Chevy & GMC Vans

My 98 van
If a timing chain breaks, do the pistons usually hit any open valves?
215K, and today the engine started and ran fine for 30 minutes, then was parked for a couple of hours.
Upon returning, the motor did not want to crank, but after a couple of tries, it cranked a couple of seconds and then almost stopped and they spun normal again. That was repeated a few tries, before I had it towed to a shop.
I also noticed the speedometer jumping from 0 to 100 MPH and every place in between at times. The seems like it must be some sort of ignition issue, nut I do not see the connection to the speedometer.
I am prepared for it to be a timing chain, but concerned about potential piston to valve interference, like many foreign motors I am more familiar with.

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The GM 4.3L v6 is a non-interference engine in that the valves open into reliefs cut into the pistons such that they will not contact the piston even if the valve is open to max lift with the piston at TDC. As such, if the timing chain has broken, turning the motor over will not harm anything.May 1, 2012
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You Da Man Crunchy !!

Thanks!
I’ve never understood why someone would design something like a zero clearance engine when you don’t have to. Things break-design it to live to fight another day.

Thanks Captain C., for that piece of information. I did not get a chance to test that information, but I am glad to know the valves and pistons, would probably would have survived a timing chain failure.
Since writing that request for information, it has been proven that it was not a timing chain failure, as I had I expected, as a possible worst case scenario.
I had the van towed to a local shop I trust, and whom I have used occasionally when a task was either more than I wanted to tackle, or over my pay grade, or urgent ( and in this case, my shop is full with another immovable project.
With 215K on the van, and not knowing the history of the first 210K, I figured a stripped timing chain was certainly a possibility.
The shop informed me that all the stumbling and misfiring and sudden stopping of the cranking was due to a bad distributor cap. That was a new AC Delco distributor cap installed last December, with less than 3K on it. I still have the receipt and the box it came in. With the new cap, the 4.3L fired right up and ran smooth.
They subjectively checked slop in the timing chain ( turning crank and watching distributor ), and pronounced it acceptable; whatever that means.
MERRY CHRISTMAS TO ME! I got off cheap this time.
Back in the late 1970's I ran a foreign auto repair shop. MY first introduction to interference motors was a Ford Escort. It was also my first ever need for 16mm and 18mm wrenches and sockets. It was also the first time I had ever seen a heavy use of a mixture of SAE and metric sized fasterners.
As I remember it, taking the busted up cylinder head of that motor required every size metric and SAE tool we had, up through 3/4' ( or 19mm) and I think I had to go buy a 16mm deep socket to fit one particular location.
Ford was in the transition period of going from SAE to Metric, but the US did not seem to want to use the European DIN bolt head standards ( commonly 8, 10, 13, 15, 17 and 19mm), nor the Japanese equivalent ( 8, 10 12, 14, 17 and 19mm). They began using nuts and bolts with a 16mm or 18mm hex, and as you have found out, we now have that combination on our vans. I still see metric wrench 'sets' for sale that are missing either ( or both) the 16mm and 18mm sizes.

Happy Holidays!

Captn. Crunch wrote: ↑Wed Dec 04, 2019 6:46 pm Thanks!
I’ve never understood why someone would design something like a zero clearance engine when you don’t have to. Things break-design it to live to fight another day.

Compression ratio. If you design the head-to-piston space such that the combustion chamber is tall enough to accommodate the valve lift even at TDC, then that dictates a certain (relatively large) combustion chamber volume. Once compared to the "stroked" volume, this will always drop the compression ratio (since Rc = Vs+Vc/Vc (the ratio of stroked volume plus combustion volume to only the combustion volume).

Since easier to make more, and cleaner, power with high static compression (especially with modern FI engines and modern valve metallurgy where very high combustion temps are easily accommodated), that's what drives the design decision towards smaller combustion chamber volumes, which leads to interference engine design. The SBC and its derivatives stem from a time when this was less of a motivating factor.

Good point-good info. I’ve always been a draft horse vs. a thoroughbred race horse kind of guy. High compression leads to premium fuel and zero clearance. If I had money to spend on making hp. I’d be putting turbos on low compression big blocks. I remember watching a car show on the tv where they put twin turbos on a 454 and broke the dyno. I like low rpm torque over hi rpm hp. Guess that’s why I’m a old school Harley guy too.