Could anyone help me please.......

I am going to rotate and change my ARF World Models Spot on 50 engine mount from a fully inverted 4 stroke to a 2 stroke side mounted at an angle of about 100* so the muflfer goes down the tunnel on the underside of the fus.

However after reading the Spot on 50 build log which helped a little I am still stuck on the problem of how you ensure the middle of the crankcase is in the dead centre of the fus. This will then keep the right side thrust correct.

Do you mark the firewall with the centre line of the fus and then some how rotate the engine mount around it . But what datum lines do you use and how can you tell if it is bang on centre?? The front of the engine at the prop driver will of course be off because of the thrust angle[:@][:@]

Mant thanks

How is the firewall currently set up? Are there blind nuts pre-installed? Does it currently have marks for installing the engine? etc

Make a piece of wood with the motor mount bolt pattern holes in it with a rod sticking out where the prop shaft should be. Most of the time you can draw an X from the holes and get close to the shaft line. Then mount the wooden patttern where you want the muffler to line up and mark the holes.

I will post some pictures later today or Saturday as soon as my camera has charged.

A picture will hope make things easier to understand.

Pictures are in



The first one shows the old factory motor mounting blind nut holes for the four stroke. The other two are where I want to position the mount. The markings are mine on the fire wall and mount as I was trying anything to get lined up!!

Tape a piece of scrap balsa, or cardstock over the hole and draw the marked lines on it

Put a piece of paper over the holes, mark the holes and put a pin through the paper at the marked center.

Rotate the paper to the desired angle and mark the new holes

A very big thanks for the pictures.

Does it matter that the original motor mount holes are not centrally aligned to centre of the firewall ???
Would this put the engine placement way off centre, or am I being stupid about this???

The original holes placed the engine off center for a reason if there was the usual thrust setup. Most models have downthrust and right thrust. When the model has those and a cowl, the motor mount will be moved around on the firewall so the spinner will be centered in that cowl.

They sometimes make a mistake on the location, but not often.

To very roughly verify the location, look at your firewall from the side and top. If it's got downthrust and right thrust the center of the motor mount holes will be moved left and up from the center of the firewall. You can more accurately verify by tack gluing the engine/motormount on the firewall. Temporarily place the cowl and look to see where the prop shaft comes out. Needless to say, you want it centered in the front of the cowl.

You can tell from the position of the holes in relation to the tank hole (which is centered) that the engine mount IS indeed off center. The method illustrated should keep the engine aligned, but you can easily use da rock's method to double check it

ARF mfgs don't always get their tank hole positioned so you do need to focus on the mounting holes. When they form a square, as they do on your firewall, MinnFlyers connecting lines give you the perfect center of the engine mount.

It's always best to pretty much ignore the tank hole and assume you might have to "move" it some. The one on your firewall isn't actually centered relative to their original mount. If you followed it's location, one might think it was positioned for up thrust and no side thrust if you didn't look at the motor mount holes.

One of the 1st ARFs I assembled when I got back into model planes was this Phoenix Sukhoi. It's tank hole was located for upthrust and the instructions didn't explain anything. They had a picture showing the tank hole centered in the motor mount. The firewall had downthrust and right thrust built in. At the time I couldn't believe a model mfg would get such an important detail so screwed up. I saw 2 of them later and one had the spinner off center in the cowl. The other had the cowl installed at an angle.

YEP...GOT IT. THANKS EVERYONE[sm=thumbs_up.gif]

Here are the results

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