Simple Post: Whats the best way to machine 1-2mm in 6 currently existing slots of 3/8" steel mounting plates? (With household tools, drill/dremel/file). Could access a mill but it'd be a big hassle and would prefer not to.

Background/Long Post:
So I got the GTSpec Strut Bar in yesterday. No need to talk about whether it works or not. My mindset is I won't notice a difference but I'm trying to dress up my engine bay a bit and got it for a good deal.

Anyways, since my strut towers are pushed in for the additional (-) camber, mounting the plates are very tight. It'll fit over the 3 bolts, but the included nuts are designed to thread into the slot opening, but with the bolts closer together, they are sitting up against the side of the mounting plate slot, thus no room for the nut to go down into the slot.

So, I need to extend the slots inwards towards the center about 1-2mm maximum. The center hole of the mounting plate is large enough to move inwards by 1-2mm so there'll be no problem there.

My question is: With my household tools, what is the best way to machine these slots? I believe its 3/8" Steel mounting plates, don't believe they are hardened steel.

I don't have a mill, as that would clearly be the best/cleanest solution.
I have a drill, drill bits (not HSS), a dremel, various attachments, and such amateur tools.

Initially I was thinking a dremel, but since they are such a high RPM tool, it won't have much torque and may not be the best in this situation.
I'm starting to think a thin long round coarse file may work best, but not sure how long it'd take. Its 3 holes per plate, so 3/8" x 12mm material.

Thanks!

 

Coarse file would work...just take a bit of elbow grease. The longer the file the better. If you have an air compressor...I'd use a reaming bit on a die grinder for speed...then file to finish.

 

I think you are way better off finding a machine shop to do it. It will take them 5 mins, cost little and look pro.

Filing holes has got to be a tiring, hand cramping job.

 

This is true. I do have access to a machine shop and they'll let me use the mill which would be a 5 min job to clamp down etc.

I'm thinking a dremel w a cutting bit would take the tiny bit of material out that I need. I also just took out the bolt that attaches the plate to the brace and noticed its held by two slots. This may be an easier solution as I can just elongate the two slots on one side (yes itd b off center by 2mm) don't think I'd b able to tell. And this would be more hidden behind the bolt/nut and its way less than elongating 6 bolt slots on the plate.

 

Got at them with a cutting bit with the dremel. Cut at a nice low speed (well relatively low for a dremel, ~15,000 rpm) and worked at the two slots slowly.

I elongated them about 2mm or so. Eyeballed it. Then finished the hole with a cone shaped grinder bit. Nice smooth and polished. Roughed up the surface, painted OEM dark silver. Polished the strut bar, waxed, now mounted. I'll take photos when its not disgusting out.