Wednesday, April 20, 2011

Isetta - Stripped Threads

It would be extremely rare (if not impossible) to restore a vintage vehicle and not encounter stripped threads at some point. Up until now, I had only encountered two stripped threads on my white Isetta. The drain plug on the chain case was stripped, and one of the threads on the engine case that holds the coil was stripped. As I had mentioned on an earlier post, I had a lot of problems with the quality of manufacturing and fit of a muffler and heat exchange that I had purchased from Group Harrington. I had never completely tightened down the muffler system because I thought I might either try to salvage the extremely poorly made heat exchange or do some additional tweaking of the bend on the header pipe. I finally decided to give up on the heat exchange and just use the header pipe that came with the muffler. After some additional tweaking of the fit of the header pipe and muffler, I decided to tighten down the muffler system. That's when I encountered two more stripped threads. The bottom two threads of the cylinder head that hold the header pipe on were stripped.

Thankfully there are a couple of styles of thread repair systems that work very well. For repairing the cylinder head threads I using a helical insert style of thread repair kit. Most people refer to these as Heli-coil which is probably the most popular brand of helical inserts.

To give myself a little better access to the stripped threads, I removed the cooling shrouds from the engine which also meant removing the carburetor.

Repairing a stripped thread with a helical insert involves drilling the stripped hole out to a slightly larger specified diameter, and then tapping the hole to accept the helical insert.

After the hole is tapped, the helical insert is thread into the hole with a special tool that has a slot that fits over the tang of the insert. After the insert is threaded to the correct depth, a punch is used to break off the tang at the rear of the insert. I then blow air into the hole until the broken piece of tang comes out of the hole.

A photo of the repaired threads.

For the stripped threads in my chain case drain plug hole I used a different style of thread repair called a Time-Sert. Time-Sert's are more expensive than helical style inserts (unless you borrow it from a friend like I did) but Time-Sert makes kits specifically for drain plugs and spark plugs which I felt might give me a better seal than a helical style insert.

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