Topic: Cometic Gaskets, 890cc head cams and valves, MRieck gotta minute?

[CB750R]

Hey, I've dug up off the archieve a lead from Mr MRieck about cometic gaskets, and I've checked out thier site, they list one head gasket for 67.5mm pistons which I assume will work for my 67mm pistons.  But  then they list several 836cc base gaskets, but list the bore as 2.790"  will this work for my application?? and then whats up with the different thickness and materials of the base gaskets?

Listed is
0.005 copper
0.010 copper
0.015 copper
0.020 copper
0.020 fiber.

Thanks for any and all input on this, I am sorta realizing that this "bolt on new piston kit" isn't so bolt on so far!!

Question 2

I see a that the megacycle cam for a big bore kit requires "lobe clearancing" is this a fancy way of saying you need to grind the alluminum cam craddles to clear the new cams lobes? or is there more to it than that??

Cheers!!

[MRieck]

Hey there....Yes the 67.5 will work with the 890 kit. I am currently running the CMF material in that size. The different thickness base gaskets allow a person the set deck height (squish or quench). Cometic will make up a base gasket according to the OD of your cylinder sleeves (I can't remeber exact OD off hand-probably around 73 or 74mm  measure to be sure). You cannot use the Orings that go around the bottom of the sleeves and seal against the upper case- a little Hylomar around base gasket holes works well. I also put some around the cam chain tunnel cut out. Don't over do it with the sealer- a little goes a long ways. Will you be using an early cylinder or later cylinder with this kit (cut outs for oil return orings or no cut outs)? I've haven't had to clearence a Megacycle cam up to .407 lift- it is for very early cam towers (1970 or 71 maybe 72).

[CB750R]

ok I'm gonna prove my ignorance what is squish or quench mean?  and I don't have the barrells and pistons yet (in the mail) so I don't know the OD of the sleeves, or if it has cutouts for the oil return orings. 

G

[MRieck]

"Squish" or "Quench" refers to the amount of space between the flat outer edge of a piston and the cylinder head when the piston is at top dead center. A great deal of power can be extracted buy running a "tight" squish. Unfortunately alot of damage can be done if it is to tight. .050 to .040 is considered safe and efficent. The tighter you run the squish- and I have set it down to .028 on modern 600cc roadracing bikes- the better your components have to be i.e. Carrillo rods and good valve springs are the first things that come to mind. Proper cam timing is essential too. At 10,000 to 14,000 RPM everything is stretching and flexing so much there is no room for error. Squish that tight can only be used in small bore engines UNLESS you are dragracing for real. Some people say it is OK to have the piston kiss the cylinder head but I personally think that is bull#$%* engineering unless you are professionally running a Prostock bike. I'd recommend you pick up a book or 2 that explains these aspects of engine building and tuning- I'm sure it is explained in a more complete manner than I have done here. It is information that you will use throughout your life and it makes it easier to speak to other engine builders if you know the "language". It will also allow you to build a powerful and reliable bike (or car) engine.