Compression Ratio Shims and Spacers

by Jason Black

"I'm in the process of determining what cylinder spacers I need to achieve a compression ratio of 7 for my 1776. On page 17 of my Gene Berg catalog, he lists spacers and shims and I'm wondering how each is to be used. The obvious difference is that spacers have thicknesses of .040 and .060 and the shims have thicknesses like .007. Are the shims to be used in conjunction with spacers to set the compression ratio with great precision? If not, then what is their use?"

Setting compression ratio exactly begins before the engine case is bolted together, in my humble opinion, by measuring the rods and pistons and ensuring that "shorter" pistons get matched with "longer" rods and vice versa. It's easiest to do this with a good dial caliper that is accurate to .001". This helps to make certain that your deck height measurements won't be wildly deviated from each other on any *single* side of the engine. Like, 1 and 2 can be .010" greater than 3 and 4... that can be accounted for in shims/spacers by making the 3/4 spacers .010" larger. But, 1 and 2 should be very close, as should 3 and 4.

After the engine case is buttoned up and pistons and cylinders installed, the deck height must be measured. With these figures (hopefully they match each other), and the CC volume readings, you can determine what shims/spacers you need to order. Gene Berg will make any size that you need.

Without CC volume, you can't know the CR... a thick round piece of clear plastic is obtained, the same diameter as the top of your pistons. A hole is drilled in the plate that conforms to roughly where the combustion chamber in the head ends, where there is little space. If it's drilled in the middle, you'll spend an eternity trying to get the head level again so that all of the air bubbles escape.

Some people use cooking oil to check their CC volume...it's what I used. It does not want to flow through the syringe easily, though. If I had something that would flow easier, then I'd use the 1cc syringe over the 10cc, for the sake of accuracy. Go ahead and score the markings on the syringe into the plastic, since the marks come off when repeatedly exposed to oil... I also partially disassemble the head, removing and hand-lapping the valves so that no oil leaks between the valve and the seat, and reassemble. Polishing the head before this is done is also a good idea. I use the blue 3M Roloc pads on a high-speed angle air tool with gentle pressure, in combination later with tinier Cratex polishing mandrel things...

Head together, grease sparingly on plate to seal, plate on, head more or less level... the rocker arm studs prevent this unless you drill a couple of holes in a piece of plywood for them to stick through. I loosely screw on a couple of nuts on the back side, too...

Then you just keep track of how much fluid you inject into a given head. When it's full, that's the volume for that one... three more to go.

Bob Hoover recommended, I believe, that heads for stock motors be kept to within a 1/2 cc of each other, within 1 cc for 1776 and up... Polishing a little more aggressively can even this up, although I'm told that the common procedure is to sink the valves...I don't have that technology yet.

Once you know that the CC volumes are approximately equal, then you can take that data and the deck height and crank stroke and piston/cylinder diameter and figure out the CR... and figure out what shims/spacers to order/have made. Some times it is a hassle trying to make everything fit after you have effectively widened the engine. For this reason many people have their heads semi-hemi cut to lower compression with little if any shimming required. Barb and Eric recommend taking down the sharp edges on the combustion chamber to have a similar effect, but I don't know how much you can safely remove and so far i have stuck with shims. Perhaps they will post about exactly how much they feel comfortable with removing and some ballpark dimensions...

But... only one shim or spacer should be used per cylinder... any more and you run the risk of leakage. People have gotten away with it, but it is not recommended.