20601 Old State Road Haubstadt, Indiana 47639
This article will deal with the piston and cylinder as it
pertains to the 3 inch bore Hercules built engine.
There are several problems that may affect the cylinder bore.
These include being badly rust pitted, scored walls, worn out of
round and/or ring ridges at both ends of the ring travel area.
Generally, problems with the piston itself are limited to worn
ring grooves. Seldom is the piston pin hole or the piston skirt
worn enough to cause problems. As outlined later, the piston pin
and connecting rod bushing may need attention.
If the cylinder is rust pitted badly enough that it won’t
hold enough compression to fire and run, there are three choices.
You can obtain another block with a good bore from someone who
parts out engines. You can have the cylinder bored and refitted
with a suitable piston or have the old piston flame welded and
machined to size. The cylinder can be bored and sleeved back to the
original bore size so the original piston can be used. The latter
is what I do. It usually costs about $125 to $150 locally to get
These are times when an ‘as is’ engine will run fine.
When it is taken apart, cleaned up and reassembled, the compression
seems to have left. If this happens, it is likely that the cylinder
bore is worn out of round. Before disassembly the piston rings had
worn out of round, too, but conformed to the cylinder walls. When
reassembled with new or the old rings, they no longer conform to
the cylinder walls and there is blow by and enough compression loss
that the engine won’t run or runs poorly. The solution is the
same as that mentioned above.
Scored cylinder walls can be cause by two things. One is by a
loose piston pin that slides to one side and makes two parallel
grooves on one side of the cylinder. The other is by grit getting
into the cylinder. This is most apt to happen in areas with sandy
surroundings or on concrete mixers where someone is careless when
shoveling aggregate. Again , the solution is as outlined above.
If the piston ring grooves are worn badly, there are several
solutions. New pistons are being reproduced or a usable used one
can be obtained. The present piston can be put in a lathe and the
ring grooves machined to accept a standard ring along with a
contracting filler ring available through automotive machine shops.
The piston can be machined to accept custom made rings slightly
wider (1/64 inch or so) than the originals.
When fitting with new rings, make sure that they are not too thick
as to bottom out in the grooves. Newly fitted rings should also be
loose in the grooves rather than snug.
The diagram above shows piston ring groove wear (exaggerated)
and the shaded areas that must be machined away to ‘square
up’ the groove.
The piston ring and connecting rod bushing are the last
consideration in this article. If the connecting rod bushing needs
replacing, I go to a local bearing supplier and get a brass bushing
?’ inside and ?’ outside diameter. It will likely have to
cut to the proper length. Press out the old one and press in the
new. For the piston pin, I use ?’ drill rod. Put it in the
lathe and run emery cloth on it until it fits the bushing in the
rod. Grind or machine a notch for the piston pin retaining screw.
Be sure to bore an oil hole in the bushing after it is in the rod.
Once the refitting is done, slide the piston and rod into the bore
without rings and check to see how the rod lines up with the
crankshaft throw. The rod is of dropped forged steel so it can
stand some bending to realign it if necessary.
Coming up next is the crankshaft along with the bearings and