No compression

I've attempted to fire my rebuilt Chevy 307 for the first time, and it backfired while making a loud bang! I found the timing to be off a little. I fixed that, but now I only get spark and fuel - no compression in any cylinder. All pistons rise and fall as I hand crank the engine. The valve covers are off and all valves open and close. Is it possible I blew both head gaskets? Someone at work said to check the timing chain. But isn't the timing chain moving the cam and pushrods? My timing mark on the balancer is keeping pace with my distributor rotor showing TDC when #1 valves are both closed. Donivan - I'd love to hear your opinion if possible. Thanks for any tips you guys may have.

I have never done a compression check by cranking the engine by hand. That might be the problem.I would hook up the starter and try again.

mike

I did use the starter at first. The push-in compression gauge wasn't reading anything, so I switched to a compression gauge that screws into the spark plug hole. Still no reading. After that I began hand cranking the engine and feeling down into each cylinder with a screwdriver to make sure the pistons are moving.
I also tried putting compressed air into the cylinder at TDC and can hear air under the carb/intake. The intake/exhaust valves were closed. I'm guessing head gaskets are leaking.

you have engine, and distributor timing to check.just because the valves are opening and closing, dosen't mean the "engine "timing is right.slowly rotate your engine over, and watch your rocker arms at #6 cylinder. as top dead center (cyl # 1) at the front pulley and timing cover comes up the valves in cylinder 6 should overlap. this means one will close as the other starts to open on that cylinder. have your distributor cap off to note rotor position too. if the engine timing is right, your marks at the crank pulley will be within the length of the tab on the timing cover, and your rotor will be pointing to # 1 cylinder according to your cap, and the valves for cylinder # 6 will be in overlap.
i have seen the gear on the bottom of the distributor cut the roll pin off when an engine backfires and your dist timing is way off. if its not this then possibly your timing chain jumped at the gears. good luck.........jeff

I really don't think you have two blown head gaskets.Sounds more to me like something with the cam timing. The valves are Open / Closed at the wrong time.That bang could have been a piston hitting a valve.

mike

I'd pull the timing cover and take a look see. Make sure everything is in "time" before proceeding, wouldnt want to hurt a new motor.

Donivan - the hydraulic lifters are new. Yesterday I loosened all 16 rocker nuts and found 15 psi in the cylinders after a quick comp test. Before that loosening, I had 0 psi. This engine is a stock rebuild. Nothing Hi-performance. Yes - there is oil coming up to the rockers. My rockers are adjustable.
JKR - I have put a timing gun on this engine and it blinks right on the mark at TDC. My distibutor rotor is keeping perfect time. I have pulled it out and the gear at the bottom seems okay.
Mountaincowboy - you're the second one to ask about the timing chain.
McFly - your big bang theory sounds logical. When I apply compressed air in to a cylinder I hear it under the intake. I assumed the valves were open, so I undid the rockers and the air still goes in. I assume the gaskets are leaking.
Thank you all for your help.

Sounds like the valves werent lapped! The only way to tell for sure is to pull the heads and do a leak test. You may have blown a gasket, either way the heads are coming off. I spose its possible that the heads werent torqued, but unlikey.

Today I removed the timing chain cover. Chain is fine and the gear marks line up with the distributor at TDC. I put a little oil in a cylinder and did another compression check. My psi hardly went up. I was wondering if my rings were leaking. That led me to think it's a gasket or it's the valves leaking. So I pulled the intake and heads off. All gaskets look fine. No marks on the piston tops from hitting valves. Tomorrow I'm going to stick another set of heads on that I have and see if the compression improves. Then that will confirm my valves are the problem.

Donivan - This engine came from a one-owner 55K '69 Malibu. A guy I know who has built many engines suggested a teardown. After doing so, he said the engine didn't need it as everything was clean inside. I never was able to hear the engine run in the car it came from. So I reassembled it and put in new bearings and gaskets. New oil pump and freeze plugs. I'm not racing this engine, just want my '69 G20 to be close to original as possible. I honed the cylinders and used the original pistons & rings. Was that a mistake I'm going to regret? What is the proper procedure for a leak down test?

