Settle an arguement please someone. If my cam belt breaks on my NA J Spec auto, is it a big expensive bang or an embarrassing stop on the side of the road?

 

Chris Davies

Big bang, not valve safe engine

Settle an arguement please someone. If my cam belt breaks on my NA J Spec auto, is it a big expensive bang or an embarrassing stop on the side of the road?

 

Chris Davies

 

 

Chris,

 

 

you have pm for the details you wanted.

 

 

Barry.

Chis, if the engine speed is below 1500 rpm or speed no more than 30 mph then 100 % no damage after that it gets a bit messy, have replaced maybe half a dozon broken belts and every time a new belt has been all that was needed, but then they all failed in the low rpm zone.

 

Jeff TT

Chis, if the engine speed is below 1500 rpm or speed no more than 30 mph then 100 % no damage after that it gets a bit messy, have replaced maybe half a dozon broken belts and every time a new belt has been all that was needed, but then they all failed in the low rpm zone.

 

Jeff TT

 

Just a thought,anyone on here had one go at speed,reason is,according to Nissan's release brochures/spec/write ups they say"If cambelt was to break there should be no damage as the engine is FREE RUNNING" ,so whats that supposed to mean!!

 

Tony

I would take from "free running" that it would have enough clearance that the pistons wouldnt strike the head, but I wouldnt bet on it having had the head off.

Confusion reigns chaps.

 

Anyone out there had a cambelt break...Did your motor survive??

 

It seems to my (albeit hugely ignorant) brain that the clashing of pistons and valves is not a vague 'maybe' but a definite mathematical fact. It is either in the way or not.

 

Comments please

 

Chris Davies (43K and still hoping!!!!!!!!!!!!!!!)

Chris,

In any engine if the piston and valves position overlap then only the correct belt timing prevent them from hitting each other, if the engine is running very low revs the crank / pistons / etc. have a lower amount of inertia than at higher speed, if the belt breaks, in an instant the cam sensor stops i.e. no spark or fuel the valves stop only the crank will continue and this is the crucial bit if the revs are low the crank stops very quickly and hopefully before any damage, it is also likely the cams may spin to the valve shut position too.

 

Of course if an auto transmission is fitted then the drive from the rear wheels disconnects too unlike a manual were getting on the clutch quick time is a must to prevent the wheels turning the engine, sadly most drivers would try dipping the clutch and bump starting mistakenly thinking the engine has just stalled ..not good.

 

Belts are stressed in single places when the engine is stopped rather than uniformly over the belt when the engine is running, this would account for why poor condition belts often fail when the engine has just been started from cold or shortly after, I know to one zed that was left idling in the Winter to warm up and when the owner came out the engine had stopped he tried to restart it several times without success eventually giving in, the belt had gone, after fitting a new belt all was ok , that was over 12 months ago and it is still going strong.

 

Jeff TT

Ask duffer.

 

His cambelt snapped on him when he was at some traffic lights if I remember. It was just after he got his car back on the road.

 

I am sure there was no damage with his engine

To Jeff TT

What a well written explanation. Thanks. But I have to think that even at 1500 RPM, the motor is turning at 25 RP second. You have no 'reaction' time to dip the clutch so therefore the engine is bound to carry on for at least one revolution. In that time, the cams will have stopped, a valve somewhere will be open and the piston will hit it.

Do I understand you right that there is a position in the cam timing sequence when none of the valves are open and therefore, in that position, you can still turn the crank without any serious dysfunction. Is that the natural position of an untethered camshaft?

Once again, thanks for the clear thoughts

Chris Davies