Air cooled twostroke, best head gasket material?

[John Orchard]

If the above style of bike comes with a fibre headgasket, is changing to aluminium or copper a better idea? Is it better to have good heat transfer between head & cylinder? I guess the best for heat transfer would be copper, as long as it is annealed sealing should be ok.

[sleepy]

You seemed to have answered your own question. Copper

[John Orchard]

You seemed to have answered your own question. Copper

I guess my query is, is it better to have heat transfer from one to the other? Which would be the hotter, head or cylinder? Would it be better to have them the same temp? Does having good thermo conductivity stops one from getting too hot? I really would be lost without my question marks ;-)

[Hoony]

i would "guess" the Head would be hotter than the cylinder due to combustion in the head surface and the cylinder is generally a larger mass therefore can dissipate the heat better due to that mass.

but then again i am an electrician not a thermal engineer

[tony27]

Copper transfers heat quite well which is why it's used on the base of cookware, have TIG welded copper before & it takes longer to get the heat in but once up to temp it retains more meaning you have to move quicker
Aluminium head gaskets are meant to be single use & can't be annealed unlike copper

[John Orchard]

Copper transfers heat quite well which is why it's used on the base of cookware, have TIG welded copper before & it takes longer to get the heat in but once up to temp it retains more meaning you have to move quicker
Aluminium head gaskets are meant to be single use & can't be annealed unlike copper

But aluminium doesn't harden like copper, hence, doesn't need annealing.

[Momus]

Aluminium certainly work hardens and can be annealed.

You rarely see it except on low output engines.
Really low output engines, like Victa two strokes, use a graphited paper.

Copper has much better high temperature strength as well as conductivity, though is much more expensive.

The ideal cylinder head joint might be lapped aluminium to aluminium if the clamp can be assured even.

[Tomas]

Some engines dont use head gasket at all. For example my Jawa 50cm3 (Jawa 21 Special) has no head gasket and i never had any leaks(compresion ratio is 9.5 to 1 and have 3 piston rings) 

[John Orchard]

Some engines dont use head gasket at all. For example my Jawa 50cm3 (Jawa 21 Special) has no head gasket and i never had any leaks(compresion ratio is 9.5 to 1 and have 3 piston rings) 

Alloy to alloy? Do they need a coating, like VHT copper coat, to seal?

[sleepy]

Some engines dont use head gasket at all. For example my Jawa 50cm3 (Jawa 21 Special) has no head gasket and i never had any leaks(compresion ratio is 9.5 to 1 and have 3 piston rings) 

Alloy to alloy? Do they need a coating, like VHT copper coat, to seal?

Most Bultaco's run without gaskets and you use lapping paste to lap the surfaces to get good contact. They just use the width of the sleeve shoulder so it is even the whole way around, no sealant is used. As noted above Aluminium does work harden and can be annealed but it is very much dependent on what grade of alloy it is. Pure aluminium is very soft but impossible to get in sheeting so you are stuck with a few different grades. To anneal alloy you blacken it with an Acetylene flame then heat it evenly till the black disappears and that gets rid of any heat treatment but will work harden again to a certain degree. On high performance 4 strokes is is common to "o" ring the block or head using stainless steel wire in a groove to get a much more robust seal and sometimes having grooves machined around the face help as well.   

[skypig]

A YZ80D I ran on alcohol back in the day ran with no head gasket.
I used a “coat of silver paint” as instructed. (From memory)

[Momus]

SilverFrost paint presumably has very fine aluminium particles in the oil/solvent carrier and makes itself a thin gasket.

The aerosol copper coat product I use on worn steel shim type 4 stroke head gaskets is effective as well.

[Tomas]

Some engines dont use head gasket at all. For example my Jawa 50cm3 (Jawa 21 Special) has no head gasket and i never had any leaks(compresion ratio is 9.5 to 1 and have 3 piston rings) 

Alloy to alloy? Do they need a coating, like VHT copper coat, to seal?

No. Cylinder liner is cast iron. Liner is pressed in aluminium cylinder and there is a flange on top of the liner. Cylinder head has a recess and thats it. There is 4 M6 studs holding it together. Shops are selling aliminium washer like gasket but from factory there was nothing. I do not thing that anyone at the factory bothered laping it together or anything like that 

[LWC82PE]

Copper head gasket and use blue Hylomar on it.

[pokey]

Sure you can lap bare surfaces and you can O ring  with Viton or SS which are variations on lapped surface and they all work to a degree. Easiest and most reliable and forgiving is the copper gasket.

if your going for land speed runs yeah go for a lapped surface variant as you will be pulling it down regularly but if you want reliability go for a gasket and while aluminum is useable copper is easier and more forgiving and it conforms so doesn't require a sealant. Sealants are  for non perfect surfaces with varying extended conditions like enduro or road bike where reliability is key. Do your measurements and factor in the the compressed gasket thickness and you can rely on it not to be the cause of a problem.

As for heat transition each component is designed to dissipate its own heat. The EXH port is the hottest and that heat is sucked into the header. Most air cooled heads have a reasonably large mass and relative large surface area  to conduct heat ala porcupine plus it has the cooling charge of fresh fuel every 2nd stroke so it never gets as hot as the EXH port

No one ever had an air cooled engine failure from a head causing heat issues as its the EXH port that catches the ring and the EXH port side of the piston that typically shows the start of Det and this is where a good pipe comes into its own and good oil to wick the heat away.

It all depends on what your going to do with that engine.