Zero clearance

What exactly is zero clearance in terms of valves and lifters? Does it mean that the rocker arm pads are always in contact with the valve stems?
Thanks in advance

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"azazel scratch" wrote:
What exactly is zero clearance in valves and lifters? Does it mean that the rocker arm pads are always in contact with the valve stems? __________________________________________
Yes, it means exactly what you said.
Zero clearance is not a desirable condition in valve trains. It is a starting point for valve adjustment.
For mechanical lifters, you position the rockers at zero clearance above the valve stems (at their TDCs), then increase the clearance to the gap recommended by the maker (example: 0.5 mm). If clearance is not provided, running the engine will burn the valves because thermal growth of the valve stems will prevent the valves from closing.
For hydraulic lifters, you position the rockers at zero clearance above the valve stems (at their TDCs), then tighten them to an interference recommended by the maker (example: 3/4 turn). If interference is not provided, running the engine will permit the pushrods to loosen and pound on the lifter and rocker sockets.
Rodan.
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Thanks for the info. I believe my head is completely unuseable after it was machined. At TDC some valves are already depressed and some have gaps between the stems and rocker pads, so the valves aren't closing enough or opening enough.

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Scott,
Thanks for your input. The tips of the valves were grinded. I tightened the rocker arm bolts one at a time only when the cylinder for that bolt was at TDC. After tightening all 6 bolts I moved the engine to TDC for each cylinder one cylinder at a time with a socket wrench. The engine has hydraulic lifters and the valve lash is not adjustable. It is at TDC I see some valves are depressed by the rocker arm pad and some rocker arms I can lift off the valves effortlessly. For example, at TDC for cylinder one the exhaust valve is depressed but I can lift the rocker arm off the intake valve easily. At TDC for cylinder three I can lift both rocker arms off the valves with one finger and I am not even pushing the lifters all the way down.

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"azazel scratch" wrote:
The valve stems were ground. I tightened the rocker arm bolts one at a time only when the cylinder for that bolt was at TDC. After tightening all 6 bolts I moved the engine to TDC for each cylinder one cylinder at a time with a socket wrench. The engine has hydraulic lifters and the valve lash is not adjustable. It is at TDC I see some valves are depressed by the rocker arm pad and some rocker arms I can lift off the valves effortlessly. For example, at TDC for cylinder one the exhaust valve is depressed but I can lift the rocker arm off the intake valve easily. At TDC for cylinder three I can lift both rocker arms off the valves with one finger and I am not even pushing the lifters all the way down. _______________________________________________________
There are two TDCs for each piston. At compression stroke TDC both valves should be closed. At exhaust stroke TDC the exhaust valve may still be partially open for final scavenging.
It is possible that the setup is okay and that collapsed lifters will pump up and tighten the loose pushrods when the engine is started, but if it is not okay, the pushrods will bend or break.
Some engines with hydraulic lifters have no adjustment. Proper pushrod insertion into the lifters requires maintaining the distance from the valve stems to the head face (if the block is not resurfaced). When valves and seats are machined the distance increases. When the head face is machined the distance decreases. After head facing and valve/seat grinding, stems must be ground to bring the distance back into specification. Is your engine one of these?
This installation should not be this much trouble. Can you describe the setup and what you are doing in more detail. In example, why didn't you notice the too loose/too tight valves when you tightened each rocker pad, instead of later when you turned the engine by hand? Someone familiar with that engine may be able to could instantly solve the problem, if you will identify the engine.
Good luck.
Rodan.
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Than you Rodan,
The head surface was machined and the seats and valves ground. I can see the newly machined surface of the head but I'm relying on the integrity of the shop that they machined the seats and valves.
When I check TDC I'm always on the compression stroke when both valves should be closed. Turning the engine by hand with the spark plugs removed I can see when the piston is at its farthest distance of travel and when the rotor is pointing at that particular cylinder.
I thought that I could just reinstall the head after I got it from the shop and the engine would run great. It was only after I heard the engine and saw the smoke that I tried to find out what could be wrong. I did remove the push rods and checked them against a glass surface. They are all straight.
The engine is a 250 4.1 straight six.
I'm going to call the owner of the machine shop tomorrow and inform him that the problem isn't retarded timing. I believe he will come to the house to listen to the engine.
Thanks again

