Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing Valve or tappet clearance

I recall obtaining 98% / 100% in my O Level exam in Motor engineering, yes i was if not the first certainly one of them in the UK that took the exam as it was introduced into Secondary Modern, when my father viewed the results he went balistic because i had not achieved 100% in this field, after all i had worked and played in our family garage since i was 9 years of age.
My father contacted the school to investigate why i had only resulted a 98%, the Teacher informed him i could not explain why we required Tappet clearance, sure i new we had to have tappet clearance because i had reset so many, but WHY the clearance had to be there (for heat expansion) was beyond me, rather than be happy with 98, he was unhappy about the lost 2%......thats Fathers for you

If clearances are to allow for heat expansion only, why when tappets are set hot do we still allow clearances? Surely they should be almost nil?

Having an ex Royal Marine for a father was not a whole ball of laughs either ;-). In my early years in training I did a 2 year Further Education Teachers Certificate, it required a number of hours of assessed teaching practice and I did mine at Solihull Technical College. The old adage is true, 'if you want to learn something teach it to someone else' all the stuff I struggled with at college myself and probably scraped a pass in (trumpet blowing I did get Distinctions in all my City and Guilds exams) became clear and understandable when I revisited the subject in my late 20's as I prepared to teach 'Motor Vehicle Engineering' to sixth formers. It's a very humbling process to prepare for questions from a group of switched on 17 year olds and was excellent grounding for many years as a trainer.

Because it's not just for heat, it's to allow an oil film to form otherwise you would be metal to metal contact, it allows for correct seating of the valve head in the seat and also for the valve and valve train to rotate in operation on many engines which is evidenced by the rotational wear patterns in many components, they often run off centre to achieve this.

Clearance has 4 properties :

. compensation of dilatation (heat)
. timing
. allows a gap between rocker/tappet and valve. If you have a gap the initial speed of the opening is greater and more sudden and it increases the efficiency of the gas flow.
. noise

A compromise is made by the maker in front of the usage of the motor.