Comparison of 2 heads- please take a look

Discussion in '2-Stroke Engines' started by Fletch, Jan 2, 2011.

  1. Fletch

    Fletch Member

    These are both slant heads. The first is Grubee Skyhawk and the second is an engine from Zoom (not sure the model).

    I'd like to know if the zoom head is OK with those "imperfections" or if it will crack down the road?

    My second question (still being a novice to engines) is which head is better designed (better performance)? IOW...which one would you rather use?

    I had them side by side and I'll let the pics speak for themselves, but I will add a few things:

    The markings that look like cracks in the zoom head you can't feel with the exception of the largest one, and it is very minimal (I sanded and polished).

    The skyhawk head seems more pronouncedly circular shaped (both combustion area and spark plug indent), and even appears to me to be somewhat smaller in diameter?

    Lastly because you cannot see this... The part that mates with the cylinder... the square "lip", is much shallower on the zoom head (about 1.25mm) when compared to the Grubee (about 2.75mm). I'm sure this allows for more compression as it is like shaving the head or using a really thin gasket. The grubee is warped though and the zoom is nice and flat.

    Any opinions on which I should use or comments are greatly welcomed. I'm still learning, and I may try both heads and see which gets better performance too.

    Also FWIW, the zoom head has taller cooling fins.


    Attached Files:

    Last edited: Jan 2, 2011

  2. GearNut

    GearNut Active Member

    The "cracks" are casting flaws from manufacturing and poor quality control. They will do no harm except give carbon a good place to start building up.
    I prefer the looks of the Grubee head due to the better quench area which will promote more turbulence into the combustion chamber resulting in a more compete burn.
  3. Fletch

    Fletch Member

    Thanks GearNut I appreciate the info. The Grubee head has about 1/2 of two of the large fins cracked off. I'm assuming it's still OK to use? The trade off seems to be the cracked fins and warped head with better quench area vs. the head with casting flaws and less complete burn that is flat.

    I shaved the skirt off the piston last night, port matched and opened up the intake and exhaust a bit. I moved the exhaust up and the intake down and out a little on the sides. All this by hand with a dremmel. Really inaccurate and sloppy, but I'm hoping it will help some. I've got an expansion chamber from SBP that I just installed and I switched from 44 to 41t sprocket. Stripped an intake threading and decided to just JB weld the ported billet intake on the cylinder. Will be dry tomorrow so I've got my fingers crossed. I haven't decided if I'm going to replace the base gasket yet or just use some Indian head gasket shellac. This is only my second build and I'm already messing around with a lot of stuff. I better quit while I'm ahead :grin5:
  4. GearNut

    GearNut Active Member

    Any good news?
  5. Fletch

    Fletch Member

    I decided to go with the head from zoom because it was already running with it, it is flat and I didn't want to sand down the grubee, and the grubee sits farther away from the piston because that square lip is thicker. Maybe if it didn't have two fins cracked off I would have sanded it down, but I was concerned about cooling issues aside from the fact it looks bad.

    I read contradictory things on here all the time. Oil ratio's and 'break in' are two big ones, but there are lots of others. Some say that rubber in between the fins is ok because these engines don't get too hot, and some say that the fins aren't large enough to do the job. Then you've got mag wheels vs spoked, porting or not, etc. etc.

    Engines hauling though with 50 miles on it. I opened up the intake and exhaust ports and trimmed the piston skirt (redundant, I know). My top speed with a 41t sprocket as of today is 39mph with the expansion chamber and all the other mods. I'm guessing it will get a little faster, but that's a good speed for me. The power band on the exhaust is at about 4/5 wot.
  6. GearNut

    GearNut Active Member

    Thank you for posting an update. I am glad to hear that you are enjoying the little beast.
    The engine should pick up a little speed and power as it breaks in. I am a bit concerned about the porting and piston skirt trim though. There is such a thing as too much of a good thing. Only time will tell though, and the most important aspect of it is that you are happy with the results.
  7. Fletch

    Fletch Member

    Are you concerned because of the what I did specifically (pics in other thread) or just about porting or trimming the piston in general?

