losing the timing gear while running... Damage?

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losing the timing gear while running... Damage?

Post by Dr Deuce » Mon Aug 28, 2017 7:59 am

My latest CCKW we call 'Pat' for the previous, deceased owner/friend lost the cam gear on our way to Boston a number of years ago. I was in front of Pat with my CCKW and saw him pull over. I stopped and went back to see what happened. He said it just died on him. I popped the distributor cap and he pressed the starter. It did not turn. He had to have it hauled back home. He had it almost all done including pressing the new fiber on correctly timed when he got cancer and died.

Yesterday, on 'Pats' 1st parade and display. everything was great. After the display, I started the engine and it ran like crap! It also smoked and I could tell it was missing at least one cylinder. I checked the distributor wires and everything was OK..

When I got it home, I removed the valve cover and everything seemed except for one cylinder which had disconnected from the pushrod. I removed the whole pushrod assy and discovered 3 bent pushrods which I replaced. I ran out of time and have not tried it yet.

QUESTION: Is the 270 (original or 50's) an 'interference engine' meaning that the piston can hit the valve head if the valve timing is off as happens when you lose the the cam gear?


BTW: I always install an electric fuel pump and make a plate for the opening. A friend suggested that after WW2 he worked for a Chevy dealer and they would regularly install a fitting on the timing cover for the oil filter drain hose to provide extra oil to the fiber gear. I did this too on 'Pat' and will do it in the future on all my 270's and 302's.
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Re: losing the timing gear while running... Damage?

Post by motto » Mon Aug 28, 2017 3:54 pm

Valve damage in event of timing gear failure is not a feature of the CCKW 270 engine. If the head has been planed to increase compression, all bets are off.
The most likely cause for bent pushrods is stuck valves. If stem clearances are minimal and the engine is not used for a time this can be a problem.

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Re: losing the timing gear while running... Damage?

Post by Dr Deuce » Tue Aug 29, 2017 8:26 am

I have been thinking that too, except I had it running for a couple hours driving around before this happened.

Very strange...
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Re: losing the timing gear while running... Damage?

Post by motto » Tue Aug 29, 2017 2:29 pm

As you say, very strange. However it is not beyond the realms of possibility that the damage was done on the initial start, (bent pushrods and possibly damaged timing gear) with failure taking place on the second start after cooling off. I would certainly check for sticking valves before attempting another start and if no problem then a compression check also. Damage to the timing gear remains a possibility and is not so easy to determine.

David

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Re: losing the timing gear while running... Damage?

Post by D.R.H. » Tue Aug 29, 2017 7:15 pm

Having the valve lash set too tight will also result in bent push rods. That is why the tech. manual recommends that they be set after the engine has been run and "normalized". Then HOT clearances should be followed. It is also possible that the cam bearings are worn to the point of camshaft cavitation. This will cause your experience as well.
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Re: losing the timing gear while running... Damage?

Post by Warren Duchesne » Wed Aug 30, 2017 5:05 am

I am not sure how long it sat up but old fuel turns gummy and can stick valves just in the manner you describe . It usually sticks the valves after the motor has been ran and then restarted . Warren Duchesne

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Re: losing the timing gear while running... Damage?

Post by kw573 » Wed Aug 30, 2017 12:36 pm

It is also possible that the cam bearings are worn to the point of camshaft cavitation.
How does camshaft bearing cavitation cause bent pushrods?

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Re: losing the timing gear while running... Damage?

Post by Dr Deuce » Sun Sep 03, 2017 9:43 am

Before I take anything apart, I bought a Home Colonoscopy Video Scope (ha ha) so I can look into the cylinders sans plugs and down at the cam. I am not sure which engine this is, 40's or 50's. The intake manifold (port size) says 40's, but I have heard of people using special stepped intake port guide rings to allow the mixing. Also, the original 40's engines had 'turbo-tops' with a large lump cast into the top. The 50's had flat topped pistons. NEVER mix the heads and Pistons!
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Re: losing the timing gear while running... Damage?

Post by radarsgunner1 » Sun Sep 03, 2017 10:24 am

I stripped two cam gears in my truck both were new manufacture. jim carter sells aluminum cam gears, which is what I'm running now.
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Re: losing the timing gear while running... Damage?

Post by Dr Deuce » Sun Sep 03, 2017 1:21 pm

Before I bought it, the previous owner "Pat" had lost the gear. He put a new fibre gear on it. I have the drain from the oil filter going into a fitting on the timing cover to give extra oil.
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Re: losing the timing gear while running... Damage?

Post by fernando mendes » Sun Sep 03, 2017 2:26 pm

motto wrote:
Mon Aug 28, 2017 3:54 pm
Valve damage in event of timing gear failure is not a feature of the CCKW 270 engine. If the head has been planed to increase compression, all bets are off.
The most likely cause for bent pushrods is stuck valves. If stem clearances are minimal and the engine is not used for a time this can be a problem.

