Optimising the 2-piece head - Pocket Bike Forum - Mini Bikes
 
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post #1 of 8 (permalink) Old 04-03-2017 Thread Starter
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Optimising the 2-piece head

So after doing lots of reading and doing my best to work my way through what I found, I wanted to get the input of people on here to see if I am on the right track. I have found it a bit daunting and difficult to work out what is correct in regard to 2-stroke tuning, as a result I have tried to research as much as possible on one part at time. What I would like to explore with people is the finer details of the 2-piece heads and also the quench zones / squish.

To make it easier to read I have broken it up into sections, if any disagrees or has anything to add to any of this then please do so, I have a lot to learn still.



1- Issues with existing 2-piece head / barrel kits: I have been advised previously (thanks CAM2) that using a 2-piece head has it's downsides such as it's ability to seal properly due to the different expansion rates of metals.

Are there other downsides? Can the sealing issue be overcome with doing some lapping like in Mexicano's thread?

Link:A "Racing Engine" Built

Or something else like more head cooling? Just how much leakage is there anyway?


2- Dome profiles / quench zones: So on the dome insert for the head I have here it has a simple taper machined into it at 15.5 degrees instead of something that matches the piston shape. I think this flawed because that's a shallower angle than the edge of the piston. I believe this means that a ring of the combustion gasses will be trapped between the angle of the head dome and the outer part of the piston curve at TDC. If so this could cause wasted combustion gasses, unburnt mixtures and also possibly promote both detonation.

Of all the times people have stated the squish distance they must be talking about the narrowest point then?

I see 2 ways for me to improve this, one is make an elaborate radii cutting fixture for my lathe to make the head dome match the piston, this would also let me machine the surfaces of my pistons to an exact shape and it would likely be a useful tool to have on hand for other projects. It could also potentially work wonders for me cutting my own combustion chamber pockets and therefore let me alter the quench zone to chamber opening ratio.

The second option is quick and easy- I read that unlike 4 stroke engines it helps to have some angle between the piston and the head in a 2-stoke, as in like the squish area making a wedge shape. I am at a total loss as to why this is though as it shouldn't be able to properly quench the mixture being like that. If this is correct then although crude just changing the angle of the head dome leading to the chamber to around 20 degrees would then move the narrowest point of the squish to the edge of the piston. From this narrow edge it would gradually increase in the squish height as it gets closer to the chamber.


3- Spark plugs: The original barrels use a M14 size spark plug and the "upgrade" 2-piece kits I have seen use an M10 instead. Given there was an option and comparing the same heat ranges and so on is there any advantage to using either of the two sizes? I am guessing not but happy to be told otherwise.


4- Offset Chamber design (See attached pic): This is when the combustion chamber is offset towards the intake. This is to help equalise the temperatures of the head and piston because the gasses are normally prone to heat up one side more due to them heading towards the exhaust port. It has been covered to some degree here:

Link: Cag heads and compression ratio

Any reasonable merit to this? or just small improvements to be had? I should have enough ability to make my own offset dome, if I succeed or not it would be a good mental exercise either way.


5- Dome to head heat transfer: Another thing I noticed is that there the main part of the 2-piece heads only touch the dome on it's top outer edges. This is the only place where the head dome can transmit heat to the main part of the head. Is this done to put as much pressure on the top edge of the bore to help it seal or something? For me I would of thought it would be better to have a larger surface area between the two and then use some thermal paste.


6- Other: Anything else I missed that could be improved? Lets say that manually making a custom head and dome isn't out of the question then what could be made even better? For example I could possibly make a head with a heap more surface area in it's cooling fins, I don't know if this would help for that much though.


That's it for now, sorry for the long post people
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Last edited by Sam-Q; 04-03-2017 at 10:15 AM.
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post #2 of 8 (permalink) Old 04-03-2017
Re: Optimising the 2-piece head

For heat transfer I spray all my gaskets, copper or paper with Permatex, Copper Spray-A-Gasket, Hi-Temp Sealant which is designed for heat transfer & eliminating hot spots. On my custom pistons I grind a 1 1/8" flat spot on the top of the piston, so when it fires the force is deflected at the cyld wall and not down the edge of the and past the rings.
I don't like the round alloy heads cause the compression cap sits on top the cyld lip. The Winged Billet B/B kits have a cap that fit over the lip so the cap cannot move around, especially with all the extra engine vibration.

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post #3 of 8 (permalink) Old 04-04-2017
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Re: Optimising the 2-piece head

Now youre finally seeing what I was saying about the headkits and how they need adjustments etc...If the chinese took the time to do a proper matching of the compression dome to piston profile in their CNC program rendering etc then that cheapo headkit would cost triple of what you paid....

Before installing my headcap I smooth the sharp edges of the dome...Its not overly critical worrying about the kernel of the flame too much...

If you look at the newer Stihl industrial 2 stroke engines with a 360 degree cylinder they belt out some pretty stout power and they utilize a flattop piston with a curved dome.

