does not state what screw should be used. The fact that the relay can be placed in a number of positions makes me suspect that there is a standard type of mounting bracket and screw in a car.

Does anyone if such a standard exists and if so what type of screw would fit the brackets?

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This type of addon relay is intended to be mounted to any convenient existing bolt that you have available. As there is no standard for what size bolts to be used in an engine bay or around the body, there is no standard size recommendation for mounting this relay.

What you will do is find an existing bracket or mount in the engine bay, then back the bolt out and stack this relay against the bracket, putting the bolt back through. You can often find a spot on the sides of the strut tower that will accommodate.

One thing to keep in mind about these relays, is they are can be sensitive to water and excessive heat. Avoid mounting in "splash areas" and near the exhaust manifolds to prevent premature wear to the relay.

==== BONUS ====

To make the install a little nicer you can use one of these relay sockets instead of spade terminals. Relay Socket

Slip a piece of heat shrink tubing over the end of your wire and over the end of the relay socket's wire, connect the two wires with a crimp on butt splice, then slide the heat shrink tubing over the end of the splice and heat the tubing to seal it all up.

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I doubt there is a standard. Most screws of this type are tapered metal screws which will fit the mounting hole. Every manufacturer could use a different screw, depending on the depth needed and what they were able to get cheapest. All it has to do is hold the relay in place, which means the head of the screw needs to be larger than 5.2mm. That is, of course, if they use a screw at all. Most common relays don't have the tab in question and don't get mounted that way. Instead, they utilize the friction of the tabs and socket to hold it tight for a lifetime of worry free operation.

  • Thanks for the feedback. Need a definitive yes or no to accept the answer, (upvote ok?) – SeanJ May 11 '15 at 14:00
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There is no standard fitting for relay mounts, and so thus the generic hole. Hella makes a 20/40 amp mini SPDT that has a metal bracket rather than a plastic bracket.

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They also sell connectors with pigtails with a weatherproof fitting.

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I'm not aware of any standard mounting that uses the screw fixing - generally standard mounts for relays are of the socket type, that the pins on the bottom fit in to, similar to that in the fuse-box

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