whats ahead?

[happyjourney]

I was reading a pre build checklist at either this or the other forum and it had that you should sand approximately 1mm off the head to increase compression i beleive. I am not acquainted with motors at all much before taking up this project of my first build and made the mistake of asking which was the head, the top part that the spark plug extends through or the lower jug which holds the piston. I was chided quickly with a statement of something to the effect of "if i told you to touch your head would you have to ask where it was" that post was then followed up with some over my head jargon but i never got an explanation. viewing my troubles i didn't have the courage to ask again and still i need to know which part to sand. do i sand the top portion of the cylinder with the head studs extending through it or the cap as i've said with the spark plug extending through it. also i've seen pictures in my browsing of people who had bored out or shaved off more clearance area inside of the cap that the spark plug extends through. what would be the benefit of that? perhaps the shaving off of metal from one side or the other of the head peices would extend the spark plug further towards the gases and oxygened area that would need exploding? is that the reason it creates more compression? I would think that making the inside areas bigger and inlet and outlet areas bigger would give more power not smaller which is effectively what i'm doing when i shave off metal from one side or the other (of the top or bottom). i don't want to get into a whole big chemistry lesson if i don't have to but i would like an idea of what is occuring.

 

[oldnube]

i am new also, but the head is the part with the spark plug. You want to mill (sand with very find paper) only the areas where the gasgetsets take a sharpie and lightly mark that area then follow the directions given. When the marks are gone your surface is "true". This makes for a better seal and improves compression.

 

[happyjourney]

a sharpie huh. sounds like a good tip. i'm not with my engine right now but i'm going to check it out again when i get home. i'm going to have to buy some finer sandpaper.

 

[motorpsycho]

no, no, no...back up!
ok first off, the head is what's bolted to the top of the cylinder. (where the spark plug is).
you have to remove the head (4 nuts) from the cylinder.
There will be a thin metal gasket between the head and the cylinder which you should replace once you pull the head off of the cylinder.
flip the head over and look at the area where the gasket sits. you can set the gasket back on the head with the head upside down to see the exact contact area.
THIS is the area on the head that will need to be sanded or milled.
you have to do it on a perfectly flat peice of glass to get the surface true.
tape a peice of 220 grit sandpaper to the glass and then sand the head (mounting surface) in a figure 8 pattern. you don't want to remove too much material because if you remove too much, the spark plug could end up hitting the top of the piston. If you remove too much you could end up with so much compression that the clutch won't be able to grab enough to turn the engine over. If you remove too little, you won't get any results and you end up having to do it again.
you want to remove like 1-2 mm at the most i think (i have never done it myself, but i know how to do it).
there are 2 reasons for doing this.
#1, it trues up the mounting surface of the head where it seals to the cylinder, BUT if the cylinder is not true, this won't make much difference.
#2, removing material from the head mounting surface will lower the combustion chamber in the head (where the spark plug electrode is) closer to the piston. It's not the fact that the spark plug is closer to the fuel/air mix that gives you more power. It's the fact that removing material from the head mounting surface will create less volume in the combustion chamber which will raise the compression. Lowering the combustion chamber closer to the piston creates higher compression. more compression = more power. think of filling a gallon jug with paper towels. Now try putting that same amount of paper towels in a 3/4 gallon jug, it won't fit easily but if you force it all in, it will fit and be more compacted together. this is the same idea, you are reducing the volume in cylinder / combustion chamber, but you are putting the same amount of fuel/air into it as before.
BUT too much compression will require running higher octane fuel, and it will make the engine a lot harder to start and it will run hotter.
if you are unsure about any of this, maybe you should just leave it alone.
You have to know what you're doing and you have to know when to stop removing material. You also have to be careful when you re-install the nuts that hold the head on because they can strip out real easily, you have to get the head torque right (I have no idea what it is on one of these engines). sometimes you can break a head stud when trying to torque the head nuts, which would require removing all 4 studs and replacing them with stronger ones.
there's more to it than just sanding the mounting surface of the head.
I'm no expert on this by any means, but i have been around engines all my life (40 years) i've built engines for drag cars, motorcycles and all kinds of high performace stuff.

 

[happyjourney]

i'm not with my engine right now but my gasket as i remember seemed to be a thick peice of wax paper not a metal one. i plan on replacing that one with a metal one from sick bike parts and i plan on upgrading the studs coming out of the head and the intake and exhaust. i believe i can handle smoothing out the bottom of the spark plug holding cap which is called the head i suppose. does the sharpie trick sound like a sufficient depth to you? i want to do this right. I've spent alot of money on accessorizing parts mostly and am in love with the project i'm building and I'm hoping to keep this thing runnin.

 

[oldnube]

Thank you Motorpsycho for clearifying and expanding what I was wanting to say. There are 3 prebuild checklist for 2 strokers check out "What Every NOOB needs to read" by AL FISHERMAN and his pic thread on takeing them apart (both are "stickie". I know the sharpie tip will get the surfaces true I dont know if it is too much or too little. Maybe someone will help us both out on this.

 

[happyjourney]

the gasket was metal not paper. thanks oldnube for giving me the ideas on the prebuild checklist i'm going over many of those lists and trying to prepare the engine properly.

 

[Big Red]

The Head

Another little thing to think about. If you don't apply pressure evenly across the head while sanding you could make the head sit unevenly on the cylinder. I've found when trying to hand sand and keep it as true as possible, you will need to rotate the peice you are sanding in your hand. Sand in a figure 8 as suggested back and forth eight or ten times, then rotate the peice in your hand 1/4 turn, Repeat. turn 1/4 turn in same direction as last, Repeat. While holding something like that I've found that you will always apply more pressure with the palm of your hand than your fingers. Rotating the peice helps solve some of, (not all) of this problem.
I discovered this problem while taking down the back side of a rear engine mount to fit a tight mount situation. First time it did not fit flat between the engine and frame. I was much more carefull the next time, and rotated it as I sanded it down. It came out a LOT closer. Heads are a lot more critical than an engine mount and need to be done as carefully as possible.
Big Red.

 

[Big Red]

Head Gasket

the gasket was metal not paper. thanks oldnube for giving me the ideas on the prebuild checklist i'm going over many of those lists and trying to prepare the engine properly.

Hey happy, I've also read on this site, (somewhere) that you can get a thinner head gasket that results in the same as milling the head. A thinner gasket will lower the head closer to the cylinder and will also result in higher compression. Wish I could remember where I saw it. Perhaps someone else will remember.
Big Red.

 

[happyjourney]

i do remember seeing a post somewhere about a thinner gasket as well. I remember also when looking at the inside of the head cap that on the gasket fitting area on the inside of that area where the gasket would go there was an elevated ring i thought. more details later when i go look again this evening.