Yea but in non interference (freewheeling? Never heard that one before) engines the combustion chamber is either A) HUGE with a related low power output or B) Above the valve like in a briggs and stratton or Flathead. Even the Dodge Hemi will touch valve to piston with a broken chain!
The primary reason for belts is: they can be used externally. A chain requires oil and thus gets buried in the crankcase behind the water pump etc. And on a OHC or DOHC chain drive - the chains are so long even the slightest stretch can retard the valve timing as much as 2degrees!
A belt can be changed in ~30 minutes with no loss of coolant or oil. A chain takes about 7 hrs (to do it correctly) and requires draining and filling the radiator, the oil and disposal of both...
The secondary reason is (believe it or not) they DON'T stretch. That is until they wear out which occurs faster than a chain towards end of life. A chain wears from day one. If it stretches .005 - in 200K miles - that was at a rate of .005 divided by 200K AND any gain never left (ie if it stretches .002 it STAYS stretched .002).
A belt will have ZERO stretch until around 45K and may gain .003 by 90K. It CAN get stretched momentarily under hard acceleration - but returns back once inertia is more equal between crank and cam gear. Like a rubber band returning to its shape after use.
As a machinist I prefer chains. Even though on a OHC or DOHC if you surface the head - you have moved the cam closer to the crank and that is then "made up" by the chain - resulting in retarded valve timing. A lot of rebuilders forget this as two degrees does not make it run bad - just lower economy and lower power but compared to the pre rebuild it is BETTER so overlooked!
As a consumer I prefer belts - (and have even belted some chevies for drag racing) just MUST do the maintenance on time!
Whenever I have seen an interference engine with bent valves - it was NOT because of "a broken belt". It was because of failure to perform scheduled maintenance. THEN the belt broke! I have never seen an engine come in with all of the valves bent a broken belt and less than 40K on it. I am sure there are some, but VERY rare.
J Wm Bishop EA, ASADE
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