    I'm definitely happy with the results, but you're right about time will tell. I've got my fingers crossed! The only thing about my builds is that they don't fire up right away (2 different engines). I think it might be because of the ported billet intakes? I have to give them gas and sometimes it takes 2-3 tries. I've read other people have the same issue. I don't believe I have any air leaks. I've read that you shouldn't need the choke at all to start. My plug is a good tan color. The only other thing I can think of is that I'm not getting a good spark, but I have an upgraded wire, and NGK plugs. The only thing I can think of is HOW I have it wired- white closed off, one kill wire in between the blues, and one that is grounded to the engine. My suspicion is that maybe it needs to be grounded in a different place? I don't know though because the kill switch works fine. My best guess would be the ported intake not being at the right angle, so the fuel isn't being sucked in right away on starting.
    Last edited: Jan 6, 2011
  8. GearNut

    GearNut Active Member

    I just think that your intake will be open too long and you will get excessive blow back through the carb and reduced primary compression. I have a ported cylinder that I have been slowly (very slowly) working on in my spare time. I intend to rebuild the whole engine with the quality bearings throughout before ever starting it. I am still stumped on locating quality big end bearings for the connecting rod though.
  9. Fletch

    Fletch Member

    How will I determine if this is happening outside of measuring compression? Will there be some kind of gradual loss of power?

    Are you saying this because of the pics of how much I opened the intake, because I both trimmed the skirt as well as lowered the intake?

    I have an extra block I can use if I have reason to believe this one is going to cause major problems.

  10. GearNut

    GearNut Active Member

    Measuring compression will not tell you anything as far as primary compression is concerned. Primary compression happens in the flywheel compartment, below the piston, after the intake port closes and before the transfer ports open. All a compression test will show is the compression in the combustion chamber, above the piston.
    There will not be any gradual loss of power, just an overall lack of power at a certain RPM. In extreme circumstances the loss of power could be across the entire RPM range that the engine is capable of operating under.

    I am stating this because yes, you both trimmed the piston and altered the port timing in a random fashion.
    Now, altering both areas of port timing control in itself is not necessarily a bad thing if the proper measurements and calculations were done to determine precisely how much altering should be done.
    I am in no way suggesting that you swap out your modified cylinder and piston for another set if you are happy with your current results.
    I am really only trying to say that modifying both at the same time without doing the proper calculations to determine the best modifications usually results in a lackluster performance gain at best and typically hurts overall performance. I am also trying to describe the hows and whys of it all as well to hopefully convey to you what exactly is going on. Someone here has a link in their signature to a book, "Performance Tuning by Graham Bell". If you can find it, download it and read it. You will gain a heck of alot more accurate knowledge on this subject than I can share with you as I am not really a good teacher. I try to share what I know as best as I can, but I know that I flounder with getting my point across and sometimes I confuse more than help.
  11. Fletch

    Fletch Member

    I see... Thanks for the explanation of primary compression. I actually think you do a great job of getting your point across.

    Would there be any problem with taking a used piston from another engine I have and using it in this one to see what the difference is without the skirt trimmed?

    I realize I was basically gambling with the random way I did the ports. That was a one time deal.

    I read some threads where that book was mentioned as well. I basically saw that measurements were in fractions of millimeters, and I knew I didn't have the tools or machinery necessary in order to do it accurately. I had already randomly enlarged the exhaust port when I put the expansion chamber on. That was before I read/knew anything about it effecting timing- just a "why not?" move out of ignorance at the time. Then I read about the intake lowering or trimming the piston. I trimmed the piston and didn't intend to lower the intake port, but I just got carried away with the dremmel when I was trying to smooth the flash casting. I didn't have the right tool on the dremmel so it ended up removing too much. Then I just said what the ****!