As long as the ring end gaps were within specifications I guess you could reuse but once you hone it I would have changed them. Did you hone it yourself ? If you don't get the proper cross hatch on the cylinders, the rings will not seat correct and compression loss will occur.Also during assembly, did you stagger your ring gaps ? I am in no way mechanic but have built an engine or two usually with the help of a friend .

Do what ya want but reusing the rings may have been a bad choice they would have a set to them , ya did all the work already I would have tossed in a new set., with new rings comes making sure the valves are seating okay to , , with only 55k on the engine I cant imagine the guides being bad so that should be a plus on those , maybe just put some new valve seals on 'em. also was there any ridge at the top that could have damaged the rings going back in the old bore?

The only thing I did with the heads was remove each valve and clean off the carbon deposits, and install them back in the same hole with a brand new oil seal. As for the cylinders I did hone with a nice cross pattern. The rings were set at proper angles so the gaps didn't overlap each other. I should have put new rings on. My decisions were based on the mileage of the engine. So I'm guessing you hone a cylinder for new rings only? Thanks everyone for your help and suggestions.

KC Did you check to make sure the valves sealed properly before you put the head back on.since the head is off now lay it down on a flat surface (upsidedown) and fill each chamber with Kerosene and check for leaks.I learn this in H.S. never had to do it another time. MY shop teacher was into what he call a"scotch rebuild" which sounds like what you may have done.

mike

I still say the valves need to be lapped.

Thank you Mike for the tip. I will try that. What does "Scotch Rebuild" mean? Today I flipped the head upside down and filled the chamber with about 72cc of water. Nothing leaked. I put the 307 head back on after cleaning all surfaces and checking them with a straight edge. No warpage found. The air test now pushes a little air through the intake and exhaust ports. I then swapped the head for one from a 350 that ran fine when last used. This time the air test shows no head leaks. The 350 head reads 25 psi compression compared to 0 with the 307 head. The air is now entering the oil pan. I am convinced the rings are leaking and the 307 heads need a valve job.
Donivan - your comment on 155K just might be true. I was told 55K by the seller and the car sure was clean and unrestored. Answers to your questions: The 307 heads were on the correct sides of the engine. The valves look clean except for residue from dried fuel. I found no warpage with a straightedge. This is my first attempt at assembling an engine. Thanks for not laughing at me! It would be too easy to go buy a reman crate engine. I like a challenge.

Kc, you live and you learn.Every day I learn something new.One thing I learn a while back is cutting corners cost you more in the end.But I fall victom to this same advice every day.It is tuff to diagnosed a problem this way but baised on all I have read it sounds like the rings . As Donivan pointed ouy 55,000 mile seems a little far fetched. Don't feel bad one time I built a 455 pontiac motor,all kinds of money did everything right so I thought. 0 oil pressure, after a few long weeks i found I forgot to install an internal galley plug under the distributor .Never knew it even existed.It is all part of the joy of owning a classic vehicle. Ain't it great ??

mike

A '' scotch rebuild'' was to take an engine apart, clean it ,check all tolerences than put it back together. Before it became a polictaly correct world it was a cheap or half ass way of doing something.By the way I an a 1/4 scotch.

I just checked my slips from the autoparts store. a new set of cast rings was 48$ from AutoValue. I'm doing what I thought was a low buck rebuild on my 350. gaskets & rings and a handful of rod bearings from a bud who works at Federal Mogul. What I'm taking a hit on is the heads, the guides were shot, machine work is pricey and there are very few around here who even do it anymore seeing as most of the newer cars run throw away engines and parts.
From what I have gathered , higher mileage heads are apt to have badly worn exhaust guides but once you get into tthe 70's they are set up for unleaded fuel so Im having a set of 882 castings redone, only way to know what ya got without buying new.
I got a kick out of the Scotch ref. Mike, around here we pick on Danes and Hollanders for the same reason, squeeze a nickle and make the buffalo $hit.

I can't help with this but I have to say that I have learned a lot from Donivan not only in this post but most of his posts. Thanks Donivan for all the great tech info.