The clearance between valve stems and valve guides is quite critical. If this clearance is too small, exhaust valves are prone to stick. If the clearance is too large, manifold pressure is prone to draw oil-rich vapor from the valve chamber along the stems of intake valves and into the combustion chamber. This condition leads to carbon buildup (see below) on compression rings, spark plugs, and the valves themselves.
You’ve fought your way through a partial teardown of your engine, to include removing the valves, and you’re now facing a decision of whether or not to replace the valve guides. The goal of this tech tip is to help you make that decision, to offer instructions and tools for replacing any guides that are worn beyond the specifications shown below.
To get started, all you really need is the pair of go/no-go gauges from our catalog to measure the ID of your guides. If it becomes necessary to replace or adjust the ID on any of the guides, we also list a valve guide punch, a valve guide reamer and new guides.
Valve stem to valve guide clearance specifications: Universal’s manual (PN 200150 dated August 1991) provides a valve stem to guide clearance of .002” to .003”. However, when specifying the reamed ID of the guides, the manual calls for a range of .3145” to .3150” which calculates to a clearance of .003” to .0035” (with a standard valve stem diameter of .3115).
Since the tendency of valves to stick in an Atomic 4 is somewhat more prevalent than oil being drawn into the combustion chambers from the valve chamber, we settled on an official valve stem to guide clearance of .003” to .004”. This is the specification that appears in our MMI Service and Overhaul Manual.
Measuring valve stem to valve guide clearance
We suggest using the .3145” and .3165” plug gauges in a go/no-go process to measure your clearances. The .3145” gauges are marked with green tape and the .3165” gauges are marked with red tape for easy reference. Below are the three possible test results with our recommended remedial actions:
- The green gauge goes into the guide with minimum resistance, but the red gauge will not go into the guide:
Indication: This condition is normal.
Remedial action: None, valve guides do not need to be replaced. The diameter of the hole in the guide is at least .3145” but less than .3165” (a nominal .003” to .004” clearance between valve stem and guide).
- The green gauge will not go into the guide:
Indication: The hole in the guide is too small.
Remedial action: Clean and/or ream guide and recheck with green gauge. If the green gauge will then go through, guides do not have to be replaced.
- The red gauge goes into the guide with minimal resistance:
Indication: The hole in the guide is too large.
Remedial action: Remove and replace with new guide.
Replacing worn valve guides: Old guides are driven down and out from the top of the block using our valve guide punch and a medium sized hammer. Please note, if you plan on leaving your camshaft installed, you will have to remove the adjustable bolt from the top of the tappet to make space for the old guide to move free from the block. New guides are driven in to a depth of .940″ below the top deck of the block. Our valve guide punch has a reference mark at .940″ to use when seating the new guides. Simply lay a razor blade or other straight edge across the valve opening and stop hammering when the mark on the punch lines up with the straight edge.
New valve guides from our online catalog are pre-machined to fit without the need for reaming after installation. However, slight variations in the diameter of the hole in your block might leave the ID of the guide slightly undersized. We therefore recommend using the green gauge to check your new guides after installation. If the green gauge will not fit, our reamer can be used to fine tune the ID of the guide to the minimum diameter of .3145”. Please note that our reamer is designed to be used by hand.
The new guides are lightly grooved on the inside (resembling fine threads) for all but approximately 3/8” on one end. The non-grooved end of the guide is to be installed in the up position.
…you’ve fought your way through a partial teardown of your engine, to include removing the valves, and you’re now facing a decision of whether or not to replace the valve guides. The goal of this tech tip is to help you make that decision, to offer instructions and tools for replacing any guides that are worn beyond specifications.
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