    I forget who, but someone mentioned 1.1" as the magic number for the exhaust port from the top of the head? Is this generally agreed upon? I ask because I'm thinking that on this new engine I have, I will just trim the skirt and leave the exhaust alone if it is about 1". It's going on a new bike with shift kit, and I definitely don't want to mess it up. Would there be any risk in that?
  12. GearNut

    GearNut Active Member

    There should not be any risk in notching the piston alone, but the notch should not be the full portion that shrouds the intake port when the piston is at TDC (Top Dead Center).
    I have read that the best gains are when only half of the offending skirt that is shrouding the port is removed. That way the port timing is not drastically changed.
    FWIW, the best gains that folks have found are from properly cleaning up all the the ports in the cylinder and leaving the piston alone. 1.1" from the top of the cylinder is supposed to be the "magic" number for the exhaust port.
    Also do bear in mind that altering the port timing will in fact increase the power output but only at a certain RPM range. It makes the engine run weaker at all other RPM's but at that "sweet spot". In order to get the best gains you need to tune the exhaust, carburetor, port timing, compression ratio, and ignition timing to all work in perfect harmony with each other. This is a big undertaking for a novice but can be an exciting thing to do if you take your time and learn exactly what to do.
    I myself am not going to that extreme, but I am only trying to improve the efficiency of my spare engine by removing any and all restrictions left by the poor quality control from the factory. Any power gained would be welcomed and inevitable as there are alot of restrictions in the flow in and out of the ports as the factory leaves them.
  13. Fletch

    Fletch Member

    Thanks GearNut! So basically if I mark the piston with a marker when it is at TDC, there is going to be a half moon shape like the way I cut the first one. If someone does half, I'm assuming that they keep the same shape, but move it down half way? So all you do is remove the flash or edges on your ports? You don't change size at all?

    I gotcha with the fine tuning- makes sense. There is a range (about 1/4- 1/2 WOT) where the engine revs much higher than it pulls. I don't know what that is called. So basically I can let back on the throttle to reduce the revving and maintain speed/acceleration. This engine was doing that before any mods though, so hopefully it is just a result of being new and not having developed full compression yet?

    What do you think about my idea of using the old unmodified piston? Can you switch used pistons into used engines?

    Last question: Would it be reasonable/safe to use the grubee head with almost two missing cooling fins?
  14. GearNut

    GearNut Active Member

    You are correct on the piston notching. Do the same shape as the magic marker trace, but only half the height of the traced area as measured from the bottom of the piston. Smooth all edges after cutting to reduce turbulence and reduce chances of scratching the cylinder as the piston travels inside it.
    As for my cylinder, I am removing all casting flash from all ports, raising the ceiling of the intake port by approx 1mm and arching it (only in the tunnel of the port, not the actual port hole in the cylinder), mirror polishing the exhaust port and satin polishing the intake port and transfer ports. I am also smoothing out the rough factory edges where the chrome plating meets the aluminum at the port holes in the cylinder. As for changing the port timing, I am trying my best to leave all that as close to factory as I can. The factory port timing gives the engine a good broad power band rather than a peaky narrow RPM power band. I am looking to end up with a work horse engine, not a race engine.
    Do bear in mind that none of this is being done with a Dremel tool. It is all being done by hand with custom shaped pieces of paint stir stick, firm sponge foam glued to them, and gradually finer grits of wet/dry sand paper. Yes I use a soap and water solution to prevent the sand paper from loading up with metal and promote faster cutting action.

    As for the engine revving much higher than it pulls, be sure that your clutch is not slipping.

    As for using an old piston in an old cylinder, I would never, ever do that to a quality engine. But as far as these cheap engines go, so long as it has no bad scoring anywhere on it, fits well in the cylinder (0.002" to 0.003" clearance between piston and cylinder), and has identical measurements between the piston crown and centerline of the wrist pin, go for it.

    As for the Grubee head with 2 broken cooling fins, I would have to see it to make that call. I do not like broken cooling *anything* on my engines, but I have gotten away with it on many of the old beater dirt bikes I had as a kid.
  15. Fletch

    Fletch Member

    Right on, thanks for the detailed info. I'm really amazed how you know all this stuff. Then again, the most I've ever done on an engine of any kind before this was check the oil.

    Another member in my other thread with the pics of what I did "cutting on my engine..." recommends using a 2" ball hone to "cross hex" the cylinder. I didn't find "cross hex" on Google, but I'm guessing he means "cross hatching". It looks like you do it at 2 different angles to get 45 degrees:

    I was going to order this Flex-hone:
    Which is self-centering and aligning. I just have to figure out the bore size I need.

