Here we go again….. Another day, another customer with chronic head gasket problems on a big bore that was built by yet another “pro” :confused: What was the answer? Studs, copper head gaskets, O-rings, and copper spray?

So let’s dive into engine building 101;
Everything on an engine needs to work as one harmonious system. The cam should be balanced with the intake and exhaust for best power, easy enough. Sealing combustion promotes maximum efficiency, while minimizing combustion by-products in the crankcase that cause engine component deterioration. This is all basic stuff.

Head studs and so forth do have their place, but are rarely needed. Studs using high torque values will absolutely pull a cylinder out of round causing increased blow-by, thereby increasing engine and/or ring wear. Ring wear in turn will eventually lead to more blow-by, power, and effiency losses over time.
The best studs will fully bottom out in the factory holes to distribute the added load over a broader area.

Copper gaskets are notorious for “cold weeping”, where the gasket will leak coolant around the outside of the head until it gets warm enough to expand and seal. They are usually made of a thicker material, and will require extra machine work to get the piston to deck clearance back where you want it. They usually require more torque to do the job (see studs).

O-rings? Every O-ring setup I have seen on an ATV engine is grooved on the cylinder side only……. This absolutely defies any logic I can muster, so I won’t even comment on it, except to say that turbo race cars use a receiver groove in the head.

So what is the answer?
Have the head and cylinder machined flat and as smooth as possible, just like you would for any decent rebuild. Use a flat piece of thick safety glass or granite that is nice and flat. Use a piece of wet/dry paper on the surface (I use 3M spray adhesive), and “lap” the head until it is smooth. I recommend a minimum of 400 grit final finish (600-800 is even better). Even the best machine work is not always smooth enough for Cometic (MLS) gaskets, they are extremely sensitive to surface imperfections. Better safe than sorry.

The next step is to chamfer all bolt holes, which not only helps even out and spread the load, it also eliminates any “summits” that would pull up around the threads. Any type of riser around the bolt that would come with fastener torque, will lower clamping force across the rest of the gasket.

You will want to do this to the head and cylinder. What you’ll end up with is a better performing engine that lasts far longer for a few bucks worth of the right tools rather than a few hundred dollars worth of “band-aid” fixes.

As everyone here knows… I design engine parts. I have finished up my new line of pistons, and during this process, some truth has been shed on the compression ratio numbers game.

I use the only manufacturer that I could find that uses 3-D chamber and dome scanning. This scanning, along with “proper” math has brought me to the following conclusion: there is no such thing as a true 14:1 piston in a stock bore 700! I will explain how I got to that bold statement, and let the games begin!!

I designed 3 different domes;
11:1 uses a -.9cc effective dome
12.5 uses a 7.9cc effective dome
14? uses a 9cc effective dome, which is the largest effective dome that will fit into the chamber. When I say “effective” dome, I am referring to the actual dome size after valve reliefs have been machined.

Notice the jump from 11:1 to 12.5:1 requires a dome size increase of 8.8cc’s. That is what is required to increase the compression ratio 1.5 points. I could possibly gain 1 more cc by not using a trough style valve relief on the exhaust. Now you must realize that as the dome gets larger, valve reliefs remove more material. The dome also has to become progressively narrower at the top to fit the chamber, making it even harder to gain dome volume. Notice the piston with a claimed 14:1 only has a net increase of 1.1cc……. I do have very large valve reliefs for use with .550″+ lift cams, but the piston company I use actually has a dome shape that fits the whole chamber.

Okay, now add in the fact that I have the only off the shelf piston that sits .015 “in the hole”, while most sit in the hole .050-.065″. This gives me the ability to have the highest compression with the smallest domes. There are some custom pistons out to compensate, but the math stays the same. The valve pockets have to be machined deeper to retain clearance, and the dome only goes into the middle of the valve pockets….. Again, not happening. Now we can add in the fact that most manufacturers conveniently forget to add the 1.2 or so cc void around the piston head to the top ring…… You can see how quickly this becomes a “theoretical compression” numbers game.

Now to the defense of dealers and engine builders….. most do not have the equipment to measure actual net dome volume. This is a very difficult task. So they have to trust what the manufacturer tells them. The manufacturers give a “theoretical” number that often assumes “0″ deck, or sometimes doesn’t account for valve reliefs, and the truth gets lost in the wash.

I will be posting this on all the forums that I frequent, and will be more than happy to go “in depth” about the math I use for my conclusions. The obvious question would be; What is the highest compression I have seen in a stock displacement engine? 12.8:1. Please do not ask questions about dome shapes if you do not want an honest answer. Lipstick on a pig? (that phrase cracks me up)

Piston

Rings

This is a 14:1 piston that ran most of last season in a circle track motor. It was a stock bore/stroke motor pumping out 70 or so horses. The head was torqued to 35 ft-lbs with stock bolts.

The area highlighted in red has been making contact across the full width of the ring, the blue area is where it made partial contact, and the yellow highlighted area never made contact with the cylinder wall. Since the top and oil rings are chromed it they would not wear in as well as the second ring, so they were making even less contact. The only saving grace was the sheer ring tension against the cylinder walls. ::)