MC8 AC Installation

PNWorBUST72 · April 14, 2018, 03:50:55 PM

So how do you guys suggest I do this since tI need to cut the main center beam here? I want to frame them out now while I have the ceiling open and I can add them in as I need later.

Gary Hatt - Publisher BCM · April 14, 2018, 04:50:10 PM

Now that you already made the cut you have already lost the structural integrity so you may as well cut out the rest. But when you weld in the new pieces you will have to jack up the roof in this spot a bit so it is all aligned properly before welding otherwise you may have a low spot in your roof which may eventually pool water and leak. It is always better to weld the new pieces in first then cut out the member you want to remove.

thomasinnv · April 14, 2018, 04:52:57 PM

Box them in with square steel tube welded in place, then cut the hole. the main roof center steel is structural so don't cut them all out at once, do them one at a time complete.

buswarrior · April 14, 2018, 06:41:09 PM

Heed the warning about allowing a water pooling droop towards this now unsupported hole.

Add the bouncing weight of the AC unit...

Restore structure and ensure the height of the roof centre line before welding.

Often, the roof AC are mounted off centre to maintain that centre spar.

Happy coaching!
Buswarrior

PNWorBUST72 · April 14, 2018, 07:00:13 PM

Nothing has been cut, thats all marker.

Suggestions on size/layout of the steel to replace this? Is the pooling just because it will bow without being raised first? Is this something a beginner should mess with or bring it to an RV shop or something?

Gary Hatt - Publisher BCM · April 14, 2018, 07:33:54 PM

Quote from: PNWorBUST72 on April 14, 2018, 07:00:13 PM
Nothing has been cut, thats all marker.

Suggestions on size/layout of the steel to replace this? Is the pooling just because it will bow without being raised first? Is this something a beginner should mess with or bring it to an RV shop or something?

If you have the ability to weld it and have some common and mechanical ability you should be able to do it. If you have problems, a professional welder can always make it right. The thing about bus conversions is that they are a learning experience and the more you can do by yourself, the better you will feel at the end when you tell all of your friends, you built it yourself. So what if you make a few mistakes, we all do.

bevans6 · April 15, 2018, 04:44:14 AM

Never cut that structure (you haven't yet) because it is a significant part of the strength of the bus. Cut a 14" square hole offset to one side, box it as you like (I used pressure treated wood) and that should be fine. If you do it near a cross-beam, it will be quite stiff. I used wood for two reasons - to maintain the slight radius of the roof in that area, and to avoid corrosion issues between steel and the aluminium roof. The thick gasket of the AC unit takes up any of the slight radius.

Brian

scott332 · April 15, 2018, 05:08:35 AM

I framed mine in with 1 1/2" x 1 1/2" 16 ga. steel tube just like you have drawn. Making sure to weld the original center brace to the cross tube that I was going to keep. The center brace that was going to be removed, I didn't bother welding to the cross tube. The original framing on my bus was also 16 ga. steel, so by adding 2 tubes approx 7" to each side, I don't feel there was any compromise to the structure of the roof.

buswarrior · April 15, 2018, 05:53:55 AM

There were busnut pioneers who believed that a wood frame for the AC had an impact on vibration transmission into the coach.

Roll your own, smoke your own?

Those who failed to get a centre hole braced properly don't come back on here to brag about it.

In all busnut things, remember that these Boards document success, rarely do we get balanced reporting...

Structure has to both hold up the unintended added weight of the AC, and not let the roof stretch fore and aft. MC8 are good at letting their foreheads droop, and then their faces, broken welds in the roof above the front cap, and that's in unmolested coaches. The roof does provide some support/tension to what hangs off it, not as much as a GM, but some.

Be sure to make your changes tight and in line, no matter your direction.

happy coaching!
buswarrior

PNWorBUST72 · April 15, 2018, 06:29:07 AM

I am not ok with the ac's being off center...I am anal about those sorts of things.

Should I rivet the outer skin to the new braces?

My 2nd rib going from left to right is a smaller size, not sure welding a 1 3/4 tube to it will be easy...

buswarrior · April 15, 2018, 07:03:12 AM

Yes, rivet the skin, everything pulls on everything else.

Nice and tight.

hapy coaching!
buswarrior

PNWorBUST72 · April 15, 2018, 07:16:24 AM

I am thinking it would be better to install the two longer pieces, rivet the skin to those BEFORE we cut the middle bar...

TomC · April 15, 2018, 08:55:18 AM

Please rethink your obsession about the A/C's being off center. I did not want to cut the main middle support. I have three roof airs. The front is mounted to the right to give a better angle to the air to aim at me going down the road-do not have dash air and with a dash fan works just fine. The other two are mounted to the left of the middle support which lines up with my bathroom on the right sticking out 4ft, then 2ft of aisle, then 2ft of cabinets. With the airs being offset, they blow past each other, not into each other. Course mine are standard hang down diffusers, not central vented through the ceiling. Hence, my 3-13,500btu Colemans cool the bus just fine-actually I usually only run 2. Offset the A/C's and save yourself ALOT of work. Good Luck, TomC

Jim Eh. · April 15, 2018, 02:52:09 PM

Besides, you don't have to be a good welder ... just a good grinder.