    Do you think this is a good idea? Meaning- is it worth doing, and can I mess it up? lol

    What do mean exactly by work horse? I'm guessing you mean endurance of the engine and dependability?

    Good call on the clutch slipping. I adjusted the acorn nut recently because it wasn't firing up right away, and that took care of it. It's not firing up again, so that makes sense.

    Messing with the JB weld because I stripped the intake hole was stupid. It expanded as it dried which I didn't think about, and that stripped side of the intake came loose. I knew I had an air leak as soon as I started bogging down at open throttle. So I took it off and had to tap the hole one size bigger to a 7mm. Its the ported billet intake with the hex bolts that are fitted to the intake- they sit inside the holes. Tomorrow I'm off to find a similar size 7mm hex bolt.

    I don't know how, but whatever I did to the engine has really got it running strong all across the board with the exception of where the power band is on the tuned exhaust. It accelerates really quickly and the power band is like between 3/4 and 4/5 WOT.

    Is my logic correct that if you lower the intake and raise the exhaust equally the timing would stay pretty much the same... like they would cancel each other out? I ask because my exhaust is really high, so maybe that is why I'm not feeling a decline in primary compression or any one band that is really weak like you talked about.
  16. GearNut

    GearNut Active Member

    All I know about 2-stroke engines is what I have been taught from my elders, some whom built and raced them for a living back in the 60's and 70's, as well as what I learned by reading the tuning books that they recommended.
    Any particulars concerning these cheap Chinese engines I have learned from the trials and results shared by the folks on this and similar forums who build them up for maximum power.

    I have built too many engines to count, not all of them successful though. You have to learn from your mistakes to get good, and I am still getting there.

    You are correct in your search for "cross hatch".
    I would never, ever hone a plated cylinder though. The plating is thin to begin with and honing it will only make it thinner. Now a cast iron lined cylinder I have no problem honing.
    The flex hone you found is for deglazing and re-cross hatching a 4-stroke cylinder that is otherwise in good running condition. Never ever under any circumstances use one on a 2-stroke cylinder. The abrasive balls will spring out and get caught on the leading edges of the ports when used on a 2-stroke cylinder.
    One needs to use a bar hone in a 2-stroke cylinder.
    Now after sharing all that stuff, if you are worried about removing any glazing in a 2-stroke cylinder that otherwise is in good running condition, I have always used Scotchbrite pads and paint thinner or laquer thinner. Do your best to keep a 45 degree cross hatch as shown in the link you shared.
    That is important as the original cross hatch should be the same 45 degrees and you are trying to clear up the burnt oil glaze that is caught in the grooves of it.

    Your assessment of my "work horse" engine is spot on.

    I do not like JB Weld for anything that requires strength and reliability.

    If you are happy with the results of your porting and piston notching, leave it alone.
    If you want to play around and experiment with things, by all means get another cylinder and piston and do it up differently. You may be surprised how subtle changes can make big power differences.
    That is one thing that I really like about these cheap engines. Cheap to mess up on and cheap to rebuild.

    Changing the port timing in opposite directions will not cause them to cancel out each other.
    Primary compression has nothing to do with the timing of the exhaust port.
    It happens when the piston is traveling down, the intake port has closed, and the transfer ports have not opened yet.
  17. Fletch

    Fletch Member

    Thanks for saving me from buying that hone. In this thread Ken is describing to me his process for what he does on new cylinders before he runs the engine. He does mention a 2" ball hone, for helping he rings seat well. From what I briefly read about cross hatching, the 45 degree lines are to keep oil from running down the cylinder- increase lubrication. Is what he talking about the same thing you are saying to absolutely avoid doing? I know that was the wrong hone (4 cylinder) but not sure if you are anti-honing on these engines completely outside of deglazing? You describe the honing for deglazing a used cylinder, and he is talking about it on a new one, so I am confused.

