Broken timing chain, valve involvement?

[homemade]

4.0 SOHC, Main timing chain (crank to Jack shaft) broken. What are the chances that the valves have not been damaged?

 

[Sedition]

If the Crankshaft to Jackshaft timing chain has actually broken, the Camshafts will no longer rotate. As the SOHC is an interference engine, the Crankshaft will continue to rotate until the pistons collide with whichever valves happen to be open at the time.

In answer to your question; it all depends on the severity of the valve collision. So basically, if the chain broke whilst the engine was at idle then the valves will be in much better conditon then if the chain broke whilst at 5000+rpm with your right boot buried in the carpet.

Best case scenario:
If the timing chain broke immediately when the engine was first started the collision could have occured whilst the oil pressure was low enough so that as the first piston came into contact with the first valve in its path, the strike may have only compressed the valve lash adjuster and perhaps rolled the cams over a little causing no valve damage, unless of course, an attempt to re-start the engine was made.

Worst case scenario:
The primary timing chain snapped whilst overtaking an 18 wheeler at wide open throttle at over 5000rpm causing all pistons to collide with most if not all valves, coverting the entire engine into 500 pounds of scrap metal.

If I was in your situation I would at least take the valve covers off and take a look before making any major decisions in what steps to take next. If the valve stems and associated hardware seem ok, replace the primary chain, set the timing and turn her over by hand nice and gentle. If its still seems ok, I'd compression check the cylinders this will give you a good indication of how the valves are seating. If the compression is "ok" over the 6 pots, I'd put her back together and fire her up.

 

[2000StreetRod]

inspection camera

A possible alternative to pulling the valve covers and checking for valves that won't seat is to borrow an inspection camera/viewer.
Digital Inspection Camera
Pull the spark plugs and look at the pistons thru the plug ports. If the pistons have struck the valves it should be obvious.

 

[homemade]

35-40 MPH when it quit, no noise, thought it was out of gas. Many attempts to restart. No clicking or clacking. Originally thought starter sense I could only hear a whir and could not see fan movement from drivers seat. Assuming damaged valves, can you simply replace the valves and run the dinged pistons, or are the heads and cams likely damaged and pistons need to be replaced.

Thanks for the advise.

 

[Sedition]

35-40 MPH when it quit, no noise, thought it was out of gas. Many attempts to restart. No clicking or clacking. Originally thought starter sense I could only hear a whir and could not see fan movement from drivers seat. Assuming damaged valves, can you simply replace the valves and run the dinged pistons, or are the heads and cams likely damaged and pistons need to be replaced.

Thanks for the advise.

Doesn't sound too bad. It would have lost oil pressure strait away with the oil pump being driven by the jack shaft. I would deffinately compression check it before removing the heads to repair valves thay (may be) damaged. Might be lucky with this one.

I personaly would run dinged pistons but never cracked ones. Cam gear and shafts should be ok but I would check over them to make sure a rocker arm has not become dislodged or something.

 

[homemade]

No compression at all, Thus the whirring sound when trying to crank. It shut off immediately with no noise. I don't think loss of oil pressure would an issue sense it shut off abruptly. My problem is that I have no place to work on it so I will have to let it sit or pay to have it checked out. I am afraid my decision at this point is pay to have it evaluated and perhaps repaired or go ahead and replace with a crate engine. I welcome any and all advise, observations and suggestions.

 

[miker104]

If you take it somewhere and have to pay for the diagnosis then they tell you to put a used or crate engine in you are essentially out the time/money for the diagnostics. Kinda hard to tell you what to do. If you know the timing chain broke I would expect some valve damage. May want to get an estimate for repair (R&R heads, valve work, Timing chain replacement) vs engine replacement.

Not being able to do any of the work yourself will definitely add up fast. Whatever action you consider don't look at it like the car is worth $7k and its going to cost $5k to fix it. Look at how many car payments the repair cost will take up. I went through something similar and the $5k cost was less than a years worth of car payments. Much cheaper cost in my mind.

 

[matt0248]

Other things to consider too is how many miles are on the engine itself. If it is a high mile engine already you may just want to consider a complete engine replacement, either remanufactured or a low mile used engine. Just some thoughts.

 

[homemade]

Other things to consider too is how many miles are on the engine itself. If it is a high mile engine already you may just want to consider a complete engine replacement, either remanufactured or a low mile used engine. Just some thoughts.

I agree. Till now I would never have thought of 92K being high mileage. The engine is slick as a whistle. I can turn the crank with my hands, and yes the pistons reciprocate. Absolutely no compression.