Crankcase Fixings for the Five Cylinder Rotary

rotary crankcase and cylinders

The crankcase fixings for the five cylinder rotary required a lot of patience as each cylinder was held in place with 4 off 8BA bolts. However, whilst tapping one particular thread in the crankcase I had broken the tap. This annoyed me so much that the whole engine was put into a bag in pieces and lost for around 2 years whilst we moved house.

broken tap in crankcase

The offending broken tap. As you can see it is well buried and looks like I′ve already had a go at remving it

Not sure how well this will work, but I bought a solid carbide end mill.

milling out a broken tap

The solid carbide mill did get the tap out, but the end mill came apart in the process.

As you can see in this picture I resorted to the trusted TiN HSS end mill and used this to finish the machining.

This setup used my XJ12-300 milling machine and a Soba vice, a good solid combination.

The TiN end mill was still in good condition at the end of the process.

aluminium plug repair in crankcase

The casing with an aluminium plug back in place of the broken tap.

I glued the aluminium plug in with some very thick instant glue.

With the tap removed and a piece of aluminium in it′s place I then drilled and tapped the 8BA threads to bolt the heads and cylinders down. This just gave me lots of trouble as the aluminium kept stripping. So I drilled the holes out to 3mm and then fitted steel inserts into the holes that had been drilled and tapped 8BA internal thread. This gave me much more secure crankcase fixings for the five cylinder rotary engine.

five cylinder crankcase, cylinders and cylinder heads

The bolts looked too bulky so I machined the thread down to 1.4mm diameter in the centre section of the bolt.

The only slight change I would make in retrospect would be to make the steel inserts fit from inside the crankcase so there would be a mechanical hard stop and hence be able to hold a greater force.

About Nigel 358 Articles
I've been making models since I was around 7 years old and using a lathe from the age of 11, a self taught engineer with a passion for making model engines.

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