Compression tester

D

Dave/Green Bay

Senior Member
Does anyone have a recommendation for a compression tester that comes with
an adapter for a Flathead. Everything I see comes with mm adapters. Thanks Dave/Green Bay
 
Aren't all spark plugs metric thread? According to Machinery's Handbook (24th ed pg 1650), there are four standard SAE spark plug threads, all metric: M18 x 1.5; M14 x 1.25; M12 x 1.25; and M10 x 1.0
 
All ford v8 heads,flatheads from 39 aand up have 14mm spark plugs. some in the early 30s had 18 mm plugs if I remember right.
 
Ones with long rubber hoses tend to give lower readings due to hose expansion, I look for Steel cord in the hoses
 
Don’t forget to check that the adapter for the plug hole doesn’t thread too far down into the opening.....it could hit moving valves. A guy I know used one with a long adapter plug on a fresh built engine and ended up having to replace a valve if I remember correctly.
 
I've been told we ended up with mm spark plug sizes due to the influence of Robert Bosch' an early day electrical engineer and manufacturer, who was of German descent.
 
Dave when doing the test, have all plugs removed and the throttle wide open for best results. You can test it dry then add a little oil to see if it changes the readings, do one cyl. at a time or the oil can drain by rings and give different results. I bought a Bosch tester from O'Rileys. Al
 
oldford2 said:
I think that 100# may slightly expand a rubber hose but it is still 100#. Not reduced. JMHO
John
Click to expand...

When the gauges are checked to a standard pressure and read the same---then when used to check engine compression they read different, the long soft rubber hosed ones read lower, when you move into the high end recording testers the hoses are short, metal reinforced,and the passage is filled with wires to reduce volume

When testing compression it's not as much about how high the gauge eventually gets, it's more about what happens on the first few compression strokes, and testing all the cylinders the same ---you have to remember that to fire there is only one compression stroke
 
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