    I'm basically trying to figure out what I should do exactly to this new cylinder before I run it. I want to remove the fllash casting without cutting into the port itself. He recommends a cratex wheel for chamfering the edge where the aluminum meets the chrome. The dremmel won't reach that far down the inside of the cylinder so I probably need to find a way to do it by hand. I'l probably use a sanding or polishing tool on the dremmel for removing the casting from the outside of the port in, and then polish the ports. I just don' want to mess anything up! The chamfer he says is important because the chrome chips off and causes the vertical lines in the cylinder.

    Sorry about all the questons, and maybe seem like I'm being too anal about the details. I'm just looking for more than one opinion and trying to make sure I understand exactly what I'm going to do, how, and with what so I don't end up with random shaped ports like last time without smoothing the transition surface like I apparently should have. I'm a novice for sure, but i can't in good conscience just do nothing to the ports now that I know how bad and jagged they come.

    Here is the extent of the damage to the fins.

    Attached Files:

    Last edited: Jan 9, 2011
  18. GearNut

    GearNut Active Member

    A new cylinder should not need to be honed at all. If Ken feels it is a good idea then Ken can do it. There is no harm in it. It is still a free country (for the time being).

    I chamfer the edges of the ports with 250 grit wet/dry sandpaper, and finish them off with 300 grit. Use a drop of dish soap per a gallon or so of water as a cutting fluid to keep the sand paper from loading up with metal. Wash the bejeasus out of the cylinder with HOT soapy water after grinding/ sanding before installing it. When you think you have washed it good enough, wash it again.

    Cratex grinding bits are excellent for shaping the ports, but I would still hand finish them with sandpaper.

    Ask all the questions you want! That's how folks learn. If I do not know the answer, I'll say so or I will research it first.
  19. Fletch

    Fletch Member

    I'll do that on this next engine. Do you happen to have any pics of the makeshift tool you use to do it? I managed to get in there with my fingers, but not that well.

    "wet/dry" meaning wet and then dry paper? You prefer sand to emery paper?

    Would yo consider using that head with the fins in that condition I posted?

    Here is side by side of the two pistons. The Grubee is on the right and the distance from crown to wrist pin is definitely not the same. Using my keen powers of deduction it looks like the left one would come up higher in the cylinder than the grubee? Also...any idea why the cuts in the skirt are larger on the grubee and what difference that makes. I read that notching the skirt can increase noise (rattling). Because the grubee has those big
    "cut outs" (don't know what that part is called), Is that why it's so **** noisy maybe? Can I try swapping them, or be better off leaving it alone?

    Attached Files:

    Last edited: Jan 10, 2011
  20. GearNut

    GearNut Active Member

    The "make shift tool" is nothing more than a paint stir stick, usually found for free at home improvement stores and at Frazee paint stores, ect. I split it down the middle and sanded it to fit down into the ports exactly as I wanted it to. Next, for the transfer ports, I cut the end at the desired angle that I wanted, then using blue waterproof masking tape, I taped the sandpaper to it.

    By wet/dry sandpaper I mean the automotive finishing sandpaper that is waterproof.
    If you mix a drop or two of dish soap to one gallon of water and use the water as a cutting fluid it prevents metal from sticking to the sand paper. When metal or paint sticks to sand paper it is called "loading up" the paper and it will quickly ruin the sand paper. Also by constantly replenishing the water you rinse away the metal "soup" that results and you can see exactly where you have sanded and where you need to sand some more. To me it works out alot better than dry sanding, cuts metal alot faster too.

    I would not run that head. Because the fins are missing across the entire side of it, I believe that it will have a good chance of warping when it gets up to operating temperature which will result in a blown head gasket.

    Those pistons are in no way compatible on an exchange basis. Different factories use different length connecting rods and therefore different wrist pin locations to maintain a proper compression ratio. That is why they are different.

    Your keen powers of deduction are correct!
    If installed in the wrong engine, the piston would travel up and out of the cylinder so far that rings would pop out and hang up on the top of the cylinder. Of course if the cylinder head were to be installed first, the piston would just hit the cylinder head and the engine would cease to turn over.

    The notches in the non thrust sides of the pistons are there to reduce the overall weight of the piston. They are different lengths because different factories made them to fit their own engine design.

    Excessive notching of the piston skirt can cause the "piston rattle of death".
    Be conservative with your notching and you will not have any problems.