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Re: losing the timing gear while running... Damage?

Post by Dr Deuce » Sun Sep 03, 2017 4:49 pm

I had the Chevy 350 valve conversion done on my 43 CCKW years ago. The engine was getting tired and I read an article by Reg Hodgson in Army Motors where he had his done. That head may be put onto Pat as I had only put about 10k miles on it before I put in the '56 GMC 302 instead.

I actually thought the engine ran a bit worse after the valve conversion which was done by a racing car engine machine shop. Looking at the video, machining the head that way may interfere with the air flow (my guess) because it looks like the intake is now recessed.
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Re: losing the timing gear while running... Damage?

Post by Joe Gopan » Thu Sep 07, 2017 4:38 am

Some of the CCKW's having WWII era Fiber Timing Gears can be a time bomb as they can deteriorate and the resulting rotten teeth will strip. The M-211 of the 50's went to aluminum. Aluminum 270 timing gears did exist during WWII.
Many of the Euro return CCKWS have those bad fiber timing gears.
M-211 uses same Camshaft as 270 WWII CCKW, but used the aluminum gear.
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Re: losing the timing gear while running... Damage?

Post by mudflap » Mon Jan 06, 2020 9:28 pm

Hi Dr. Deuce,
QUESTION: Is the 270 (original or 50's) an 'interference engine' meaning that the piston can hit the valve head if the valve timing is off as happens when you lose the cam gear?
I have wondered about this myself for some time. While I know many sources have stated that the production CCKW is a non-interference engine, I still believe in the old adage quoted by Ronald Reagan: "Trust, but verify."

So... Using the engine that self destructed awhile back when the crank broke, I set up a little experiment. I put together a partial build, using the remains of the crankshaft, a connecting rod, piston, bearings and cylinder head.

I installed the front half of the crank and main bearings, torqueing the caps. Next the #2 rod, piston and rod bearing were installed. The pistons appeared to be factory replacements, 0.020" oversized. The top of the piston was cleaned of any carbon deposits. The crank was rotated until the #2 piston was at TDC.

The #2 intake and exhaust valves were removed from the cylinder head, and the production valve springs were replaced with low rate springs from the local hardware store. The valves were inspected. They were obviously used, but appeared to be in good condition - no large deposits on either valve. Not sure if the had every been resurfaced, however they seemed to have reasonably good sized margins. The cylinder head was installed using the production head gasket. Several head bolts were installed around cyl. #2, and tightened to around 25 lb.ft.

A dial indicator was installed and zeroed on top of each valve.

270 GMC Valve Travel Measurement Lores.jpg
270 GMC Valve Travel Measurement Lores.jpg (60.28 KiB) Viewed 1232 times

Each valve was then pushed down until it stopped, with the following results:
  • The intake valve stopped at 0.440"when the valve retainer came into contact with the valve guide.
  • The exhaust vale valve contacted the piston after 0.408" of travel.
I was unable to find any published data relative to camshaft lift or rocker arm ratio specifications, so I measured some of the components I had on hand. I had two spare cams - the lift on one measured 0.270", the other was 0.240". The lift for intake and exhaust valves was the same on either cam.

After measuring the rocker arm, I came up with a ratio of 1.4:1. This may be off a little, I know, as the effective ratio is somewhat different due to the change in geometry as the rocker arm contact point slides across the valve stem when in operation. However, using this value means that theoretically, at full open the valve moves a total of 0.378" if used with the 0.270" lift camshaft.

Since the intake valve cannot possibly hit the piston, all is good there. However these data show that worst case, there is only 0.030" of clearance between the piston and the exhaust valve - at least on this configuration. Other variables exist that might negatively affect clearance:
  • Deposits on the piston and/or exhaust valve.
  • Condition of the cylinder head (a virgin valve seat will cause the valve to sit lower than a head which has had the seat machined ).
  • Condition of the exhaust valve (the margin will be thicker for a new valve than for one that has been resurfaced).
  • Valve lash adjustment.
  • Camshaft variation (not sure which camshaft represents the correct lift).
  • Resurfacing of either the cylinder block deck, or the cylinder head.
  • Engine to engine, and part to part variation.
Granted, the cam would have to jump several teeth in order for TDC to coincide with max exhaust valve position. However, if it does, the above indicates that the safety margin in this case is only 0.030". It would be interesting to know if the bent pushrods that you experienced were intake or exhaust.

Best Regards.....

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Re: losing the timing gear while running... Damage?

Post by Dr Deuce » Tue Jan 07, 2020 5:04 am

In the case of Pat, it was the middle two intake valves that bent the pushrods and broke the lifters.

My guess is that because the middle two intake valves just behind the carb probably run a bit richer and that the 'aged fuel' runs fine when hot or the engine running, that when the engine is turned off, the rich mixture on those valves combined with the engine cooling is what causes the cooled, rich fuel on those intake valves to glue them to the head.

My theory anyways.
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