Timing chain broke

[BigBens1969]

69 with a 307. Timing chain gears wore down(original chain and gears, said gm on them) and the chain slipped off. Replaced the chain and gears and discovered all of the pushrods were bent. Replaced all the pushrods and reassembled. Truck still won't start, just turn over and backfire periodically. Any ideas what it could be?

[jjzepplin]

Welcome to the forum!!!! So how did you adjust the rockers?

[Mike C]

If the pushrods are bent, even odds some valves are as well. Time for some due diligence with a compression gauge.

[BigBens1969]

Tightened all till there was no play in the pushrod. Rotate 90° tighten any loose till no play, rotate 90° and so on for 2 full rotations. Hand turning the motor. Mike C that's what I was afraid of.

[jocko]

Welcome. Did you properly phase the cam and crank (i.e. line up the dots on the cam (6 o'clock)/crank (12 o'clock) gears while ensuring on the #1 cyl compression stroke (both intake and exhaust valves closed)?
Keep in mind that when the chain breaks, the cam stops pretty much immediately - the crank may spin slightly further. Proper phasing is a must. (and you may have done this, just wanted to mention just in case)
Concur with Mike, better check compression - but I have had this happen and not bend any valve stems - bent about half the pushrods though. I was lucky. Well, relatively..

[TheSeeker]

This happened to me but slightly different because my truck has a 6 cylinder 250 and the timing gears no chain. It exploded on me at 50 mph coming off the freeway ramp. There were no bent pushrods on mine but I did have to drop the oil pan and flush the motor of all the shards of the nylon original timing gear. I don't know if the metal from chain breaking would find it's way to the oil pan on a V8 but might be worth checking the condition of the oil.

The timing on the motor needs to be verified and set to correct spec before you should diagnose any other parts. Focus on the system you changed/replaced and go from there.

When you changed the gears and chain did you match them to exactly the same spots as the originals you took off?

[BigBens1969]

I alighned the dots on the timing gears when installing them so I think(not positive) the timing is correct. Didn't verify #1on tdc when installing though. I will check that tomorrow. Is there any way to tell without pulling the cover again. ( Tdc on #1 and check the distributor location in it's roation, will that work?) I also had the oil pan off to do the rear main seal while I was working on it. Cleaned it out real well. Not a whole lot in there.

[AK49Watson]

Does it backfire through the carb or out the exhaust?

[BigBens1969]

Out the exhaust

[geezer#99]

Good chance you’re out 180 degrees.
Easy test is to put #1 plug wire where #6 is and rewire from there.
If it runs then you can change it back but this time by moving the rotor.

[AK49Watson]

Find TDC on #1, rotate back to 8-12BTDC, and drop the distributor in with the rotor pointing at your #1 cylinder post. This will will ensure your timing is good enough to start. Adjust the valves using the EOIC method (Google it, there's good guides out there) it takes longer but is fool proof and 100% accurate. If it still doesn't start, time to break out the compression tester.

[bigmac73]

okay I have to ask this question , since it was the original Timing Chain and gears in a 69 307 and you could have possible valve train issues (bent valves) , why just a quick fix and why not rebuild the whole engine, or upgrade time? (basic shortblock and head work)

How many miles are on this engine?

I fully understand budget and time issue as well , but just curious since no one has asked the question

[Bigdav160]

Chevrolet v8 pistons will hit the valves when the timing chain goes out. I've seen it on stock engines when I was a tech.

[BigBens1969]

Budget constraints are what's holding me back from upgrading the engine.

[randy500]

Originally Posted by jocko

Welcome. Did you properly phase the cam and crank (i.e. line up the dots on the cam (6 o'clock)/crank (12 o'clock) gears while ensuring on the #1 cyl compression stroke (both intake and exhaust valves closed)?
Keep in mind that when the chain breaks, the cam stops pretty much immediately - the crank may spin slightly further. Proper phasing is a must. (and you may have done this, just wanted to mention just in case)
Concur with Mike, better check compression - but I have had this happen and not bend any valve stems - bent about half the pushrods though. I was lucky. Well, relatively..

Ignore these instructions...when the dots are lined up the engine is not on #1 compression stroke.
When the dots are lined up the engine is on #6 compression stroke.
For whatever reason this is not explained in any repair manual that I have read.

[randy500]

Originally Posted by geezer#99

Good chance you’re out 180 degrees.
Easy test is to put #1 plug wire where #6 is and rewire from there.
If it runs then you can change it back but this time by moving the rotor.

There is also a good chance he is not 180 degrees out...

[randy500]

The best course of action is to follow standard repair procedures...

While I believe turning the motor 90 degrees, bla bla bla will work to adjust the valves, why not read the repair manual and follow the procedure? If you did that you probably would be driving it now....

Then once the valves are properly adjusted, find TDC by using a known procedure such as watching the intake valve on #1 close while turning over the engine by hand, then bring the timing marks to 10 degrees advanced, install the distributor with rotor pointed to #1 and wire per firing order.
Or feel for compression with your finger in the #1 spark plug hole while turning over the engine by hand.

Since you bent all the push rods you likely have bent valves, you should do a compression test as part of the repair procedure.
You should have done that right after you installed the chain and gears to determine if you have bent valves. If you would have done that and did have bent valves you could have saved yourself a bunch of time assembling an engine that wont run....

[bigmac73]

Originally Posted by randy500

Ignore these instructions...when the dots are lined up the engine is not on #1 compression stroke.
When the dots are lined up the engine is on #6 compression stroke.
For whatever reason this is not explained in any repair manual that I have read.

I agree line them up dot to dot then do one revolution so that the dots are at 12 o'clock that should be at Nr 1 , if I am wrong some one correct me please

[Ironangel]

Dot to dot is for timing your cam to your crank and should never be confused with ignition timing...Period! That is exactly why many mechanics find themselves 180 degrees out...Cam timing is NOT ignition timing...Separate the two and the zombie apocalypse is thwarted...~Ghostrider~

[BigBens1969]

So checked compression on 2 cylinders and got nothing so pulled the heads and valves are bent. So now what's the opinion, replace valves or get upullit 305 heads for $80? Would anything else need replaced other than the valves?

[clay68c10]

Inspect those piston tops just to make sure they're not cracked. They are probably ok but it's worth a look.

As long as the seats and guides look decent I'd stick some replacement valves in, lap em and run it.

[AK49Watson]

Originally Posted by BigBens1969

So checked compression on 2 cylinders and got nothing so pulled the heads and valves are bent. So now what's the opinion, replace valves or get upullit 305 heads for $80? Would anything else need replaced other than the valves?

You're going to want to disassemble the heads and make sure the valve seats are even OK before you spend money on new valves. Who knows what happened under there. Even the pistons for that matter, they could be smashed too.

[CaptRMW]

You may want to steer clear of the 305 heads unless you know the chamber size. Your heads are likely 76 CC and some 305 heads were 55 CC if my poor memory serves me and would only make a bad situation worse.

[jocko]

randy, ironangel - when I screw the pooch, I gotta own it. You are correct - I combined the cam timing step with the ignition timing step. Thanks for that catch, that was bad gouge. BigBen, my apologies for the confusion.