This engine is about to fail - do not fly it & take all steps necessary to prevent it from flying - it could hurt someone.
Tightness when hot is caused by a fretted crankcase. ie the two halfes of the case have been wearing (usually at the centre main bearing), have lost material and is starting to squeeze the main bearings and crankshaft. This repidly gets worse, and what happens next? The main bearings start to rotate, oil supply to the crank is interupted - engine gets very hot - engine seizes. Game over. This will happen, and I have known this to happen from first signs to engine stoppage in less than 2hours in service.
It is fairly common and very dangerous. If your engineer says its OK - then sack him - he is wrong and being negligent.
Get the engine to an overhaul shop & tell them - fretted crankcase!
(This effects all Lycomings, but is more common on the O-235 as the design of the through studs is different to the bigger engines & is more prone to severe fretting).