Cylinder base gasket

Discussion in 'General Questions' started by pferraro07, Oct 17, 2009.

  1. pferraro07

    pferraro07 New Member

    I think I have to replace my cylinder base gasket. My bike is about 7 mths. old and it appears that the gasket is just melting away, a little smoke comes out where the gasket is. What would cause this ? As far as mods, I have drilled some holes in my pipe and it seemed to increase power, could this cause my engine to get too hot and melt the gasket ? Also, I noticed my head bolts being loose so I tightened them up, it idled better after that, but still have top end power problems I assuming is due to the bad gasket.........................if so is this an easy fix, I have already ordered a new gasket but have never replaced one.....................?

  2. arceeguy

    arceeguy Active Member

    If you see it leaking, just replace it and the head gasket along with it.

    For the base gasket, you can buy some gasket material and use the old one as a template.

    Use a torque wrench to tighten the head nuts. (8 lb/ft) It is easy to overtighten the studs and strip the threads in the aluminum block.

    If you still experience top end power issues, you may need to rejet the carburetor. Does the engine sound/feel more powerful at part throttle when you are near top speed?
  3. 210061741

    210061741 Guest

    The gasket prolly blew out due to the head bolts coming loose.
    You could use a pop can to cut an aluminum gasket out.
    I did this on the head.
    It gives you higher compression.
    Now if you have a slant head you might need to watch your clearence from the piston to the spark plug.
    But with the center fire head it would be fine.
    Thinning the bottom gasket will change the timing a little too.
    Most seem to have had good results.
    Higher compression will give more power.
    But i run higher octane fuel.
  4. Al.Fisherman

    Al.Fisherman Active Member

    After doing research, the cheap studs (stretch easily) that come on these engines torque to 3-5 foot pounds (6mm) and to 12-17 (8mm) foot pounds. Converting to inch pounds equals 36 to 60 for 6mm, and 144-204 inch pounds for 8mm. I trash all the head, intake, and exhaust OEM studs and replace with US grade 5 (grade 8 is overkill). Now a grade 5 can take more torque then the OEM studs, but the case can't. I use 50 inch pounds for 6mm and 150 inch pounds for 8mm and red locktite, and haven't had a single problem. On my first build and before I changed the studs out, I blew a head gasket. To be honest I don't know if it was the cheaply made studs or they were not torqued to specs. SBP sells a thin gasket and I've had issues with them. The piston would hit them (slant head), as the diameter of the bore hole was smaller (about 1/16 to 1/8"). I don't know if they were a bad batch or what. Make sure that the head mating surface is true (flat). Out of 3 engines I haven't seen one that the head DIDN'T need surfaceing...As far as I'm concerned!!. Factory engines usually have 2 head gaskets...Remove one and save.
    Picture 2 Showing partial shave. RED lines shows milled area and BLUE shows unmilled area.
    Picture 3 Showing completed milled head...same thing was done to the cylinder.

    Now for something you are going to think I'm out of my head...Increase your plug gap up to .040...I have two engines that run much better with a .040 gap.

    We have experimented with a boost bottle...To tell the truth it made a noticeable difference on the two of the bikes I built (had to turn out the idle screw out at least two turns). One we didn't experiment with as a before and after test. I don't know if that was the reason we went to .040 on the plug. Much better top end now. We have a hill in the neighborhood and I use it as a testing guide.

    The bottles are easy to make (PICTURE 1)from PVC. Each end cap holds 5cc and 3-1/8" of schedule 40 holds 57cc's...for a total of 67cc's. Obviously this is for the so called 80cc engine. If you use schedule 20 the length needs to be shorter as the inside diameter is larger.

    Attached Files:

    Last edited: Oct 17, 2009
  5. machiasmort

    machiasmort Active Member

    I had to take mine off and used 3 layers of gasket material from Auto Zone to replace it. Just make sure once it's back and assembled that the piston don't hit the head!

    Worked fine, Higher compression and the bike runs better on high octane!

    One other note, when r+r'ing the head(jug), be careful not to break the piston rings... There's what they call a keeper pin that keeps them from spinning in the grooves of the piston!
  6. Flapdoodle

    Flapdoodle Member

    I wasn't going to share this, but it has been many miles so far without problem. I don't use a base gasket. Instead I used non-hardning Permatex form-a-gasket after cleaning the surfaces well with acetone. Extra care was used in tightening the head bolts as the Permatex settled in.
  7. arceeguy

    arceeguy Active Member

    Any problems with piston/head clearance without the gasket? Did you need to do any mods?

    After the ngine warms up and everything expands, the effective torque on the head studs goes up quite a bit, thats why you don't have to torque them down very tight.
  8. 210061741

    210061741 Guest

    If you have a center fire head there will be no problems with clearence.
    If you have a slant head you better check your clearence with clay.
  9. Flapdoodle

    Flapdoodle Member

    No problems of any kind.
  10. Pablo

    Pablo Motored Bikes Sponsor

    If you aren't using a base gasket, you don't want to use a less than stock thickness head gasket with the slant head.

    Here is the basic pattern for the base gasket: Gasket.pdf

    You will need to assure it prints at the correct ratio to use it as a pattern.

    As for our head gaskets, there is some small manufacturing variance at the gasket cutter, but I can assure you it's not "about 1/16 to 1/8"" - what I do think is there is a larger variance in bore diameter between engine mfr's than we accounted for. Mr Fisherman is sending us one his used gaskets so we can confirm bore.

    For sure agree on the slant head lack of flatness - if you buy one, lap it first - don't waste your time even checking it, let alone installing it.
  11. 210061741

    210061741 Guest


    Nice share thanks for that.

    I wish i had a small punch set cause that would make it much easier to make theese.
  12. GearNut

    GearNut Active Member

    Budget small punch set:
    Aluminum or copper or whatever metal you can get tubing of appropriate size. (I have used many, IE :the threaded tube out of table lamps to metal bodied ball point pens) Sharpen the outer edge of one end and enjoy.
  13. Al.Fisherman

    Al.Fisherman Active Member

    You know it. Made many a punch. Lay material on a piece of wood and strike.

    Although I don't need one yet, I decided to make a hole punch before I need one. A 9mm shell casing will work for 8mm studs. I made one out of some stainless tubing.

    9 punches... $12.00 3/32 to 1/2"

    Attached Files:

    Last edited: Oct 19, 2009
  14. machiasmort

    machiasmort Active Member

    Would have never thought of a 9mm casing, great tip!!!