Hello all,

Just wanted to pick some expert brains. My goal is to replace the smog-era 350 with what I believe is a '71 (G cylinder heads) 455 I picked up off FB marketplace. While it is still in the cradle stand I bought one of those mini hi-torque starters from Jegs and cranked the engine to do a compression test. Unfortunately is seems like the engine would only crank for a second, read 70-75 psi and stop and acts like it binds up. I got this same result for all 8 cylinders. Remove the flywheel and the starter operates just fine. I can also turn the engine by hand pretty easy.

I also did a leak-down test and I am seeing serious loss on all 8 cylinders. 85-95% across all cylinders. Now I bought this engine knowing full well it was going to be a rebuild and the price I paid reflected that. My question is more out of curiosity on what might be the likely cause(s) be. The guy that sold me the engine told me he was driving and lost power but able to limp home around 25 mph and mentioned he thought the valve seals were the issue so I was pretty sure I was going to find a compression problem. I guess I was just hoping it would be an issue in just 1 or 2 cylinders.

 

Incorrect valve timing, compression is low but equally low across the board correct? Put the balancer on TDC, pull the valve covers and either cylinder 1 or 6 (not both) rocker arms should teeter open close with a slight movement of the crank in both directions.

It isn't a valve seal problem unfortunately.

It would be good to get a look in the cylinders with a borescope.

 

Yes, that's correct Sugar Bear, low compression across the board. I'll try what you suggested. Thanks for lending your thoughts.

 

How long has it been since the engine was run ? When you are doing the compression test and the engine stops do you have all of the plugs out ? Are you sure the battery you are using is up to the task ?

 

Just going by what was told to me its been almost a year the engine sat in the cradle stand. I measured 12.6V on the battery I was using. The thought about trying a different power source for the starter crossed my mind so I used a 12V charger to jump the starter and the behavior was the same. Even tried the charger on the engine start setting.

On the compression test I did have all the plugs out. In regards to the leak-down test I ran through it again but tried a different method based on a YouTube video. I had a 100 psi going in the cylinder and turned the crank CCW until the piston starts building compression. The results were better but still abysmal.
Cylinders leakage percentage. Cranked on the breaker bar until cylinder moved past TDC.
1. 40% - Cranked like an SOB not sure how I could have done this if the cylinder had little to no leakage.
2. 70%
3. 50%
4. 60%
5. 55%
6. 60%
7. 60%
8. 65%

With each cylinder at TDC I tried to listen and feel where air was leaking from. All I could really determine was I could hear the sound of air in the distributor hole, in the oil neck tube after removing the cap and in the dipstick tube after removing it.

Also Sugar Bear, I removed the driver side valve cover and turned the engine until the mark on the harmonic balancer lines up with the #1 cylinder TDC mark. Using an old metal BBQ skewer I could verify the #1 piston was at the highest point in the cylinder.

 

With #1 at TDC, what do the rockers do when you move the crank CW and CCW? On either #1 or #6 they should teeter open/close as the crank is moved back and forth an equal amount from TDC. Moving the crank about 1.
5 " should be enough to see the movement on either #1 or #6. The goal is to see if the timing chain possibly jumped.

Also could add a squirt of oil into each cylinder to see if the results improve a lot, if they do the rings are bad. Some improvement will always happen with the oil but if it improves major league it's rings.

 

Yes it does move as you said. Pulled both valve covers and put #1 to TDC. Rotated crank CCW a little and it looks like the #6 exhaust valve opens. All cylinders valves appear to open and close as I rotate the engine around 2x.

 

The 50-60% leakdown does not surprise me an an older engine that has been sitting for who knows how many years ? I have never heard of trying to turn the engine over with the leakdown tester putting pressure in it ???? I always put it at TDC and then turn the pressure up.

 

Rotate both CW and CCW just off TDC. Do both rockers teeter? If #6 is teetering then #1 is TDC on the compression stroke and #6 is on the exhaust stroke. Whether it is 1 or 6 that teeters doesn't matter as long as one of them does when 1 is at TDC. This is close enough to tell us if the valve timing is close.

 

With the timing mark on the balancer lined up the timing guide 0 degree mark it looks like if I turn the crankshaft somewhat close to 90 degrees CCW that's when I see the #6 exhaust valve start to open. When I rotate CW from TDC up to 90 degrees it looks like the #7 exhaust valve starts to open.

 

Sounds like the teeth may be knocked off a nylon timing gear. That would explain why it died while running.

 

I'd pull the front cover at this point to see the gears, chain and valve timing. Take pictures and stay organized.

 

sounds like its dry in the cylinders from sitting, maybe also why it stops turning its binding?? maybe? oil up them holes maybe

 

Thank you everyone for the suggestions. I am heading out of town with the wife and kid for the weekend. I'll hoist it out of the cradle onto a stand in a few days, remove everything on the front down to the timing chain and report back.