Blown timing belt? Probably

You guys should see what a mess the BMW M20 engine looks like after a timimg belt breaks or gets out of time. Broken pistons,valve faces jammed into piston tops,mangaled valve guides, it is a real mess.

I had a Nissan Quest (interference engine) in the shop once that broke a timing belt going down the road. Everything checked out okay, so I told the customer this and asked if they wanted to go ahead and fix it, telling them that, hopefully, everything would work out based on leakdown test results. They approved the work, I installed the timing belt and water pump (which is driven by an external belt, but still mounted behind the timing belt on this engine. It leaked all over the timing belt, causing the failure). The engine started right up and ran great, so it does happen. Interference engines do sometimes lose their timing belts without causing great damage. However, there was no convincing the owner that they got very lucky on this one. “Lucky??? #$*%@ $700 repair bill???” “Hey, it could have been a $2000+ repair bill, and according to Nissan, it should have been”

I think what is happening here is that there is a degree of interference involved here. Perhaps the criteria used by the people that say “interference” or “non-interference” needs some standardization.What I mean is , if there is even the slightest chance of a collision, the engine gets the interference lable,pretty good way to sell more belts.

Can anyone explain how the valves and pistons can occupy the same space without damage to both?

Truthfully I have seen it go the other way much more, that is an engine labled non-interference damaged by a belt breakage

Just out of curiosity… Why would the valves or pistons on a non-interference engine get damaged? I’ve seen it too (on a timing chain engine none the less). I just don’t understand it.

Because they really are interference engines, and just how today some that are really non interference are labled interference in the past some interference were called non-interference. The engine I mentioned,the M20 BMW is clearly an interference of the highest degree, no question if this engine will be destroyed every time, even if it is just being cranked. Other type engines get damaged only if the belt breakage happens at high rpm and others are damage simply from when a carbon built up allows for a collision.In these engines the contact is very,very slight, but it is enough to make so the valves do not seat right.I had a friend that timed a V-8 4 cam BMW and one cam was off and only one intake valve got touched very slightly, but it was enough to cause a very slight misfire only noticable at idle.The V-8 4 cam was from around the year 2000(driven by chains) and the M20 came from the 80’s (driven by a belt) and obviously had a much less tolerent relationship between valves and piston movement.

In short, uneven application of the criteria that makes for what should be called interference and what should be called non-interference.

Of course…timing belt goes…there is no way the engine car run…

We need to ascertain if the neon drives its water pump via the T-belt…if so then your scenario is puzzling. Methinks from your description that it does not drive the pump with the T belt. I will look this up for you. Otherwise it sounds like you blew the water pump bearing and the belt that drives the pump couldnt spin the pulley any more…typical. I need to look up that engine design…

It looks like it does drive the pump…there may be a small chance that the pump got locked up or was in the process of locking up…but didnt get there all the way and still allowed the t belt to drie the cams without issue…this is the only scenario I can think of that would match what you said. If your engine was running and you had to shut it down then the t belt didnt suffer a catastrophic failure. You might have gotten lucky here…Have the mechanic do all the checks he can to verify this. You may have squeaked or Zzzzzzzz’ed by here.