The first step to easy engine maintenance
To run properly, 2-stroke engines rely on perfect sealing in critical areas. In an ideal world, gaskets would not be needed. With two perfectly machined mating surfacesmuch like the old Dick McCoy engines of the 50sa gasket isnt even necessary. But most manufacturers use gaskets to guarantee a leak-free, airtight seal.
Designers sometimes use rubber O-rings, but they also use gaskets made of materials such as aluminum and brass shim stock, Mylar and plain or resin-impregnated paper. You should be able to obtain replacement gaskets from your engines manufacturer. If, however, the engine is too old to get gaskets for, or you just cant wait because you want to get back into the air, it is handy to know how to make them yourself.
In this article, I explain how I make two styles of gaskets out of two different materials. For sealing a backplate, I show how to cut a gasket out of Velumoid (a particular brand of resin-soaked Kraft paper). I also explain how to make a metal head gasket out of sheet brass. Lets get to it!
PAPER BACKPLATE GASKETS
The most commonly needed engine gasket seals the junction between the crankcase and its rear cover, or backplate. If there are any leaks in this area, it will compromise the engines ability both to draw fuel into the case and to transfer it from the case to the cylinder. Idle and performance can suffer. Often, when disassembling the engine, this gasket is torn or damaged and needs to be replaced. Here is how you do it!
Use a vernier caliper to measure the thickness of the original gasket material and find gasket material of the same thickness. I prefer Velumoid, which is Kraft paper soaked in resin. It is available from some automotive parts dealers, but there are many types and brands of gasket paper, and dont worry about the brand because its the thickness that matters. You can also use other things such as playing-card stock.
I start by measuring the diameter of the backplate where it fits into the crankcase. Next, I mark an appropriate circle on the gasket material using a circular template or a drafting compass. After determining the bolt-hole positions, I use small, inexpensive punches to cut them out. You can use an XActo no. 11 blade to cut out the small holes. Having cut the small holes out, I move on to the main opening. Here are a couple of tricks I use to cut these holes. One is using a small pair of cuticle scissors. These are small, sharp and have a curved blade excellent for making gaskets! Just pick up a pair at the local drugstore. My other trick is to use a no. 11 blade in a compass to cut the hole precisely and quickly. A drafting compass with a removable leg works great.
Next, position the gasket on the backplate, press it into the crankcase and mark its outside profile. Then, again using a sharp knife or scissors, cut it out. You might be tempted to use the crankcase and backplate as a cutting guide, but its easy to scratch these parts, and the assembly can be awkward to hold while youre wielding a sharp X-Acto knife. It is much safer to mark the part and trim it on a cutting board or with the cuticle scissors. Thats all there is to it. Give it a try. You will soon see that making your own backplate gasket is easy.
METAL HEAD GASKETS
Most engines today have a round aluminum or brass head gasket to seal the cylinder head to the cylinder sleeve. Some older engines used asbestos/rubber gaskets, but these were prone to blowing out or leaking after a time. Soft (annealed) brass or aluminum makes an excellent head-gasket material. Metal head gaskets usually survive removal, but you may want to make some for other reasons such as shimming the head to run more or less nitro or to cure a predetonation problem, etc.
Start with shim stock of the correct thickness. I like K&S brass shim material, which is easily obtained at most hobby and craft shops. Measure the head or the old gasket for thickness, and inside and outside diameters. Now use your compass or circular template to mark out the gasket. You can use the same small scissors or an X-Acto knife as you did with the paper gaskets. Watch that you do not create a burr on the material or kink it by mishandling it. If you do kink the metal, do not try to flatten it by
tapping or hammering it. This can lead to the metals being of an uneven thickness. Just use something with a small diameter and roll out the gasketmuch like using a rolling pin on dough.
JUST ONE STEP
Being able to overhaul your own engines is a very rewarding thing. Each step of the process involves different skills and procedures. Dont forget that the gasket is as vitally important as any other part of the engine. Being able to make replacement gaskets is just one small part of the whole job, and its an easy one to start with. Until the next engine tech tip, have fun and take care of your engines!
K&S Engineering, www.ksmetals.com (773) 586-8503