tomfiorentino

Guys,

I don't know what it is, but every time I drill and tap a motor mount (glass filled nylon type) the front bolts almost blow out the side of the rail where the beams narrow. Sometimes, I can see the tapped threads bulging on the side of the mount.

I just taped a Dave Brown mount in an Ultrasport that my 13 year old is having a ball building...and the same thing happened.

The mount is sized properly, and with an OS 65ax I need to use 8-32 bolts. They grab really well, but it bothers me just the same.

Anybody else experience this issue?

Tom

earlwb

I was using 4x40 screws for the .20 size engines (.15 through .36) and I was using 6x32 screws for the .60 size engines. For me using 8x32 screws was something for using on 1.20 size engines.  Maybe it is the screws that are too large.  But then if the screws fit the holes nicely then it isn't a problem.  But I have noticed a little bulge sometimes with the screws in the motor mounts sometimes. But it wasn't as pronounced as you were describing.

I usually have the motor mounts butt up against the crankcase so that the screw holes tend to be centered more in the motor mounts. But there have been some engines where the holes are farther out or closer in that can make that a problem getting the holes centered better.

flyinwalenda

My Feedback: (5)

Yes. Sometimes a little grinding on the inside center of the mount will help keep the mount legs closer to parallel  as some engine cases tend to make them bow out,

tomfiorentino

OK thanks for that response. The 8-32 bolt fits perfectly in the 65AX so that is why I used them instead of the 6-32. In terms of butting the case, I have found that usually I can't get the spinner backplate out far enough. So maybe I should be using spacers between the mount and the firewall. I have to go to my LHS this afternoon and I think I am going to get another mount; time for a "re-do"!

One more thing I should have metioned. I have noticed that the beams of the nylon mounts have a slight "toe-in" which exacerbates the problem. It's slight, but the tolerances on the front-end aren't that great to begin with!

In any event thanks again for the responses...I'm just thrilled that my son has enthusiasm for working on this! We have the fuse framed up and over the weekend, we joined the wing panels, applied the center section fiberglassing and shaped both wing tip blocks. It won't be long now as tonight we will be putting on the turtle deck and starting the alignment rigging of the wing/stab/fuse.

Thanks again!

Tom

Scar

My Feedback: (3)

I never tap threads in a motor mount, I just drill all the way through and use an Allen screw with a lock-nut. The stresses on the mount are then all compressive, no tensile stresses with power/torque.

Just my 2¢ worth,
Dave Olson

flybyjohn

My Feedback: (2)

I have had that problem every now and then. I think that sometimes drilling the hole with the recomended drill bit does not always work. The plastic sometimes stretches as the hole is drilled and then shrinks again when the drill bit is backed out. So in a sence, the tap is going into too small of a hole and bulges the plastic out. Another thing to try is to use a very sharp tap and instead of screwing it all the way in on one go, take it in a turn and back out and then in another turn and then back out and then in another.... you get it. This may actually cut the threads into the plastic instead of deforming it out around the tap. I usually do not have all the drill gauge sizes so I use inch sizes and get close and choose the larger size that stradles the gauge size I am suposed to use when I am doing my mounts.

tomfiorentino

Flybyjohn...so sorry to hear that you have had that problem every now and then and that your drill bit doesn't always work {read the rest of your post and it is absolutely histerical}; you may want to try the little blue pill

I bet you are correct on the deformation comment though and I hadn't thought of that. In fact, I am so worried about the drill bit wandering I usually push pretty hard to move it along. That could make what you are indicating even worse yet.

Thanks pal,
Tom

dennis

My Feedback: (90)

No 55 or 60 needs anything larger then a 6-32 allen bolt. Your not mounting a Continental engine to the model. Asian manufacturers usually love 4mm bolts and we detest them. To make sense of the situation just sleeve the engine bearer hole with brass tubing or plastic tubing and then drill and mount the engine using 6-32. I assure you the engine won't leap out of the mount.

dennis

ThumbSkull

My Feedback: (57)

I use 4-40 allens/nylocks all the way up to Saito 125.
Plenty strong to hold it and if a turf incident happens, I'd rather break the bolts than the crankcase.
6-32 for up to DZ 1.70

airraptor

My Feedback: (66)

+1

you could use 10x32 nylon bolts to hold an engine on.

it doesnt take much at all.

KW_Counter

Drilling then tapping sounds like lots of work.
The more work the more chance for an error.
In those mounts I drill then use a sheet metal screw.
If it gets stripped for some reason, hasn't happened yet,
then I can enlarge the hole a little and use a bolt and a locking nut.

Good Luck,
KW_Counter

earlwb

I always drill and tap the holes in the motor mounts. it works great. So I don't have a problem with people drilling and tapping the holes. But in the past I have used sheet metal screws and also nuts and bolts in holes that go all the way through. So it isn't a big deal with whatever someone does.

Now I don't suggest using nylon screws as you cannot really tighten them up well. If you try to tighten them up good the heads break off. Plus the plastic dries out and becomes brittle over time. It is something prone to failure.

blw

My Feedback: (3)

Socket head 6-32 for up to Saito 1.25.

kenh3497

As to the beams of the mount being closer together at the front is from the plastic shrinking a bit after it comes out of the mold. Not much can be done to cure this in the molding process except to closely monitor the temp of the plastic as it is injected and cooling time before the part is ejected. It's all about cycle time, getting the most parts per hour out of the mold.

It doesn't really hurt anything. You just need to shim the rails to get the bolt holes in the right spot before drilling.

Ken