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"azazel scratch" wrote:
The valve stems were ground. I tightened the rocker arm bolts one at a time only when the cylinder for that bolt was at TDC. After tightening all 6 bolts I moved the engine to TDC for each cylinder one cylinder at a time with a socket wrench. The engine has hydraulic lifters and the valve lash is not adjustable. It is at TDC I see some valves are depressed by the rocker arm pad and some rocker arms I can lift off the valves effortlessly. For example, at TDC for cylinder one the exhaust valve is depressed but I can lift the rocker arm off the intake valve easily. At TDC for cylinder three I can lift both rocker arms off the valves with one finger and I am not even pushing the lifters all the way down. _______________________________________________________
There are two TDCs for each piston. At compression stroke TDC both valves should be closed. At exhaust stroke TDC the exhaust valve may still be partially open for final scavenging.
It is possible that the setup is okay and that collapsed lifters will pump up and tighten the loose pushrods when the engine is started, but if it is not okay, the pushrods will bend or break.
Some engines with hydraulic lifters have no adjustment. Proper pushrod insertion into the lifters requires maintaining the distance from the valve stems to the head face (if the block is not resurfaced). When valves and seats are machined the distance increases. When the head face is machined the distance decreases. After head facing and valve/seat grinding, stems must be ground to bring the distance back into specification. Is your engine one of these?
This installation should not be this much trouble. Can you describe the setup and what you are doing in more detail. In example, why didn't you notice the too loose/too tight valves when you tightened each rocker pad, instead of later when you turned the engine by hand? Someone familiar with that engine may be able to could instantly solve the problem, if you will identify the engine. Good luck. Rodan. __________________________________________________________
"azazel scratch" wrote:
The engine is a 250 4.1 straight six.
The head surface was machined and the seats and valves ground. I can see the newly machined surface of the head but I'm relying on the integrity of the shop that they machined the seats and valves.
When I check TDC I'm always on the compression stroke when both valves should be closed. Turning the engine by hand with the spark plugs removed I can see when the piston is at its farthest distance of travel and when the rotor is pointing at that particular cylinder.
I thought that I could just reinstall the head after I got it from the shop and the engine would run great. It was only after I heard the engine and saw the smoke that I tried to find out what could be wrong. I did remove the push rods and checked them against a glass surface. They are all straight.
I'm going to call the owner of the machine shop tomorrow and inform him that the problem isn't retarded timing. I believe he will come to the house to listen to the engine. _________________________________________________________
That's the best thing you could do. I'll bet the machinist will solve the problem immediately.
Good luck.
Rodan.
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On Sun, 20 Apr 2008 13:17:41 -0700, "azazel scratch"

First off, IIRC, you are working on a 250CID six which is the big brother to the 170/200 engines. You have had the head milled which removes material from the head surface as you already know. That process will reduce the clearance on each and every valve if you put the rockers and pushrods back in their original positions. - no exception to this. Now, you say one valve is open and the other has clearance when at TDC. Don't forget that the engine has essentiall 2 TDC's for each complete cycle of operation. The cylinder needs to be TDC of the compression stroke - not the exhaust stroke - to check the valves. Next, if the machinist has properly ground the valve stems to the proper installed heights, the valve should be pretty close to the ballpark for a properly set valve train. If any valves are not set correctly to center the lifter as I have described to you previously, you will have to implement whatever method your engine requires to properly set the valves. Some require different length pushrods which are available where required. Some engines can be adjusted with shims or screws or the center retaining nut for the rovker arm. I do not recall which method your engine uses. The bottom line is that your head has not bee cut too far to adjust the valves. You can cut it all the way to the water jacket and still be able to adjust those valves even if you have to order special length push rods. Jeggs and Summittracing are among many sources of a special checking adjustable pushrod used to determine the correct length pushrod for your engine. You are giving up way too damn soon. You need to step back an lookat what you are doing. Make sure you are checking this correctly with the cylinders at compression TDC. If you are not sure of exactly how to do this, you may need to do what many of us do and check out a good repair manual on the subject.
BTW, zero clearance is exactly what hydraulic lifter in your engine are designed. They keep all of the clearance out of the valve train using engine oil pressure to expand the lifter. The lifter has a controlled bleed so that it does not over adjust causing the valves to remain off their seats and leak. Mechanical lifters which are always mechanically adjusted periodically have a measureable clearance at some pre-determined place in the valve train. Many of the have and exact timing location for checking. Your engine is not mechanical lifter and needs to be checked at the compression TDC position. If the bled and compressed lifter gives you 0.60-0.80" clearance, it is good to go. As soon as the engine starts, the engine oil pressure should pump up the lifters to eliminate any clearance.
Good luck
Lugnut

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lugnut,
Once again thank you for your valuable considerable knowledge. I did check every cylinder at compression TDC. And I also looked into oversized and undersized pushrods. It looks like I might have to go that route.
Thanks

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just thought of something.........Did you use new lifters? You should, it could be some are collapsed/stuck. Sounds like it is hyd lifters that are adjustable and it is next to impossible that you would need different push rods to make it right.

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yep hyd lifters. KB
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