Author Topic: Wall 30cc Mighty Mite design question  (Read 511 times)

Offline gbritnell

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Wall 30cc Mighty Mite design question
« on: September 07, 2021, 01:39:53 PM »
Ladies and gentlemen,
I am rebuilding a Wall 30cc inline 4 engine for a friend and I have a question. The engine was designed with a counterbore at the top of the cylinder liners. Now this might not be too questionable at first glance but here's the issue. First off the big ends of the rods won't fit through the bores so the piston/rod assembly must go through from the bottom of the crankcase. Ok, I can live with that as some full sized engines had a similar installation procedures. My problem is the counterbore. You insert the piston rod assembly into the bore and slide it toward the head deck so the crankshaft can be put in place. While fiddling around with the crank to get the end covers mounted one or more of the piston rod assemblies gets pushed deeper into the bore. Now mind you the combustion chamber is in the bore so the piston doesn't go all the way to the head deck. As the piston/rod assembly gets pushed toward the head deck the top ring pops into the counterbore. The counterbore is only slightly larger than the bore so now there is no way to compress the ring to get the piston back into the bore so the ring needs to be taken off of the piston and the whole process started over again.
I guess my question is: why on earth is there a counterbore at the top of the cylinder liner? Yes this is the way the drawing shows it.
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Offline Charles Lamont

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Re: Wall 30cc Mighty Mite design question
« Reply #1 on: September 07, 2021, 02:35:16 PM »
If, when assembled, the top ring just goes partially into the counterbore, it might be there to avoid the formation of a ridge with  bore wear.

Online Jo

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Re: Wall 30cc Mighty Mite design question
« Reply #2 on: September 07, 2021, 06:02:56 PM »
Just reviewing the drawings:

The bore of the Mite Mite is 0.875. At the top is a counter bore of 29/32 or .906 going down  3/16.  :thinking:

Looking at the cross section the two valves seem very close to the sides of the bore: the valve stems are spaced at 1/2" and the valves are 3/8" diameter. so the extra bit is to prevent them from catching on the slides of the liner  :)

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Offline Jasonb

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Re: Wall 30cc Mighty Mite design question
« Reply #3 on: September 07, 2021, 06:33:02 PM »
I suppose it's best to assemble with the head or a temporary plate in place then the pistons won't go too far up the bore.

Though if making it now you could just up the bore a bit so valves clear and do away with the counterbore as you are unlikely to be running it in a 30cc engine class competition.