Drilling Bolt Holes in the Engine Mounts

[JHildreth]

I always have a lot of difficulty marking and drilling engine mounting holes accurately.  I do use a drill press for the drilling operation, but getting the hole positions marked accurately is somewhat problematic.  Especially so when the engine 's case overhangs the mounting holes requiring the marking tool to held at a slight angle.  I am looking for suggestions for a more reliable marking and drilling process.

Joe

Let me expand this plea for help somewhat.  If a hole is mis-drilled slightly (one hole diameter of less), how can its position be corrected?

[Dan McEntee]

  I use the tool that either Great Planes or Top Flite makes that has a centering cone and a pilot drill for marking center. i position the engine and mark one hole. I drill that hole and put a bolt in it and secure with a nut, or use a blind nut. Then I recheck engine position and do another hole, catty-corner on the other side of the engine, drill that hole and secure again. Then I mark the last two holes, drill those and there you have it. I don't recall ever having any problems with this method.
   Jim Lee makes drill jig templates for the popular engines. If you use only one or two brands or styles of engines, these are well worth the cost of having them in your tool chest.
   Type at you later,
    Dan McEntee

[mike londke]

12 bucks and worth every penny. http://www.greatplanes.com/accys/gpmr8130.html

[mike londke]

and Dan posted about it the same time as me LOL

[Steve Scott]

  I use the tool that either Great Planes or Top Flite makes that has a centering cone and a pilot drill for marking center. i position the engine and mark one hole. I drill that hole and put a bolt in it and secure with a nut, or use a blind nut. Then I recheck engine position and do another hole, catty-corner on the other side of the engine, drill that hole and secure again. Then I mark the last two holes, drill those and there you have it. I don't recall ever having any problems with this method.
   Jim Lee makes drill jig templates for the popular engines. If you use only one or two brands or styles of engines, these are well worth the cost of having them in your tool chest.
   Type at you later,
    Dan McEntee

That's how I do it and 3 of the holes come out perfect.  Why the 4th doesn't I'll never know.   

EDIT:  Actually I suspect my drill bits aren't exactly sharp so I do get some wander when drilling...

[Tim Wescott]

I get the hole dimensions, and then mark out on the bearers with a ruler and a scribe.  I use a machinist's scribe, then cheat and use it as a center punch to start the holes.  I make a 1/16" starter hole, then finish to size.

Works for me, 4 times out of 5.

[JHildreth]

This looks like a good tool to have.  However, it seems that to work properly it would have to be held perfectly vertical to the engine mount.  I have some engines whose case, up along side of the cylinder, overhang the mounting holes.  In this case the tool cannot be vertical.  What is done in this situation?

Joe

[Steve Helmick]

You might go to a machine shop supply store and buy the drill size(s) you want in "stubby", aka "screw machine" drills. Much stiffer, they'll give you more accurate hole size and location. I think most or all are "cobalt" steel, which is much better than the crap you get at Harbor Freight.

It's also very possible to buy a new drill and have it drill oversize because it's not sharpened exactly the same length on both flutes. It's a good idea to do a test hole before you drill your prime parts.  Steve

[Steve Scott]

Another thing I've noticed is the holes in the lugs of the Asian engines appear to be metric - a 3mm bolt fits snugly.  Using a typical 4-40 bolt allows quite a bit of slop.

[Gary Dowler]

There are several ways that will yield acceptable results.  I used to struggle with this very topic until someone shared this trick with me. Its time consuming but nothing is more accurate.
Install engine in mount. Carefuly mark 1 hole. Remove and drill that hole.  Reinstall and bolt in place, then carefuly mark the next hole. Remove and drill that one hole.  Reinstall with 2 bolts. Carefully mark hole 3, remove and drill.  Reinstall with 3 bolts then carefully mark hole 4. Remove and drill.  Reinstall engine and all 4 bolts simply slide into place.
There are faster ways, but nothing this side of a CNC mill will yield a more accurate result.

Gary

[Dan McEntee]

This looks like a good tool to have.  However, it seems that to work properly it would have to be held perfectly vertical to the engine mount.  I have some engines whose case, up along side of the cylinder, overhang the mounting holes.  In this case the tool cannot be vertical.  What is done in this situation?

Joe

     The only thing that should be "sticking out" would be the exhaust, and I don't think I have ever seen any modern engine that has an exhaust stack long enough to over hang the mounts. The shaft of this tool is usually right up against the exhaust, but is pretty vertical, and even if it is leaning a couple of degrees, it will still make a center mark that is close enough on center for a typical 4-40 bolt to fit easily. It works very well and if Great Planes had not come out with it, I'm sure Jim Lee would have thought of it!!
   Type at you later,
   Dan McEntee

[john e. holliday]

Well Jim Lee has drill guides for various engines in his listing.   Type in     Lee Machine Shop    on your browser.

