Drilling firewall for motor mount

[rvamateur]

I suspect this is not a new issue but I have burned up a couple of bits just getting the first two wholes drilled..............have tried cobalt and titanium plated...........what say you?

 

[flion]

If you can get a pilot bit, that works pretty well. Drill the pilot holes, remove the mount, and then use a unibit to enlarge the hole in the firewall and finally put the mount back and finish with the pilot bit or a normal bit. Also be sure to use lots of lubricant and drill slowly. Slow through steel, fast through aluminum.

 

[lammers4699]

Drilling for firewall mount

I had no problems drilling the first hole with a step drill. I temporarily mounted the engine mount and found that the holes don't line up very well. I am not sure how you can drill with a 3/8" drill through the mount into a hole that is 1/4" offset. I wonder how other people did that. Do you just enlarge all holes and then struggle to get all the bolts in or is there another way? It is not possible to enlarge the holes in the firewall with a step drill with the engine mount installed, so much I found out already. Any suggestions?

 

[derLuigi]

you can use drill bushings (outer diameter matches the motor mount, inner diameter matches the drill you want to use at the moment). Making a set of them (having access to a lathe helps, they can easily passed around later) you can open up the holes step by step and really matching the motor mount.

 

[flion]

I would have totally ignored Van's holes except as a general guide to aligning the mount, and drilled my own pilot holes as I said in the post above yours. In your case, since the first enlarged hole is putting your mount out of line, I'd ignore it, reposition the mount with the guidance of the remaining holes (they won't be perfect, you just want the least offset on average) and then use the mount to make your own pilot holes and enlarge from there. Then you can deal with the first hole you drilled, either by struggling a bolt in there if it's not too far out or by re-drilling. If you have to re-drill it should be acceptable to fill the enlarged hole by putting mold release on the bolt, putting it in place on the firewall, and packing structural adhesive into the gap. Remove the bolt after it has set and flush the adhesive to the firewall and then install the mount; the bolt and mount pad will keep the adhesive in place and the adhesive will keep the bolt from wobbling about. You could also get someone to weld the hole in the weldment and re-drill but that would probably be a real pain in the butt.

 

[rleffler]

There are a couple long thread on the subject back 4-6 months. Just use search to find them.

I didn't have any issues with drilling the bottom holes that were offset, but still within the diameter of the intended final hole. The motor mount kept the bit aligned. I didn't have any issue with the bit taking material of the motor mount. Just make sure you start with a sharp bit.

bob

 

[Phil]

I stepped up the bit and used plenty of lube. You can't spin the bit too fast or that steel gets hard quick and you'll burn through bits. Also, once it starts cutting, don't stop the drill until you're done. If you heat the steel up and then stop the bit to take a peek at the hole the metal will get hard as soon as the cooler air hits it.

I found the best way was to step it up in size, use plenty of lube, and once it starts going don't change speed or stop until you're done.

Phil