FAQ

Fixing Cylinder Bore Pits with JB-Weld

Close to fifteen years ago Ken and I were doing a re-paint and general restore of a 3 hp. Fairbanks Morse Z that had a lot of blow by and did not hold compression except at top dead center. Pulling the piston quickly revealed the problem. The wrist pin (gudgeon) had worked loose and worn two sets of parallel grooves in the cylinder wall the full length of the stroke. The grooves were too deep for honing to take care of. Since the piston and rings were in good shape we didn't want to bore and/or sleeve the cylinder so we decided to try a repair with this miracle product we had heard of called J-B WELD. The technique used is as follows:

 

  1. Thoroughly clean the cylinder wall with a good non-oily solvent.
  2. Using a die grinder or Dremel type tool grind the defects out some to increase the surface area for good bonding and also remove any porous cast iron that might retain some absorbed oil. Use a coarse stone or carbide tool to leave a roughened surface for better adhesion.
  3. Clean the cylinder again with a good solvent and paper towels. Our current favorite solvent is a spray can of brake cleaner.
  4. Mix the J-B WELD according to instructions and fill the defects with whatever tool is appropriate. Tools might include wooden ice cream sticks, putty knives, fingers, etc.
  5. Remove as much excess epoxy as possible.
  6. Cover area with wax paper and use a wooden dowel as a rolling pin to force the material into the defects so there are no voids.
  7. After the J-B WELD sets remove the wax paper.
  8. Use emery cloth to remove the excess and high spots. Start with a medium grit and work down to a fine grit. We made a curved sanding block with a radius just slightly smaller than the cylinder bore to prevent damage to the cylinder wall.
  9. Hone the cylinder.

Since the repair this engine has run many hundreds of hours including some hours at full load. A few days ago we removed the piston to see how the repair has held up and were pleasantly surprised to find no apparent degradation. Non of the epoxy had come loose or burned out and the surface was smooth and even with the cylinder wall in all four grooves.

Recently someone suggested pushing a well oiled piston ring through the bore with an appropriately sized piece of wood to keep it square before the epoxy sets up. This would act like a squeegee to remove most of the excess and would make step 8 above much less time consuming.

Since the above repair we have filled pits in the cylinders of two or three other engines using the same technique. The only problem we have noted is that the J-B WELD burned out of some pits that were immediately adjacent to the exhaust valve on one engine. We ended up ignoring these pits as they did not affect the operation of the engine.

Larry Evans
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Thanks to Ken for providing us with the benifit of his experience in this area. I myself have an engine which has some significant pitting and next time I strip it down will give this method a try. I've used JB-Weld for some other tasks in the past where it is exposed to heat and pressure and it does stand up well. I currently have it in one engine where I've used it to fill a small hole between the cylinder and the water jacket - seems to be holding up so far. Paul.