Valve Guides

B

barnnone51

Member
How much movement should a valve have in a valve guide? It appears that there is some gap causing the valve to rock back and forth. So how would you tell if it is time to change? it is very little but there is movement. I have had a lot of blow by. Thinking of up grading to later valves and adj lifters.
 
I don't recall seeing any specs specific to a Model A engine, but 0.001-0.003 would be acceptable, with the larger tolerance for the Exhaust valves. More important would be how the valves are seating.
A lot of blow by would typically be attributed to rings. A leakdown test would help you to determine what's happening.
 
A rule of thumb is that intake valves need a guide-to-stem clearance of .001″ to .003,″ and exhaust valves need from .002″ to .004″ guide-to-stem clearance.

Measure the guide ID's with the guides installed using a ball gage. Measure the valve stems with a micrometer of vernier caliper.

I would question any check that specifies an amount of valve wiggle because the amount of wiggle is a function of the valve guide's length plus the stem-to-guide clearance.

 
Caution:
(The following is intended to be humorous)

I once had an old Dutch mechanic tell me the clearence should be; "loose mit no slop"

Sort of a "mechanics tight" type of measurement!

Brian W.
 
Worn valve guides in a valve-in-block engine is not as critical as an overhead valve system. An overhead type would allow oil to leak into the combustion chamber causing oil consumption and smoking. Worn guides in a Model A engine can cause improper valve seating to occur over time. The main concern is the exhaust valves can begin to leak which would affect compression and thusly engine power. Installing new guides and proper honing of the valve and valve seats are super important.
 
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