Even when I convert McCulloch 2 strokers into racekart engines I get really nice power with a flattop piston over a curved pistontop

I tried a flattop piston in a 1pc build....I made a model of the area left utilized some mathematics and came up with how much air and fuel mix was needed to cram into that space to create power..

Unlike a 4 stroke which needs that squeeze cycle in order to excite the molecules on a 2 stroke velocity and the mixing action of the crank agitate the fuel more than enough to add a spark to in order to complete combustion with enough force to send the piston down and around for another bout...

For the sealing issues although I dont mention everything in my builds again because no-one else other than maybe EPR and you that would utilize it...

I lap the head to the cylinder always aswell as I true up the top of my cylinder with an oil stone into a super flat and with nice sharp crisp edges before lapping....

When I insert the dome into the cooling fins I use silver based heat sink compound between the two to help aid heat transfer that gets lost from the tooling grooves between the two....

I did the math on how much area is being lost between aluminum to aluminum contact just from those tooling grooves alone..........Its quite substantial.

In order to get a 100% complete seal on the dome its best to get some soft aircraft sheet aluminum and have 40 and 44mm rings laser cut...

On the cylinder youre gonna notice a compression relief notch that bleeds off pressure...............That should be a bigger concern because 43cc engines can be built way more powerful and rev alot harder because they dont have that notch. and a lighter rotating assembly............

Thats another thing I dont discuss because who else is gonna take 1/16" aluminum welding rod,two 12v golfcart batteries and jumper cables to arc weld the notch then clean it up with a dremel besides maybe again you and EPR....

Theres always a give and take in this stuff....You cant perfect every area....

Two strokers need extreme velocity to create massive suction to cram lots of fuel into the cylinder over and over to make power..........

Only way to know whats gonna work and doesnt work is to build multiple engines in multiple ways then test them...............

If I documented every small reedported and piston ported 2 stroke engines I built since joining here Id have close to 5 dozen threads...........................

Last edited by CAM2; 04-04-2017 at 08:22 AM.
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post #4 of 8 (permalink) Old 04-17-2017 Thread Starter
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Re: Optimising the 2-piece head

Sorry for the delay chaps, was held up on other things and I had my usual physical issues.

I had a good think about things in regard to making the previously mentioned radii cutting tool and decided what I am likely to do. It's a pretty big job to make a custom fixture / tool for my lathe that can cut external convex shapes and while it would be nice to be able to be able to skim the domes of my pistons to make them all the same known radius and identical to each other it's otherwise not an a tool I see me having much of a need for. On the other hand making something just for cutting concave shapes like for example modding combustion chambers is not only disproportionally easier but it should come in handy for other jobs too. I ordered a cheap centre alignment tool holder to suit a narrow Model-V insert that I think would work pretty well:

MVVNN1616H16 16 x100mm Lathe External Turning Tool Holder For VNMG1604 | eBay



Quote:
Originally Posted by EPR View Post
For heat transfer I spray all my gaskets, copper or paper with Permatex, Copper Spray-A-Gasket, Hi-Temp Sealant which is designed for heat transfer & eliminating hot spots. On my custom pistons I grind a 1 1/8" flat spot on the top of the piston, so when it fires the force is deflected at the cyld wall and not down the edge of the and past the rings.
I am still researching piston related information so I will make a mental note on this one mod. This is the first time that I remember anywhere of hearing such a suggestion, dos make sense though. I will go through your old posts again to see if I can find a picture of this mod.


Quote:
Originally Posted by EPR View Post
I don't like the round alloy heads cause the compression cap sits on top the cyld lip. The Winged Billet B/B kits have a cap that fit over the lip so the cap cannot move around, especially with all the extra engine vibration.
When I was previously looked at the winged heads I totally missed the fact that they have the difference of having the dome with the lip you mention. It's not an obvious thing to see in the photos and you know what those Chinese sellers are like for their limited information.

Regardless I don't follow on how it would make a difference in how it can move around though? The reason being that if the dome of the other type fits into the head recess without too much clearance then the head will hold it in place and also be what located it over the top edge of the bore instead?

But even if it's a small improvement I was thinking of making a new dome anyway so I could just make one that does have a locating lip and with a better fit to the outer bore edge. From there there is enough material in the head to enlarge the diameter of the recess to suit the new dome. This would also have the bonus of giving more of a surface area between the dome and head for heat transfer and all up I don't think any of it would be overly time consuming to do. If I come up with something I will post some pics on here.
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post #5 of 8 (permalink) Old 04-17-2017 Thread Starter
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Re: Optimising the 2-piece head

Quote:
Originally Posted by CAM2 View Post
Now youre finally seeing what I was saying about the headkits and how they need adjustments etc...If the chinese took the time to do a proper matching of the compression dome to piston profile in their CNC program rendering etc then that cheapo headkit would cost triple of what you paid....
My thoughts exactly, apart from a piston with a really wonky dome I in no way can complain with what I was able to get verses how little I paid.


