Compression test Question

A-Jay

A-Jay

Senior Member
Will the standard type of unit that screws in to the spark plug hole work on a model A. Can someone tell what the thread is ? Would these tools give more than one size adapter ? I guess its best to remove all plugs first ? (Newbie) Thanks Guys!
 
The set requires the 7/8" adapter. Most sets sold today do not have that particular size. You either need to make one out of an old spark plug, or buy an adapter from Brattons or Snyders.

Remove all the plugs first. Take a dry test on each cylinder and record readings.
Squirt a shot or two of oil into each cylinder, and record readings again, to see the difference.
 
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Thread is 7/8" - 18 TPI. Warm up the engine, remove all plugs open the throttle wide and test each cylinder. Should be around 50 to 60 psi for a stock engine with no more than 10 psi difference between highest and lowest.

It is unlikely that your compression tester will fit the plug hole. The vendors sell adapters to go from the 7/8 thread down to the modern size (14mm). Brattons # 17711.

Dick
 
Thanks Guys for the info. I am regular at Brattons, thanks for the part number. The last time I did a compression test I think was in High School auto shop, Back when the Redskins were winners !:eek:
 
You can also find an older gauge that uses a one size fits all rubber stopper instead of the screw in type.
 
Loosen plugs 1/2 turn, start engine, rev up some --then do compression test --sometimes carbon dislodged from removing plugs can get caught under valve and give wrong reading.

many compression testers read different from other testers --I have found + - 10 lbs between testers ---name brand ones.

I usually crank for 5 compression cycles, and watch first 2 --the first full pump usually is 75% of total in tight sealing engine
 
OK I checked out Brattons they sell the adapter but you have to buy a set of four for 24-25 dollars. Should I buy four, I was thinking only needing one ? Would any body know if this plug adapter is a thing that NAPA or other regular auto parts places would sell ?:confused:
 
A-Jay said:
OK I checked out Brattons they sell the adapter but you have to buy a set of four for 24-25 dollars. Should I buy four, I was thinking only needing one ? Would any body know if this plug adapter is a thing that NAPA or other regular auto parts places would sell ?:confused:
Click to expand...

Now check part number 08471
 
Mike V. you found it, thanks that will work perfect with the thread in type. Also thanks to all for the advise on performing the compression test. I would have never thought to hold open the throttle.

I think I may have a leaking head gasket. There is light colored waxy stuff in a few places where the head meets the block.
 
A-Jay said:
I would have never thought to hold open the throttle.
Click to expand...

You need air to make compression -I didn't know either when I started. It gets easier (and more fun)as you go along

Many Model A's I have seen (including one of my own) have a wheepy head gasket which marks the block but run just fine and have good compression. My experience of blown head gaskets is that you know you've done it because they don't run well ! -Karl
 
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