hi comp head experience (long)

Discussion in '2-Stroke Engines' started by emdude, Jun 21, 2009.

  1. emdude

    emdude Member

    Hi All:

    Been a while since I posted. Just came back from a brisk morning ride and I thought I'd share my experiences with the "high compression" head I put on my 80 cc china girl.

    OK, so I ordered one and after initial inspection this is what I believe:

    The head sold for retrofit onto the 80 cc engines is clearly NOT meant to go onto this cylinder. I am convinced that it is made for a smaller cylinder bore. If you examine it carefully it becomes very clear that the casting is made to onto a cylinder bore of around 43 mm, and not the 47 mm bore common to 80 cc engines. Every comparative measurement with calipers makes the difference between the two heads very clear. The head is also clearly a bit smaller on the outside, so to speak, than the stock head. This has been observed and posted by others. The slant entry of the spark plug and the different squish zone in the head absolutely provide higher compression over the stock domed head.

    I emailed the vendor with my findings saying that they must have made a mistake and sent me the wrong head for a smaller engine. They disagreed and stated that this is the same head used on their current high compression 80 cc engine. And it is. However this engine I now think has some key differences to the many 80 cc engines out there that many of us have. More on that later.

    This does NOT mean it won't fit and work...or can be made to work.
    Aside from the fact the at the head I got had holes for 8 mm studs whereas my engine has holes for 6 mm studs it was a bit of a challenge to get I on. Since my 80 cc engine is one of those in which the piston is out of the top of the cylinder by a quite noticeable amount at TDC this posed a particularly challenging problem getting the head on in such a way that the "flat" part at the outer edge of the head does not make contact with the piston when it is at TDC. This "flat" part would normally of course be far less of an issue if used on a cylinder with a 43 mm bore as the head was intended for.

    So, for a first run I needed to use THREE!!! head gaskets to get sufficient clearance. Two just didn't cut it in my case. And of course it took a bit of "adjusting" to get the head into the correct position for piston clearance. I don't know how others do this but I put it all together with the engine in the bike and with the spark plug removed turn the engine over by moving the bike back and forth while keeping my hand on the head and feeling for contact. I adjust the head position until there is no contact. I progressively tighten down on the head bolts making sure there is no contact when fully tight. This has proven fairly reliable.

    OK, first run was pretty impressive. But I was not happy with having to use three head gaskets. So I decided to attack the problem from "below". I took it all apart and made two relatively thick cylinder base gaskets and installed them under the cylinder. Pattern available for download at sick bike parts. The piston now no longer protrudes from the top of the cylinder bore at all. In fact it is a tiny bit recessed when at TDC, so the material I used for the base gaskets could have been a bit thinner I suppose.

    I was hoping to now be able to get away with not using any head gasket at all but this was not possible. The aforementioned "flat part" at the edge of the cylinder head and the slight domed curvature of the piston still allowed contact. Had this head actually been made for a 47 mm bore it would have worked great. So I still needed to use one head gasket. But one is better than three.

    So how does it run? GREAT. This is definitely and improvement worth looking into in spite of the work that may be required to get it to function on any given 80 cc engine. The engine definitely has more torque in the low end, something we all want I'm sure and gets up hills and up to top speed faster. As one would expect from an increase in compression. And the nice thing about increasing compression in any internal combustion engine is that it is the ONE modification that can be made that increases both horse power AND efficiency. Horsepower and torque increases are NOT gained at the expense of fuel economy.

    The bike does NOT go any faster, but does get up to top speed quicker. This because I had my carb tuned just perfect so that the top speed of around 35 MPH is limited by engine vibration. You just can't go any faster with out fear of the engine going to pieces. Prior to the high compression head I could cruise along at "vibration limited" top speed at about ¾ trhotlle. With the new head I can do so at ½ throttle or even a bit less. Crank balancing is the only thing left to make my engine really spin.

    Further thoughts:
    Raising the cylinder using thicker base gaskets does of course modify the port timing. Exhaust and transfer port timing is increased while intake timing is shortened. I don't think that the amount of change was sufficient to change port timing detrimentally. The casting on my cylinder is a bit odd in that the intake port never actually opens up completely, i.e. the piston skirt at TDC is never fully out of the way. This has been reported by others as well I think. So it could be argued that at least the thickness of the new base gaskets has to come off the bottom of the piston skirt. But for now I will leave it alone.

    Also I know from experience that increases in intake timing over a reasonable stock value on a two stroke go towards top end power. Excessive intake timing by being too aggressive in piston skirt removal will lead to an engine running with plenty of top end but no bottom end. Low end power loss can then only be recovered using a reed valve...or a new piston.

    The transfer ports with the piston at BDC never fully opened. Now they do. I don't think this made a difference. Effective compression has to be a bit lower since the exhaust port closes later but this small amount is easily made up for, and then some, by the higher compression provided by the head.

    I suppose the only way to know for sure would be to put the stock head back on, maybe with no head gasket as this should work fine now and see how it runs. Hoping the compression is still close to what it initially was.

