View Single Post
Old 3rd May 2021, 11:25 am   #19
emeritus
Dekatron
 
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 5,316
Default Re: Rusted in grub screw.

Heating or cooling will work best where two different metals are involved, making use of their different thermal expansion coefficients (TEC). The approx. TECs for brass and steel are 18-19 and 11-13 respectively. So for a steel screw in a brass bush, heating will enlarge the hole on the bush by more than the increase in diameter of the steel screw, making the fit looser. Conversely, for a brass screw in a steel bush, cooling in a freezer would be required, as applying heat would make the brass screw expand more than the bore in the bush, making it tighter. In the latter case, or where screw and bush are of the same type of metal, if corrosion is present, there might be an advantage in heating due to differential expansion of the corrosion products loosening them, but it would be necessary to cool it down before trying to unscrew. Differential expansion will be much greater when one of the items is aluminium (TEC = 21-24). Replacing a cracked alloy piston in my car involved heating to about 140°C in an oil bath, whereupon the steel gudgeon pin dropped out under its own weight. At room temperature it was an immovable interference fit. Thus heating is likely to work with a steel or brass screw in an aluminium bush.

Last edited by emeritus; 3rd May 2021 at 11:39 am. Reason: typos
emeritus is offline