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Low compression after head rebuild

sdew99

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Hi all, I have a 1979 cb750k that I am rebuilding and have run into a roadblock.

I disassembled the head and cylinder due to low compression so piston rings were replaced, new cylinder gasket, new cylinder head gasket, etc.

After reassembling, I performed a compression test and was between 50-90. I then realized that my exhaust cam was slightly out of time, so I went back and removed the cams, and made sure they were in time (level with cylinder head at the TDC mark and cylinder one lobes pointing towards spark plugs).

Performed another compression to have the exact same results as before. Did a wet test, and the compression rose to 120-150 across the board.

Dreading the thought of removing the head and cylinders again to inspect the rings... rings were staggered and seemed to fit into grooves without any issues so hard for me to believe they are the issue but all things are pointing that way.

Wondering if anyone has any insight to other causes that could be contributing to low compression.

Thanks!
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If the valve clearances were that low that's a very bad sign. How did the valves look? It's possible they are not sealing right because they're worn, or warped, or tulip-shaped, or burned up.

However! You should completely ignore the results of your compression test. Put at least 500 miles of break-in on that engine and don't worry about how much compression you're making until then. (Factory says 600 miles.) The rings and honed cylinder bores have not had time to get familiar yet and your engine simply will not make good compression until they do! You haven't presented them with any gasoline explosions to push them into place yet.
 
If the valve clearances were that low that's a very bad sign. How did the valves look? It's possible they are not sealing right because they're worn, or warped, or tulip-shaped, or burned up.

However! You should completely ignore the results of your compression test. Put at least 500 miles of break-in on that engine and don't worry about how much compression you're making until then. (Factory says 600 miles.) The rings and honed cylinder bores have not had time to get familiar yet and your engine simply will not make good compression until they do! You haven't presented them with any gasoline explosions to push them into place yet.
The valves definitely had a build up of carbon, but came clean. I have the head taken to a machine shop to have the seats refinished so I wasn't personally able to observe the shape of them. I have attached a couple photos showing the valves.

Ok, I have not attempted to start the engine yet as it is still out of the frame.. even with those numbers would it be worth while to see if it will start?
 

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The rings were offset. It is possible that when the pistons were put back into the cylinders they may have moved.. but impossible to tell without taking it apart.
True, they could have. But if memory serves they 'might' rotate a bit in place as the piston does its thing.
 
That first image is troubling. These valves from the factory did not have a very deep hardened coating and if they are put on a grinding machine, that coating will disappear in a flash. Underneath it the steel is not hardened. If all your valves got ground and look like that right valve in your picture, I'd be very worried that your valves will turn into tulips in just a few thousand miles.
That's secondhand information, mind you. I don't actually know it from personal experience and that's me repeating what I've read on this forum from the old farts, but it does make sense because of how the old valves were manufactured. I've got some spare valves lying around from a junked head and I've got half a mind to grind off the nitride, then take a file to them and see how soft they really are underneath.

The low compression reading on a brand-new top end rebuild doesn't mean the same thing as a low compression reading thousands of miles in. In your case, the rings haven't even been seated properly into place by the force of combustion, nor have they worn the cylinder walls to match their profile. Put that engine back together and it should start right up if everything was reassembled correctly.
 
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That first image is troubling. These valves from the factory did not have a very deep hardened coating and if they are put on a grinding machine, that coating will disappear in a flash. Underneath it the steel is not hardened. If all your valves got ground and look like that right valve in your picture, I'd be very worried that your valves will turn into tulips in just a few thousand miles.
That's secondhand information, mind you. I don't actually know it from personal experience and that's me repeating what I've read on this forum from the old farts, but it does make sense because of how the old valves were manufactured.

