Newbie needs help with skimming the head

AuntySpam

AuntySpam

New Member
Hi everyone, I recently purchased a 48cc Grubee slant head motor. I've been reading a lot about what to do before even mounting the motor in the bike. What I am unsure about in skimming (or milling) the head is this. There is a circular flange around the combustion chamber. Is that what I'm supposed to sand?

Sorry if this is so elementary, but I don't want to screw up the motor's head. (Mine is already screwed up!)
 

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Try this. Use a broad magic marker or paint that ring with machinist dye. Get a sheet of glass. About one square foot is a good size and some sandpaper, maybe about 280 grit. Lightly run the cylinder head over the sandpaper that you have placed down on the glass, grit side up. Use a circular motion as if you're waxing your car. After a couple of circles inspect the head and if there are imperfections, or non-flat surfaces you'll quickily see the high and low spots where the ink or dye was removed or is still there. You want a flat, uniform gasket sealing surface so repeat the ink and dye application and sand again. Keep doing that process until the surface is flat/flush.

Rotate the cylinder head in your hand so you don't sand in an angle. Some will tell you to remove that ring of metal and go for a flat surface across the face of the head. That will increase your compression, slightly, but it isn't necessary.

Here is some discussion on 'lapping' the cylinder head > http://www.google.com/cse?cx=partne...earch.php?searchid=1424503&ss=5828j2240606j21

You can check the gasket mating surface of the cylinder using the same method described above but you'll need to remove the cylinder from the engine case. If you do that make sure you know how to reinsert the piston/rings when you reassemble things.

Tom
 
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Thanks 2Door and Craccius! Craccius, I think it's easier for me to do it now since it's not mounted in the bike yet. But thanks for your input. And 2Door, thanks for the idea with the machinist's dye or the broad magic marker (which I'll use since I have that). I've already got the barrel and piston separated, so I'll skim that, too.
 
What, do you not understand?
Tell us a little about yourself and your motorized bicycle. Maybe we can help if we know you're involved with the hobby.

Tom
 
A suggestion: Not just any sandpaper will do. I spent hours trying to sand down 0.2mm from my head with regular sandpaper. 120 grit silicon carbide sandpaper will do it quickly.
 
Careful with the belt sander checking with the vernier's as I go is how I shave my heads.
Then 180 grit wet and dry paper till you get the scratches out and finish with 400 grit.
I use cheap WD spray to stop the paper clogging.
 
I actually use 100 grit emerycloth as regular sand paper wears to fast then switch up to 220 grit and finaly 400

But srsly a standard head running high comp is only for short rides and i would be carfull about overheating! !!!
The stock head once decked far enough down to gain any reasonable comp will have its squish band sanded off and now have a horizontal flat that protrudes into the chamber by about 4mm so you need to payy attention to the piston to head clearance.

You could just get a puch 50cc hi hi head for 30$ they work great my fastwst bike yet is running one
 
Angle fire heads have not got enough cooling to run high comp, the straight fire heads however work well after being shaved, but it is best to machine/dremel a squish band after shaving the head.
My Puch heads also needed modifying as the stud hole spacing is a little less than a 66cc China Girl, I got the Hi comp puch head rather than the Hi Hi comp, Then shaved and re worked the combustion chamber giving a 5mm squish band.
 
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