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Spitfire 1500 compression test

Chris Bracey

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Remove all spark plugs

Have battery in good state

Hold throttles open 

With gauge screwed in spark plug hole 

Crank to turn engine over till youmget best reading , may take 3 to 6 turns

Note the   result

Repeat on other 3 cyls , in any order 

Youre looking for similar readings on all 4 

There are other methods so  wont confuse with that 

Resilts can vary but expect something between 120 to 150 psi  

After each test you need to release the pressure reading 

All 4 want to be within 10% of each other 

Let us know what you find


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There is no "target compression".   A compression tester is not a precision instrument, and is not calibrated.      It is used to compare pressures, across the block, so that you can tell if one, or more, cylinders have leaky rings or valves.      It is also very dependant on precise technique, atmospheric pressure, etc.     You could test two cars/engines on the same day, same place, and know that one had better compression than the other, but that's all.

Technique:   I would add to Pete's list:

Throughly warm the engine.

Wire the throttle WIDE OPEN.

Repeat the test until you get consistent results.    You've never done it before, so you need to practice.

The last can run down your battery!   A good way to avoid this is to use jump leads from a running second car, so that keeps topping it up, while you are doing adjustments.


If you do find low compression on one (or more) cylinders, then the diagnostic test is to pour a teaspoonful of engine oil into the bore.    Turn it over a couple of times and then test again.    This seals the rings and will increase the pressure, UNLESS the low compression is due to a valve problem, when the pressure will not rise under the 'oil test'.

Good luck!  John

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Johns right     but  problem is to warm it up it might go terminal   before its.......... hot  

and add to the what if's    a loose cam sprocket  makes a horrendous rattle .

but wont explain the loss of coolant and low oil levels   , sorry  its  still looking ............ sad


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