The low compression reading on a brand-new top end rebuild doesn't mean the same thing as a low compression reading thousands of miles in. In your case, the rings haven't even been seated properly into place by the force of combustion, nor have they worn the cylinder walls to match their profile. Put that engine back together and it should start right up if everything was reassembled correctly.
Ok, that is good to know about the valves. I am not very familiar with the material they are made of so I never thought about the consequences of grinding them down. They are all like that now, so I will have to keep an eye on them and possibly look at replacing them if that starts to happen. Thanks.

Alright.. that's a good point. This is my first rebuild so I wasn't aware that the compression could be lower than before a rebuild. I guess that would explain why the wet compression test had a much higher compression.
 
When I did the valve work on my own 1980 K I only did a light lapping with fine grinding compound because I had read about how thin the coating was. That reconditioned the seats and valve faces well enough for my purposes at least -- the oxide spots were (mostly) gone and the factory valve angle and lips were still intact and a good amount shinier. I then reinstalled the springs and spark plugs and flipped the head upside down, and poured some WD-40 or rubbing alcohol or some other thin liquid (I forget which) into the crowns on top of the valves. None leaked through into the ports. Getting them all water-tight was definitely good enough for me!
I'm also a cheapskate and balked at the price for having a machine shop cut all 16 valve seats, plus having to buy all new valves...the lapping kit was $14 at my local auto parts store!

Here's hoping the old farts talking about those soft valves were exaggerating. Now that I'm curious, I think I will grind into one of the junk valves I have lying around and see how soft it is underneath that hard outer shell.
 
When I did the valve work on my own 1980 K I only did a light lapping with fine grinding compound because I had read about how thin the coating was. That reconditioned the seats and valve faces well enough for my purposes at least -- the oxide spots were (mostly) gone and the factory valve angle and lips were still intact and a good amount shinier. I then reinstalled the springs and spark plugs and flipped the head upside down, and poured some WD-40 or rubbing alcohol or some other thin liquid (I forget which) into the crowns on top of the valves. None leaked through into the ports. Getting them all water-tight was definitely good enough for me!
I'm also a cheapskate and balked at the price for having a machine shop cut all 16 valve seats, plus having to buy all new valves...the lapping kit was $14 at my local auto parts store!

Here's hoping the old farts talking about those soft valves were exaggerating. Now that I'm curious, I think I will grind into one of the junk valves I have lying around and see how soft it is underneath that hard outer shel
 
Whoops.. somehow edited your comment.. first time using the forum so i apologize for being a newbie haha.

I definitely wish I would have done some more research into the valve thing.. I really hope they were exaggerating because it wasn't cheap to have them done. Had I known, I definitely would have just lapped them. Oh well.

So in your opinion, I should try and get the thing running regardless of the low compression? Would there be any other evidence if the rings weren't the right size or hadn't seated into the piston grooves?

There's no unusual sounds coming from it when I'm cranking it over.. if that means anything at all haha.
 
So in your opinion, I should try and get the thing running regardless of the low compression? Would there be any other evidence if the rings weren't the right size or hadn't seated into the piston grooves?

There's no unusual sounds coming from it when I'm cranking it over.. if that means anything at all haha.
Yep. The rings will make abnormally low compression when first assembled after a top-end is done -- that's normal for every piston engine. Again, you haven't put any real force into the piston or the rings to shove them into the cylinder walls and wear them in.

Dirtdigger is also right; you could try another compression tester. You can also check on two things that can foul up a compression test: one is that the carbs need to be off or the throttle has to be held wide open, and another is that your elevation can change the compression readout due to the difference in ambient air pressure. Look up what your town's elevation above sea level is and also look up the compression test conversion factor for altitude.

If the rings weren't the right size, like if your machine shop had bored and honed the pistons to the next oversize but you were still using stock-size piston and rings, you'd hear a ton of piston slap as they bounced around inside the cylinders. It'd probably sound like a rock tumbler when turning over. You'd probably also hear small things bouncing around in there if any of your rings got broken.

If it were me, and I knew a machine shop had done all the work, I'd completely ignore that compression test and see if it runs then test again after the break-in was complete.
 
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