hi james a- wow never heard that advice. seems most heads today are aluminum. all plugs i have seen are steel. is your concern long term galvanic reaction between dissimilar metals? many newer cars of various makes specify 100k plug changes. havent seen on the board at least in the last 4 years problems with plugs seizing to the aluminum head and ripping the threads, but it sounds possible. just installed new plugs in a merc marquis 01 which had almost 100k and were the originals.they were worn and the gap had expanded to about .065. spec calls for .054. noticed no damaged threads, installed cold and used anti seize. what say you? thanks tons oldduke

hi sages- if i discern the answers correctly, i will wait until the head and exhaust manifold is touchable without a burned hand. will be careful not to tighten new plugs too tight. thanks tons oldduke

hi sages- thank you all for your interesting insights. plugs left in for 100k may or may not be a problem, especially since many new cars specify 100k plug changes. my original question though. does anyone have an opinion on how long you should allow the engine to cool before r and r of the plugs? thanks oldduke

Hi,

It's not quite as simple as that. Holes enlarge only if they're in a mass that is free to expand in all directions. If they're constrained, as a cylinder head is with bolts, they may become smaller. You can verify this by putting a piece of metal with a hole into your vice and heating it. The hole will expand in the direction that's parallel to the vice jaws, and contract in the perpendicular direction, becoming distorted. If it's constrained in all 4 directions, the hole will stay round but be smaller.

I know that the cast iron holding the bolts also expands, but aluminum expands at twice the rate of cast iron, so there is constraint. The same principle can be used to explain why aluminum cylinder heads warp so easily.

Other factors to consider are that the spark plugs expand when heated, and that aluminum looses strength much more quickly than steel as the temperature increases.

I think the best argument for taking spark plugs in and out cold is that the threads on both were cut at room temperature, and that's when they should mate the best. Because steel and aluminum expand at different rates, the higher the temperature, the poorer the fit. This causes wear, mostly on the softer aluminum, and eventual stripping.

Peter