setting valve gap

[redhead 10]

I am in process of checking the valve and tappet clearance on a kohler K321, which said in book to have valves on compression stroke, isn't both valves closed on comp.stroke? if you sat gap this way won't valves be open from know on, or am I missing something.
Someone explain how to set, and on what stroke? THANKS

[sorekiwi 761]

Yes both valves are closed on the compression stroke.  That is why you set the clearance at that point.  When the cam rotates, this clearance is taken up and then the valve starts to lift.

[gwest_ca-(File Mod) 10,531]

Yes both valves are closed on compression stroke or you would not have any compression. With the valves down that is when you want the specified clearance. Make sense when worded a different way?

Garry

[KC9KAS 4,741]

You are measuring the gap between the end of the valve and the "lifter" while the valves are closed.

[MikesRJ 552]

The other thing to mention here is that "valve wear" consists of wear at several locations. The following assumes that the valve seat (the part in the engine block) and valves contact face (also called a "seat" as shown in the image below) are in known good condition and do not need to be re-ground or replaced due to breakage or wear. It also assumes the camshaft lobes are also not worn out. The main wear locations are on the upward face of the valve seat, and the bottom surface of the valve itself. Also, you can have wear on the valve stem and/or the camshaft lobe, but if the engine oil is replaced regularly this is usually very little compared to the valve and seat. As the engine wears, the valve "sinks" into the valve seat, because of wear at both the seat and valve, effectively closing the gap between the valve stem and the camshaft lobe. The most important factor of setting this clearance is to make absolutely certain that the end of the valve stem is ground absolutely flat and perpendicular to the valve stem. Most home shops do not have the fixture, or the correct grinding wheel. to do this "correctly", and we try to do it by hand on a normal grinding wheel we find in most of our garages. Though this might work, in the long run it will shorten the life of the engine more than if you did it on the correct equipment, or had it done by a local machine shop. The procedure is pretty simple. You rotate the engine till both valves are closed (compression stroke), and both valve stems are no longer in contact with the camshaft lobes. You measure the gap that exists between the valve stem and camshaft lobe, and you write that gap on the top valve with a Sharpie. You then look up the spec. for your engine, remove the valves, and either grind them yourself or carry them down to the machine shop to have them ground. The difference between the number you wrote on the top of each valve and the spec. is how much will be removed from the valve stem. n example, I find the intake valve clearance is 0.004" and I know the valve and seat are good, I take off 0.004 - 0.006" off the end of the valve stem, re-install the valve in the engine, and measure the gap again. I should now be between 0.008 - 0.010 clearance when the engine is rotated to the same location. I always shoot for the middle to give me a margin of error, but always "sneak up to" the correct dimension VERY slowly so I don;t trash a valve or be forced to re-cut a valve seat to make up the difference. Same process for the exhaust valve. When they are both within spec. I move along to the next step in the rebuild process.

A LITTLE BACKGROUND
Say I find an engine which needs a valve job done before I can trust the engine to be installed in a restore. I certainly do not want to get everything painted and installed, then find the engine is failing and have to take it all apart again, messing up the paint job on both the tractor and the engine. Therefore, when I redo a machine, the engine is about the third thing on my list to restore after the wheels and frame are completed. I dis-assemble the engine completely, knock out the valve guides and seats, replace them with new parts, and cut the seats and ream the guides to size. If the seats are cut, then the new valves have to have the matching angles cut to the seat area on the bottom of the valve, so that when assembled and are closed in the block, the valves make an air-tight seal. But your not done yet. Once the seats of both parts are cut to match, you still have to "lap" the valves and seats as a pair. This is a process where you put some lapping compound (very fine polishing paste) on the seat and valve faces, spin the valve in place (usually using a drill or milling machine) and polish the two surfaces so they match exactly. This is not normally possible to do in the home garage because not many of us have the appropriate tools, the drill press or mill, and the lathe to do it right. Sure you can do it with standard shop tools, and it will work, but it will not be as long-lived if you don;t do it with a certain level of precision. This is why its best for most of us to take it to a machine shop and let them do it with the right tools and equipment. Anyway, so now the vales and seats are cut and lapped to perfection. This is the stage at which we are talking about here. Now the valve stem to cam lobe clearance is adjusted to spec. The only way our engines are going to make their maximum HP potential is if the valves and piston(s) are air-tight and close-fitting within their respective tolerances.



K321 Valve Clearances:
Intake: 0.008-0.010 in.
Exhaust: 0.017-0.019 in.

Edited July 26, 2015 by MikesRJ