Most plunger rods are machined from heat treated 4150 steel. They should be made from the largest diameter possible to prevent droop. This is when the rod and tip are at rest before the beginning of the stroke. Scrub marks on the leading and trailing edge of the tip as it passes the pour hole would indicate droop.
Some plunger rods are designed with threaded insert studs to receive the tip. If this part breaks then a new stud can be inserted rather than scrapping the entire part. It is good to use pipe nipples on cooling ports because the ID's of brass barbed fittings are usually too small for good water flow.
Adequate lube is very important. Lubricants are good insulators and they form a thermal barrier which subjects the tip to less heat transfer from the shot sleeve. Constant lube helps cool the tip and sleeve and this reduces individual expansion rates.
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