I am assembling an ARF with a nylon motor mount and a Saito 72 . There are no pre drilled holes and i want to make sure i do everything correctly. #1 does anyone know if the great planes dead center tool is worth buying? #2 the holes i drill should they be tight when the bolt goes thru like the same diameter as the bolt or should they slide freely all the way thru?

The Great Planes centering drill works very good. You should drill a hole a few sizes smaller than the drill and than use a tap to cut threads in the mount. Stop tapping a few threads short b of coming through and then use the bolt to cut the last few. This way the nylon of the mount will act like an aircraft locking nut.

I agree, I like my dead-center tool.. I use it with every mounting. I get the engine in the position I want, then I use a few simple clamps to hold it on the mount, while I make my marks with the dead-center tool. Your local hobby shop, or even hardware store should have a tap/drill set... I'm thinking its a 6/32 size needed for that size engine.. double check me on it.. If your kit came with bolts.. theres a good chance they're metric of some sort... I probably wouldn't use them.. I'd get some Dubro machine/allen bolts... and the Dubro tap/drill set... Plastic mounts actually tap out very nicely if done carefully.... DO NOT try to squeeze a bolt into them though...inotherwords... use the drill bit included with the tap, cut the threads. Don't try to cut threads using a bolt... that will split the plastic... Of course you can drill the holes oversize and use lock-washers/nuts.... that works also.. and is a way to save a mount if your threads don't work out. Good luck.

The Great Planes Dead Center Drill is one of the handiest tool you could put in your tool box! They are great for getting the holes correctly positioned on many model mounting jobs: engines, eng. mounts, servos, control horns, switches, you name it! Buy one!

+1 on what was said!

I agree on the Dead center tool. I use it for engine mounts, servos and even non-aircraft projects. Well worth every penny.

I've never had one and have mounted a good number of engines. Yes, the tool will help you avoid mistakes and will make things easier, but modelers were mounting engines before the dead center tool came along too.

As for the hole size, a lot of that depends on your mount and how the front end of the plane is made. You can drill and tap the plastic, or you can make the holes a slip fit for your screws and put a nut on the bottom. Use loctite or a a nut with the rubber insert to make sure they don't vibrate loose if you go the nut route.

I've had good luck using my Transfer Punch Set. Good to mark the center of holes from 3/32" to 1/2". Got my set from Little Machine Shop online, but Harbor Freight has a similar set for about $10.

The tool from Great Planes looks like it would do the job, obviously it is liked by others that have used it.

+1 on the good information already posted about threading the hole.

The proper size tap drill for a 6-32 tap is a #36 drill or about .106". A 4-40 is a #44 drill. I have been using a 1/8" pop rivet with a long drill inside to locate my holes for a while. They are mostly for 4-40 bolts though. I also do one hole with the rivet/locater thing by hand to just make a dimple, then drill the rest of the way through, and tap it. I bolt it in place, and then spot the next hole, drill and tap it, and put the bolt in. then spot the rest and finish up.. I found it best to use a drill press after many years of trial and error, this is about the best way I know of. The rivet thing is just a redneck type of the Great Pains setup.

I'll second the transfer punch idea. Very handy to have. The Dead Center tool is great too. Get them both!

http://www.harborfreight.com/catalog...transfer+punch

The GP hole locator works extremely well and once past your second airplane it will prove one of your most valued tools. And one that you would not consider being without from that point on.

John

I used 6/32 bolts to mount an OS .72 Surpass 4 Stroke. The bolts fit the mounting holes well, and held the engine firmly in place. I used the Dubro drill/tap set. It worked like a charm.

I have used this tool on three planes for the engine mounts and servos. It is one of the most indispensable tools I have!

Hi!
All you have to do is to glue the engine to the mount using Ca medium glue and use a long drill and tap one of the holes! Then remove the engine ,which is easily done, and screw the engine firmly to the mount .
Then...drill the other three holes (the engine still atached to the mount) and tap the holes.
3,5mm -4mm diameter screws are adequat to use for a .72 Engine.

And remember if the drill gets into the way of the engine, invert the engine and drill threw the bottom of the mount!

Good one! Never head of this idea.

+1

I have used this procedure before with really good results. I have included a photo of one example; however, you need to check to make sure that the clearances on the top of the mounting lug are the same as on the bottom. On many engines, the intake passage from the crankcase to the intake port of the cylinder will cause a small offset in the location of the mounting holes. It's hard to see in this photo, but the screws on one side of this mount are closer to the edge than on the other side.



It's probably not a problem for a four-stroke, but it is on many two-strokes.

Funny i just have to mention..After reading this thread a couple weeks ago, purchased a centering tool off e-bay for like 5 bucks, figured what the heck.
Last night I was installing a couple servos in a beater plane for snow fun, I carefully pre drilled 4 tiny holes in the servo rail with my homemade pin vise just by holding the servo there while free handed the holes and got one just a little too close to edge where it split while installing the servo hold down screw, today i'm sitting here at my computer looking at the new tool i bought still in it's packaging, I had already forgotton i had it, t sure would have been able to use it last night.