[dave siegler]

transfer punches

https://www.harborfreight.com/28-piece-transfer-punch-set-3577.html


also I have a CAD file for mos of the engines I use.  print it out, and you have a drill template. 

[Wayne J. Buran]

You might go to a machine shop supply store and buy the drill size(s) you want in "stubby", aka "screw machine" drills. Much stiffer, they'll give you more accurate hole size and location. I think most or all are "cobalt" steel, which is much better than the crap you get at Harbor Freight.

It's also very possible to buy a new drill and have it drill oversize because it's not sharpened exactly the same length on both flutes. It's a good idea to do a test hole before you drill your prime parts.  Steve

As an ad to the screw machine length drill. The aircraft drills have a split point designed not to wander because most are used in portable applications in aircraft skins. A good tooling distributor will have what you need. Hope this helps.
Wayne

[Bob Heywood]

Transfer punch, center drill, pilot drill (1/16), and drill to size...

[Avaiojet]

I put the engine in place with the correct offset, generally one degree. I clamp it in place with this plastic clamp I have. I recheck the engine offset to make sure it's correct.

I have Stabilo pencils I use in my work. Blue for this or black. White for black R/C mounts.

I whittle down the wood on the first 2" of the Stabilo exposing only the inner wax stem.

I actually mark all four holes by spinning the pencil. Leaves a perfect mark.

I then eyeball the center of that mark and push a sharp awl into it leaving an indentation.

I remove the clamp and the engine and drill all four holes using a tiny bit just for a starter hole.

I put the engine back in place for a look see. If everything looks good I drill each hole for the proper size.

I like this because I only have the engine in place once.

Doesn't take long at all and I've never had any issues or screw ups.

Last three models, still in the works, I use an R/C firewall mount. Easier because the offset is by shims behind the mount and the engine fits tight in the mount.

CB



CB

[Brent Williams]

I see that this tool will be in my future.  Probably the Lee drilling guides as well.

Thanks for bringing up the topic. 

http://www.greatplanes.com/accys/gpmr8130.html

[George Albo]

I see that this tool will be in my future.  Probably the Lee drilling guides as well.

Thanks for bringing up the topic. 

http://www.greatplanes.com/accys/gpmr8130.html

After reading this post, I ordered one. It's on it's way.  I used to use the appropriate sized drill bit that fit in the mounting hole and then twirl it hard until it made a divit. I then drilled out the mounting holes. This tool will make things easier I believe.  Can't wait to use it!

[Dave Harmon]

There are several ways that will yield acceptable results.  I used to struggle with this very topic until someone shared this trick with me. Its time consuming but nothing is more accurate.
Install engine in mount. Carefuly mark 1 hole. Remove and drill that hole.  Reinstall and bolt in place, then carefuly mark the next hole. Remove and drill that one hole.  Reinstall with 2 bolts. Carefully mark hole 3, remove and drill.  Reinstall with 3 bolts then carefully mark hole 4. Remove and drill.  Reinstall engine and all 4 bolts simply slide into place.
There are faster ways, but nothing this side of a CNC mill will yield a more accurate result.

Gary

This is exactly the way I do it Gary except I cut the head off whatever size bolt is right sized for the engine holes, then chuck the cut off bolt in the drill press and put a point on it with a file.
Then set the engine precisely on the mount or fuselage then press the sharpened bolt in the hole by hand.
It is short enough to fit vertically in the hole so there can't be any parallax.
Works great and I've tried most methods mentioned and I have several of the center drill thingies from Tower or whoever.
They work but can lean over easily and throw off the hole.

[Phil Krankowski]

Sharpie and masking tape.  Mask the bearers, paint the lugs with sharpie marker heavily.  Before the marker dries press the engine into position.  The holes will be a blank space. 

The precision with this method is slightly lacking, but it provides marks that are "sane" so when followed up with a more precise method you can see immediately if there is an error before drilling. 

Even on wood use a center punch to make a dimple.  The bit cannot wander out of the dimple.

Phil

[John Rist]

In the past I have used CA to temperately mount the engine to mark the holes.  Works well on RC stile (black) mounts. Not sure if it is a good ideal on wood beam mounts.  Might pull a chunk of wood when removing engine.

[Bootlegger]

  How about just using an Ice Pick?     

[Scott Richlen]

That sounds like a basic instinct....

[john e. holliday]

I see that this tool will be in my future.  Probably the Lee drilling guides as well.

Thanks for bringing up the topic. 

http://www.greatplanes.com/accys/gpmr8130.html

I have 2 of those tools and still do it one bolt at a time.   

[mccoy40]

So I drill the first hole with the drill press - mount the engine with a bolt thru the hole -tighten it and align the engine on the mount so it's square - 0 degree thrust to the fuse . I then use a 12 inch drill and my hand held drill gun to drill the other three holes with it, while the engine is mounted. Just starter holes, about an 1/8 inch deep.  I then remove the engine and drill the remained of the holes on the drill press.

This gives me accurate holes pretty much every time.