I am building my first model and am currently at the engine mounting step. The engine I have is a OS .46LA. In the instructions that came with the plane it called for 1/8" holes in the mount and installation with 6-32 1" bolts. The manual that came with the engine called for 3.5mm bolts.

In either case, the bolts don't fit through the mounting holes on the engine - they are too large.

Am I supposed to force thread the bolts through the mounting holes on the engine? Is expected that the user will tap these holes to the thread size of the mounting holes? What to do?

Just use smaller bolts. The next size down is 4-40. They'll do fine.

Don't tap the holes on the engines mounting lugs, or otherwise force the bolt through the hole.

I peeked at the manual, on page 8. It says to use a 3.5 mm bolt, but a 3 mm nut to mount the engine
to wooded beams. Below that, it says to use 3 mm bolts on OS's radial mount. The 3.5 looks like like a mistake
in the manual. http://www.osengines.com/manuals/40-46-65la-manual.pdf
4-40's will fit in a 3 mm hole.

Thanks for the advice. The only problem I have now is that I drilled engine mount for the bolts prescibed in the manual for the plane. I suppose they'll be too large for the 3mm (or 4-40) bolts you are suggesting.

So I guess I have my first lesson in model building - 'Make sure you reconcile the differences between the model instructions and engine instructions when it comes to mounting the engine, and then test fit the selected bolts in the engine before drilling the mount.'

Thanks for the reply. I'll avoid tapping the engine itself and work out a way to make sure I have a good fit on the mount.

PD

Nah... they're fine, dont' worry about it.

My Sig LT-40 instructions say to use a 1/8 inch hole, and 4-40 bolts with lock nuts.
(I happened to used 6-32, and tapped the mount itself.)

1/8'th is about 1/32 larger than required for a 4-40 A 7/64 would be better. Maybe they figure that 1/8
gives people some room for error.

Here is great tool for centering holes in the future: http://www.greatplanes.com/accys/gpmr8130.html

good luck with it


edit, changed 3/32 to 7/64... 3/32 is the drill size if you're tapping a 4-40 hole... according to my tap set.

Thanks again - I really appreciate the helpful advice.