Quote:
Originally Posted by CAM2 View Post
If you look at the newer Stihl industrial 2 stroke engines with a 360 degree cylinder they belt out some pretty stout power and they utilize a flattop piston with a curved dome.

Even when I convert McCulloch 2 strokers into racekart engines I get really nice power with a flattop piston over a curved pistontop

I tried a flattop piston in a 1pc build....I made a model of the area left utilized some mathematics and came up with how much air and fuel mix was needed to cram into that space to create power..
When you previously mentioned lightening the original piston and how you also tried piston model it did get me thinking and I did some research in what was available from other engines. As a result I spent a reasonable amount of time and I did see a nice wide range of diameters available but frustratingly they mostly used 10mm wrist / gudgen pins when I need 12mm. Back to plan-A then.


Quote:
Originally Posted by CAM2 View Post
For the sealing issues although I dont mention everything in my builds again because no-one else other than maybe EPR and you that would utilize it...
Makes complete sense to me, on top of that I am sure that EPR would already know the info anyway so it would just be useful to me alone.


Quote:
Originally Posted by CAM2 View Post
I lap the head to the cylinder always aswell as I true up the top of my cylinder with an oil stone into a super flat and with nice sharp crisp edges before lapping....

When I insert the dome into the cooling fins I use silver based heat sink compound between the two to help aid heat transfer that gets lost from the tooling grooves between the two....

I did the math on how much area is being lost between aluminum to aluminum contact just from those tooling grooves alone..........Its quite substantial.
Sounds good, after reading Mexicano's thread I thought it was a good idea in how he used some lapping compound, as a result I ended buying some to use. Useful for other job / projects too.

I used to tinker with computers when I was younger and have seen hard data showing the difference in temperature from a seemingly smooth surface to one that's buffed further with wet and dry sandpaper. With the difference that made I have no doubt at all how significant it would be if there are machining grooves between the surfaces. I did plan on using some thermal compound as you have as well.


Quote:
Originally Posted by CAM2 View Post
In order to get a 100% complete seal on the dome its best to get some soft aircraft sheet aluminum and have 40 and 44mm rings laser cut...
Interesting.... I did not know about this.


Quote:
Originally Posted by CAM2 View Post
On the cylinder youre gonna notice a compression relief notch that bleeds off pressure...............That should be a bigger concern because 43cc engines can be built way more powerful and rev alot harder because they dont have that notch. and a lighter rotating assembly............

Thats another thing I dont discuss because who else is gonna take 1/16" aluminum welding rod,two 12v golfcart batteries and jumper cables to arc weld the notch then clean it up with a dremel besides maybe again you and EPR....
Yes I remember reading about those subjects. With that rather interesting experiment as I said in that other thread I personally can't see it being able to be done at all but I would like to be proven wrong about it and would very much want to know if it actually worked out.
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post #6 of 8 (permalink) Old 04-17-2017
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Hey just found this site I got an x7 bullet 110cc but I'm having trouble finding new fairings for it??
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post #7 of 8 (permalink) Old 04-17-2017
Re: Optimising the 2-piece head

Go to the Introduction Section and post there, you need 3 post to make your own on the whole Forum. Body parts are hard to find and if you do it's going to cost $350, more than the bike is worth, lol.

Empty Pockets Racing: The difficult we do, the impossible takes longer.....
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post #8 of 8 (permalink) Old 04-17-2017
Re: Optimising the 2-piece head

Sam, you continue to amaze me, lol. For around $80 WiseCo. can make you a custom piston w/ 12mm pin and make the pin height to the top of the piston where you tell them. The China ZPF W/C mini moto has a 40mm 1 ring piston w/ 12mm pin, I have 1 or 2 here I believe, the stock pin bearing is to small you can use a Cag 44mm/49cc 12mm rod bearing. There are chain saw pistons in 1 or 2 rings, Stihl and Husqvarna at 40 and 44mm that might work, I cut the lip down on top of the 2pcs B/B cyld to raise the piston, inwhich I have to modify the top of the piston to miss the compression cap, then you do the port timing, lol. I have a 1 ring 40mm Banshee Sho piston w/ 10mm pin I am trying to put on a 10mm pin FC crank and into a Cag reed port engine. CAM2 can remember, the guy that turned one leg of the FC crank down to fit the small seal on a Piston Port engine. He was trying to go with a 44mm cyld which has the exhaust channel, I bought the stuff cheap from him. I am using a highly modified & ported 40mm cyld and polished piston I bought from CAM2, hooking the FC crank into the piston port cases with either a round alloy head or winged head, have a few of both here, its boxed up right now. Working on my Euro bikes right now, GRC and Polini.
CAM2 modifies almost every part, the best way to lighten the end of the rod and piston is to bore out the center of the piston pin, which he has done, or buy a titanium one. I some titanium M5 and M6 allen bolts on my avatar GRC-GP, there strong and very light, and all the body screw are billet alum, lol.

Empty Pockets Racing: The difficult we do, the impossible takes longer.....

Last edited by EPR; 04-17-2017 at 01:57 PM.
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