Hi this might seem like a silly question, but as Iread the instructions for drilling the holes to mount my engine to the engine mount, it says that Ishould tap the holes for the nuts. Can I ask the reason for this, why can't they just sit directly on the mount, surely tapping them just makes the material the bolt has a hold on that much thinner?

Thanks
James

Depends on the engine size, the mount size, and the  mount material. what is your application??

Ken

The engine is a SuperTigre G2300 (1.40cu.in. 23cc]
Mount is a GP Adjustable Engine Mount (1.08 - 1.80 2 stroke size)

The engine is a SuperTigre G2300 (1.40cu.in. 23cc]
Mount is a GP Adjustable Engine Mount (1.08 - 1.80 2 stroke size)

jamersoniiv,

Forgive me if I am misinterpreting your question, but the way it reads to me, It sounds as if you are interpreting Tap to mean counter sinking the nuts into the mount. If that is the case then you are misunderstanding what they mean by tap. To tap a hole means to cut threads into hole in the mount so that the bolt screws into the mount.

If i am reading your question wrong please disregard this post.

Ahhh well that makes much more sense! Thanks Mr Beagle you saved me from making a plonkerish mistake ;-D

Happy to help. Here is a picture of a tap. You have to make sure that it is the right size and threading for the bolts. Most hobby stores have these and they can help you get the right size tap and bolt combination for your application.

I hope the instructions say to tap the holes for the "bolts" not the nuts.  Also, I add a nylock nut on the protrudilng bolt from the bottom of the mount.  Just gives me peace of mind.

+1 ,
Also the GP center drill is a great time saver and assures the hole will be straight.

JW

A few thoughts on tapping mounts, using SAE hardware.

For every tap size, there is a specific number drill size that must be used.

For best results use a drill press and make sure the mount being drilled is flat when drilled.

Plastic is very easy to tap, either in nylon or glass filled nylon. No lubrication is required.

Aluminum takes a different technique. You run a risk of breaking a tap if you do not do the following:

Use a light oil or kerosene for lubrication

As you go forward, you stop and reverse direction every half turn forward. This breaks the metal that the tap is removing to form the thread in the mount. For the softer alloys, you may want to tap in 3-5 turns (in the 1/2 steps mentioned), then remove the tap completely and clean before resuming the tapping process. This is especially important with deeper holes, smaller taps, and softer alloys. I try to avoid hardware smaller than 6-32, even if I have to enlarge the mounting lugs holes slightly. It's just easier to tap 6-32 than 4-40 hardware.

With either glass filled plastic or metal, I don't find any type of nuts or locking washers to be necessary.

http://www.greatplanes.com/accys/gpmr8130.html this works real well, it creates a starter hole

For an engine of size 23cc, T-nuts should be installed at the back side of the mount to hold the bolts. The tapped hole on a GP "plastic" mount may not be strong enough.

 if we're talking about mounting the engine to the mount, i have a hard time believing the sheet metal screws are enough to hold against all the vibration. i know many say they hold fine, but i never realy liked the idea much.  i drilled my mount arms for 8-32 cap screws and used nylock nuts. milled a small flat on the taper of the arm for the nut to sit on. i just don't like screws there and the additional weight of bolts and nuts was worth the piece of mind.
     for the mount to firewall i "invented" what i think is a neat trick.....
     my plane has a spinner so alignment is critical.  with my fire wall epoxied to the fuse at the offset angles, i can move my mount around slightly to get the spinner lined up without sacrificing  the offset angles by using shims to get the spinner lined up. if i do need to change the offset a bit with a shim, there's enough play in the holes to "trig" the mount over to gain spinner alignment again.
     i made a 1/4 inch thick, H-shaped plate no bigger than the diameter of the engine mount base that is drilled and tapped for the mounting bolts in the base on the mount. this H-plate clamps the mount to the firewall with the mounting bolts. the H-plate sits against the back side of the firewall and the holes in the firewall are large enought to allow about 3/32" movement in all directions. it allows me to put it all together on the firewall, then, with the bolts just loose enough to be able to move the mount around, install the cowl and get the best alignment at the spinner / cowl junction. then i tighten the bolts to clamp the mount in place and lock the mount in that location with a small fillet of 5 minute epoxy around the base of the mount where it sits on the firewall. if i have to take the engine off the plane for any reason, it automaticly goes back in the exact same place. the aluminum H-plate doesn't weigh any more than 4 nyloc nuts and the bearing/clamping area is many times greater than 4 nuts.  

