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How to Repair a Cracked Cylinder

By Bill Turner

In this article Bill tells how he repaired his cracked cylinder. He also offers an explanation of why it cracked.

The cylinder wall of this 2-7/8 inch M8 Cushman engine cracked in the fin area on the flywheel side. I did not want to discard the block so I decided to repair it. Here is the solution. You may have to modify this procedure depending on just how and where your block is cracked.

Refer to the pictures and drill holes down through the top of the block with the proper bit for a 5/16-24 tap. In this case I drilled five holes spaced as shown in the photos. The holes are located just outside the gasket area and go down on the outside of the cylinder wall. Drill through the top of the block and continue down through all of the fins as shown in the picture. Enlarge the top part of each hole for the shaft of the bolt with a 5/16 inch bit. Then tap the rest of the way down through the fins. An extra long tap will be required.

Thread bolts into the holes as tight as possible, being careful not to break any fins. Then turn the block upside down and weld the underside of the top two fins (they will be on the bottom now) to each 5/16 bolt (depending upon where the crack is). Use special rods made for welding cast iron. Do not attempt to weld up the crack because doing so could warp or heat stress the block or burn through the cylinder. After the block cools turn it back right side up and mill the heads of the bolts down flush with the block so that they will not interfere with the head or the head gasket.


A sleeve that can be used for the M8 block is an L A Sleeve Company part number LA-214. Its outside diameter is 3.0625 and allows a standard 2.875 inch finished bore. This sleeve is longer than necessary and must be cut to size.

Here is the explanation of why this block cracked.
What caused this block to crack was really something we can all learn from. There was plenty of meat in the block to run forever at 3 bore. The bore center had been maintained perfectly when it was bored. What actually caused the crack was that the top of the block had been milled approximately .050 inch. When you put a stock head on it with a stock head bolt and gasket the bolt bottoms out on the fin under the crack. This fin was drilled, but never tapped at the factory and the pressure of the bolt put a stress across the crack area. If the top of the block had not been cut this would not have come into play. You said it was definitely higher in compression than normal and if the head also was milled the problem of the bolt bottoming out would be even worse.

Bottom line When I put the head on your engine the bolts bottom out on the fins before they fully tighten against the head. When the block got hot and expanded it caused it to crack. This one was not a crack from an over-bored condition and it is an easy fix. I always run a tap through the bolt holes to clean them up, then put the head on the block without a gasket. I torque the bolts down and make sure they will tighten against the head and not bottom out on a fin or the bottom of a bolt hole. After doing this check you know that when the gasket and the washers are installed you will have plenty of clearance and they will not put pressure on a fin or bottom out in a hole.
Cheers, Bill.


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