Three weeping core plugs after a complete engine rebuild is frustrating and concerning in case one of them lets go. After replacing them with the engine in situ using brass ones and some serious sealant still didn’t resolve the problem, I decided that more drastic action was required.
Engine out and stripped to a bare block revealed that they had probably been leaking for years, allowing the coolant to erode a leak path at the bottom of the counter bore and past the corner into which the dished plug should form a tight fit and seal.
Leak path eroded over many years
I am fortunate to have an excellent father and son engine build/machine shop nearby and whilst their usual customers bring race engines which are much younger, the same principles apply. Plan was to set the block up on a vertical milling machine and with a fly cutter maintain the diameter, but cut the seat deeper into good fresh metal. It simply didn’t happen because the sound of the cutter changed each time it passed across the leak path from machining Cast Iron to, in the words of the operator, pushing Ferrous Oxide out of the way.
The new plan was to bore all the way through, increase the diameter until it cleaned up and then increase again to take the next available size of a modern cup-design core plug (example of cup design core plug shown in the picture). With pretty well the whole industry using them for years they are available in every metric and imperial diameter. The one at the back of the block for example ended up at 2 inch and the smallest ones at 35mm.
Tight push fit into a freshly machined bore
At this point and with the block accurately located on the bed he machined all of them the same way. Now the width of the contact area has increased from the 1.5mm thickness of a dished penny core plug to the 7.5mm depth of the cup, a 5-fold increase and to tap them in square with a snug fitting socket requires far less skill to achieve a perfect seal.
When painted I think they look neat and similar to the original design intent.
Block ready for reassembly
If I was rebuilding another engine which required a trip to the machine shop for a rebore or inline boring of the main bearings I would take the opportunity to do this as well.
Ed’s note: Looks to me to be a superb ‘fit and forget’ solution!