So you have decided to buy an old Volvo to drive and restore! Ya, good. Here is the best way to check out that engine before you buy, for shur. This article was authored by M. A. Wade of the Riley Register and is reprinted from the San Diego MG Club Newsletter with the permission of the editor, Mr. Steve Hawxhurst. Enjoy! 

THE APPLICATION OF A COMPRESSION GAUGE FOR ENGINE TESTING 

   A compression gauge properly used is of great utility in diagnosing and locating the causes of loss of compression. A good reliable gauge should be used. 

   The engine should be at normal operating temperature, the test being made with all sparkplugs removed. The purpose of a compression test is to determine what faults, if any, exist in the compression of individual cylinders, in order to indicate where the power impulses are uniform. This is more important than the actual value of the compression readings. 

How to make a compression test: 
1. Remove all sparking plugs. 
2. Insert the adapter in No.1 cylinder . 
3. Lock the throttle wide open, see that the carburetor choke is wide open. 
4. Crank engine with starter and note maximum reading on compression gauge. 
5. Having recorded the reading, remove gauge, pour about 1 oz. of engine oil on top of piston. 
6. Replace the gauge and repeat the compression test. The oil will temporarily seal the piston rings. If the compression does not materially rise, it indicates that the rings are tight and any loss in compression is past the valves. If , on the other hand, the compression rises by 10 lbs. or more over the first reading without oil, it shows an excessive leakage past the rings. 

   How to make a complete analysis of engine condition with the compression gauge: 

   By setting out the test figures in the form of a schedule, the condition of the engine can be completely analyzed as follows.

   Schedule showing method of analyzing engine condition by making a compression test. 

   A frequent mistake when making compression tests is to base the analysis only on the results shown in column C; that is, the difference between the dry and oil-seal readings. Such an analysis in the case of the example used, would probably result in cylinders 2 and 3 being good because they show a bore loss of 2 lbs. and 7 lbs. respectively, and that cylinders 1 and 4 although not so satisfactory as the others, do not yet need attention because they show a bore loss of 10 lbs. 

   If a complete analysis is made, it shows that although the bore loss of No.2 is very low, the valve loss is very high and calls for attention. As this means removing the cylinder head, attention can at the time, be given to cylinders 1 and 4 which have a fairly high bore loss and might justify fitting oversize rings. 

Analysis of . . . schedule:

   A "Balance Test" made with a vacuum gauge by short-circuiting cylinders in turn will show the cylinders to have the following descending order of efficiency: 3, 1, 4, 2: this complies closely with compression-test results. 

   An analysis based only on the results shown in column C where no thought has been given to fully determining the condition of the cylinders, or where it has been deemed TOO much trouble, would have resulted in the assumption that cylinders 2 and 3 are good, and cylinders 1 and 4 - although less efficient - do not yet need attention.

   The complete analysis, as covered by the schedule, indicates CLEARLY that while the bore of cylinder No.2 is excellent the valve loss is high and calls for immediate attention. Since this has to be done, attention could also be given at the time to the bore of cylinders 1 and 4, where the fitting of oversize piston rings would in all probability rectify the loss of compression taking place. 

M. A. Wade's Compression Analysis Schedule