    Further, further thoughts and some questions:

    If you've made it this far, THANX for reading. Although externally identical, or close to it , I'm under the impression that there are internal differences in these China Girl engines depending on which factory may have made them. Many have observed some of what of what I described above in regards to pistons protruding from the top of the cylinder bore at TDC and the intake port not being fully cleared with the piston at TDC.

    At the same time I have read that "newer" engines have neither of the above characteristics. Some people can put a slant head on with no problem. Others have reported that on their engines the intake port fully opens. This could have been achieved with a change in cylinder casting alone or with a change to both cylinder and piston design.

    I noticed that one vendor had two different piston designs available for the 80 cc engine but stated clearly that either was meant to go in one specific type of 80 cc engine or the other. I inquired about the difference and was told that that the only difference was the relative location of the wrist pin. If this had been raised slightly the problem of the piston protruding at TDC would be fixed. A shorter skirt would take care of the intake clearance issue as well. However I have the feeling that "newer" engines that have neither of the problems described have both a different piston and a changed cylinder casting. The question is does anyone know for sure? And would a newer style cylinder fit on an older China Girl. Or is there something that would prevent this from fitting properly onto the crank case?
    Last edited by a moderator: Dec 15, 2015

  2. bluegoatwoods

    bluegoatwoods Well-Known Member

    good post, lot's of info there.

    I only read about half of it since I doubt if I'll ever change heads. But if I ever do, I can come back to this post and learn a few things.
  3. Hawaii_Ed

    Hawaii_Ed Member

    Great info. I can add that I just bought my second kit, and got an 80cc slant kit from zoom bicycles. It came from the factory with 2 head gaskets. Have not tore into it yet since it is running great, but was thinking of trying it with one and seeing if the clearance was OK.
  4. emdude

    emdude Member

    I've been eying on of those myself, let me know how it performs once you have it running.

    But also, if you tear into it I'd be curious to know if the piston still protrudes at TDC and if the intake port opens all the way?!?!?!?
  5. Hawaii_Ed

    Hawaii_Ed Member

    got it running a few nights ago, but I also added a shift kit so hard to compare. Still, motor revs faster and is much smoother than my old one. Can get some pics of the piston this week for ya.
  6. RMWdave

    RMWdave Member

    the noise and vibrations are on my first problems to cure.
    after that ill put it to "sleep" on 105 octane lol
  7. Pablo

    Pablo Motored Bikes Sponsor

    Good write up. I have seemingly similar seat of the pants feelings as well. I only was stuck in futz zone because my clutch was slipping. Once I got it dialed in, the right plug selected (one heat range cooler) - I am loving it.

    I was lucky on the bolt-on side. Just a single SBP head gasket worked like a champ. I have 8mmx1.00 studs. Although I just remembered, one side of the mating surface was very concave - huge gap. So I had to polish it on fairly rough wet paper on glass.

    I also agree on the size of the head as a whole - but oddly the bore seemed to match right up.

    Who did you buy yours from (did I miss that?)

  8. linnix13

    linnix13 Member

    i have a china 80cc from zoom bicycles and i had it apart the other day to check on the rings and clear off like an inch of carbon and at TDC the piston does not stick out at all. infact the piston is slightly dome shaped and the peak of the dome it exactly flush with the top, the edges of the piston are recessed into the block a few milimeters. im thinking of getting that high compression end but i dont think my HT is going to last much longer, i get grinding sounds on the low end with the throttle pinned(bearings or bushings) and its missing alot(bad cdi) but the compression is still good, my clutch slips real bad and its already as tight as it will go! im approaching 3240km on original parts, i hope to make 6000 but thats a long shot!
  9. RMWdave

    RMWdave Member

    ^very nice linnix.
    do a rebuild on your bearings and whatnot and you probley can get 6000 on the same piston and rings without a bore or anything.
  10. linnix13

    linnix13 Member

    yeah i had it apart to clear some carbon and the cylinder wall looks new! i ran my nail down it and there is not one scratch, i also checked for play and there is none, exept i can rotate the piston head about 2mm but i think its always done that, i need a new cdi to because the one i have has hot glue holding it together and something is shorting in the engine and causes the power that normaly goes to the spark plug into my bike frame! man that is one heck of a jolt! so i covered every metal part i can touch with electrical tape, ima ride it till it dies than get a good 4 stroke.
  11. solitus3989

    solitus3989 Member

    Hey there - I have the '80cc' Happy time engine. (same one featured on zoombicycles).

    is http://www.zoombicycles.com/eShop/6-Parts-and-Accessories/14-Addons/121-pk80-slantcover-80cc-Slant-Head-Cylinder-Cover-A-2B.html that what im looking for?

    Also, do i have to order extra/spare/replacement head gaskets? after reading this im a little concerned about just ordering one.

    OR should i do what i did for the bottom, stock gasket that eventually failed - clear out all the old gasket and fill with high temp copper rtv?