Timing inspection hole (centered between the two cylinders on the
right crankcase) 1)
The plug can be removed with a 3/8“ hex tool (Allen wrench).
These are aluminum internal (case) and steel external (plug) threads (5/8”x18 NF straight bolt cut).
It is a common occurrence to damage these threads or for them to being found to be damaged. Whoever threaded the plug in last may have not taken the time to insure proper thread starting before using a wrench to install the plug and cross-threaded them. Over tightening the plug can crack / stretch the threads and thermal expansion can warp and gall them 2). Sometimes the threads get corroded and galled to the point that removing the plug takes the threads out with it. You can't be certain that the threads haven't been galled before you remove the plug. 3) If you've noticed oil seeping from this plug, the threads are at least suspect.
Once removed, check the threads on the plug and in the case for damage before trying to re-install the plug. These are straight (not tapered) threads on the plug and in the case. There is no need for the plug to have tapered threads with it having a head on the end. Inspect them closely and, in the case of warped or damaged threads, first try to repair or 'chase' the them. This will keep you from having to drill into the case if it isn't necessary.
Pics and App. Dims
5/8“x18 steel plug has straight threads with Teflon tape around the neck. 4)
Straight threads in the case. Same drag on bore gauge - front to back. No taper 5)
Notice the plug doesn't go all the way into the end of the case threads 6)
Measured from the case outside surface to the flywheel. 8)
Measured from the case outside surface to the app. end of the case threads (app. 1/4” longer than the plug threads). 9)
Permanent Repairs
The foregoing is under the assumption the engine is assembled. This would obviously not be as big of an issue with the engine out and the case split. Repairs of the timing inspection hole are much easier for you or most machine shops without the flywheel in the way or concerns of metal shavings getting into the engine.
A tap and a die is used to chase the threads in the case and on the plug respectively back to their original shape / cleanliness and usually done with plenty of good cutting oil.
For this application, fluidity can be detriment to the cause. With the small amount of threads on the plug and the imminent danger of getting cut shavings (swarf) into the engine case thru the timing hole, it is best to use thick lube or even axle grease to lube the tap to chase the case threads.
While chasing the threads, your following the existing 'centered' thread pattern. 10)
You only have to run the tap in far enough to clear the length of the plug, not to the end of the case threads.
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