Engine Mount too narrow

The plan dimensions and the prepunch firewall dimensions for the engine mount to firewall bolts are:

Top Horizontal 33 1/2"
Bottom Horizontal 29 1/4"
Top to bottom (hole to hole) 19 1/8" *

* The plan provides a vertical (plumb) dimension of 19". As the bottom holes are narrower than the top holes, a little trig yields a top to bottom hole to hole dimension of 19 1/8".

My mount measures:
Top Horizontal 33 7/16"
Bottom Horizontal 29 3/16"
Top to bottom (hole to hole) 18 7/8"

I am narrow by 1/4" top to bottom. This is a much greater difference than I expected. I have already drilled the pilot holes up to 1/4" diameter to mount a rotisserie. At this point I can only eat 1/8" and even that is going to involve drawing all 4 holes the maximum amount possible. Not looking forward to that.

Any suggestions? Does anyone have an engine mount not installed that can provide a hole to hole dimension measured top to bottom? If there is any hope of getting a mount closer to plan dimension I would try to swap my mount with the mothership. The mount can be sprung some by hand but the 1/4" is a lot to work with.

Thanks in advance.

Robert

As I recall, I had to flex my engine mount a bit to get it to fit. Mine is a QB.

Robert---Email Jay Pratt and see if he has one, or if he has experienced this.
Tom

Having installed number of RV-8 mounts, I have never seen one line up exactly. I have been known to put quite a bit of stress on the mount to get bolts to go in to include straps and some cranking.

Mount

Kahuna said:

Having installed number of RV-8 mounts, I have never seen one line up exactly. I have been known to put quite a bit of stress on the mount to get bolts to go in to include straps and some cranking.

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Thanks for this info. I can definitely make this one work but I will have to eat a big bowl of Wheaty's first. I expected it to be closer than what I have.

DaAV8R said:

I expected it to be closer than what I have.

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Until the moment it was removed from the welding fixture, it probably would have matched your mounting holes just about exactly.

Because of internal stresses in the tubing (relieved by being heated), and other shifting and twisting that goes on during the heat/cool cycle of all the different welded areas, it doesn't necessarily want to stay in the shape it was held while being welded.

It is for this reason that the engineering text book description of a welded assembly is " an instrument of the devil"

Fortunately, we have a huge amount of service experience to show that forcing them back into proper position doesn't seem to be detrimental to there service life.

Curious and dumb question - why cant the welded areas be heated up with a rosebud and stress relieved while in the jig?

Not a dumb question.

It certainly could be heat treated and stress relieved, but given the service history of RV engine mounts it does not seem to be necessary.
It would simply add cost for something that can be done with a heavy rubber sledge hammer.
That is how I persuaded my engine mount into submission.

Jrskygod said:

Curious and dumb question - why cant the welded areas be heated up with a rosebud and stress relieved while in the jig?

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If you had a HT oven large enough . . .

I've heard that "eyeball" temperature control by color is so poor that the attempt to achieve stress relief can be more detrimental.

Said just for reference. I'll try to find it in my notes.

You can do it.

Yep, they are not all going to fit gust right. We just stood on one side and pulled up with some arm grease till it got close, then bolted it on. If you rap a towel around it, it will help from scratching the powder coat. The powder coat and a rose-bud don't get along to well, I would not heat one myself.
Hope this helps. Yours as always R.E.A. III #80888