Discovering Your T-Series’ Compression Ratio

A very simple way to determine the compression ratio of your T-type engine is to measure how thick the cylinder head is.The measurement you want is the distance between the bottom machined surface and the upper machined surface that mates with the valve cover gasket. No, you don’t have to remove the head to take this measurement, just follow these simple steps.

A. Raise the left side of the hood.

B. Remove the valve cover.

C Carefully lift the back half of the valve cover gasket from the head. The machined surfaces you need are now exposed; the top surface was covered by the valve cover gasket. The bottom surface is just below the lower left hand corner of the metal plate covering the water jacket at the rear of the cylinder head.

D. Scrape both surfaces to remove paint cork, etc.

E. Using the proper micrometer*, carefully measure the distance between the two surfaces.

F. Refer to the tables above and you can determine your compression ratio. If the thickness of your cylinder head is a little less than the figures in the tables it indicates that the head was machined slightly to remove any warp or other irregularities.

*If you cant get hold of a micrometer, an outside caliper and a high grade metal scale may be used. Transferring the caliper measurement to the metal scale to obtain the correct reading is straightforward.

By Carroll Dorschel

(Caroll’s tips and the these specs provide an easy way to determine the compression ratio of your T-type engine. The primary reason for checking your engine’s compression ratio pertains to how well you can expect yow engine to run  on today’s lower octane fuels. While raising the compression ratio has been a standard means of increasing power output, it also raises cylinder head operating temperatures and increases the likelihood of ignition “pinging”, detonation, and running-on problems, all of which can cause serious engine damage (usually in the form of a holed piston). 

With today’s lower octane fuels, T-type engines are much happier with the compression ration below 8.5 or 9.1. To reduce the compression ratio, a second head gasket can carefully be fitted, which will restore approximately .045″ of head thickness. Simply use Copper Kote® head gasket cement and torque the head very carefully prior to starting and 2-3 times more during the first few hundred miles. For cylinder heads that have been extensively machined, it may be necessary to make up a “compression plate”. This is a solid machined gasket which is then sandwiched between two stock head gaskets. Also, keep in mind that oversize cylinder bores raise the compression ratio slightly. A .100″ overbored block, when fitted to a standard thickness head, produces a compression ration of 7.74:1. –ED)