Acouple of pics to help visualize ...

Some mounts are flat on the top and bottom. SIGglass-filled mounts are used in pic 1. It's a Saito 82 four stroke. You can see that there is access to the bottom of the mount to add nyloc nuts.

Some mounts are flat on the top and rounded on the bottom making it impossible to add a nyloc nut. These mounts require that you drill a hole then tap it with threads to hold the engine mount bolts.

Some cowls make it impossible to access the bottom of the mount even if the mount is flat on both top and bottom. These also require that the mount be drilled and tapped for the engine mount bolts.

pic 1 - 4-stroke, SIGmount, removable cowl
pic 2 - 4-stroke, Hayes AL60 mount, removable cowl
pic 3/4 - 2-stroke, SIGmount, built up imovable wood cowl

the mount i used, i can't recall the brand, but it has 4046L cast into the top of the base. the base is round and the arms are radiused on the sides and the bottom of the arms are tapered ....taller at the base to narrow at the front. i drilled holes all the way through the arms for the engine bolts and then spot milled the areas at each hole parrallel to the topsto makea seat for the nuts. i guess i never gave the cowl a thought, though, my cowl breaks at the firewall and comes completely off exposing the entire engine. working on the engine area is much easier that way.

Most likely the 4046 refers to the alloy of the aluminum. Not sure what the "L" would be for, if anything relevant.

hi bruce,
the mount is glass filled nylon or similar. the 4046 i assume refers to the engine size range it is made for and the "L" might refer to it's arm length from base to tip, because it seems pretty long to me... the backplate of the engine is 1-3/4 inches away from the back faceof the mount,... where it would sitagainst the firewall....
there's a flat shelf between the arms extendingforward ( gusset?)from the base about 1 inch and the engine'sbackplate is snugged up tight to that. it looks like it was made to have room for the old back carbed super tiger type engines

I started to think that way ablout the "L" after I posted but let it stand as you had not stated the material type. 4046 would then be either the eng range or mount model number.

I think it's an engine mount for .40 to .46 cu in engine size with long bearers.


Edit:
D'uh. Should have read the subsequent posts before wading in: sorry

Ron what you have described is the Dave Brown mount In 46 class size and indeed the L stands for longer beams. Because of the hole spacing you can access all four of the firewall bolts from the nose ring with a long driver even with the engine installed. For fixed cowls this is critical.

These are wonderful mounts that work very well in very tight old style fixed cowls or any tight situation. Far preferable over the large square adjustables, I use them almost exclusively up to 120's.

They come with self tapping screws that are sized for the size of mount you purchased. The back of the package lists the proper fractional drill bit size to use for drilling the pilot holes. There is no need to tap the holes and there is no need to file or spot mill flats, In fact that will only serve to weaken the beams considerably.

I know its contrary to popular opinion to actually use the mount as intended but the fact remains the mounts and the method has been around for many years and thats because they work.
Yes I do have even 120's mounted with the screws both a Webra 2 stroke and an OS 4 stroke as well as probably the most violent shaker in the plane of rotation I can imagine YS 110 running on forty percent nitro in some of my warbird racers.

All use the supplied screws an all stay tight. Its a good method give it a chance or you will never reap the benefits.

John

 hey john,

       yup, it's a Dave Brown, now that you mention it. boy, i hope i didn't create a grenade waiting to happen....this isn't the first time i read that the screws do work good!.  i did drill the arms for 8-32 cap screws to use with nylocs.   i looked at that long and hard after i drilled it,  but i think they still have enough meat.....well, they're cheap enough, maybe i'll just go get another and actually do it the way it's supposed to be done!.   i have to admit, i'm one of those guys with a ton of tools and machinery, know how to use them and just can't leave something alone.... i gotta make it..."better".   i'm usually  successful, but there's always a first time and i sure don't want that to bite me where the sun don't shine!!!  thanks for the warning, ...well taken.
                 ron