2XT cylinder head

[Robbi347]

Hi, can anyone verify if this is the correct profile on the 2xt head? Looking at the 1KT/2MA they have a small lip on the outside edge of the combustion chamber. Part numbers are different and the head gasket on the 2xt is 47x ( 500lc). Just trying to make sure the profiles correct and not been skimmed. Cheers

[problemchild]

it is skimmed i guess...
http://rd500lc.free.fr/tzr-250-2xt/normal/11/TZR250-Rebuild_09.jpg
http://rd500lc.free.fr/tzr-250-2xt/normal/11/TZR250-Rebuild_21.jpg

[Robbi347]

Thanks, I suspected it had been but held onto the hope i didn't need to spend MORE money on a new head. It's amazing the things you find when you split a supposedly "minted" engine.

Any good heads going free ? 

[Robbi347]

Anyone know the thickness of the 2ma head so I could see how much was skimmed and maybe I can get a copper gasket to suit?

Cheers

[David141]

Its definitely skimmed, my 2xt head has the same lip as the 1KT head, but I cant remember the measurement, and I cant get to the garage now because of lockdown....

I was looking at the 2XT race manual on here (P34), now I don't speak Japanese but it does reference the original gasket (47X-1181-00) then goes on to say KIT 1KT-1181-00 so this may have something to do with that mod....

In any case when I started messing around with 2xt stuff I didn't know about the change in gasket, it was only when I bought a set from japan i found out.... So the first time I built my 2xt motor like I would normally with like a 'partially' skimmed head. That lot worked fine not damage when I took it apart, But then I haven't really run it in anger yet.

David

[Robbi347]

Yeah I did the same, the bike came with a 2ma gasket so I just replaced with the same. I've had a few issues with starting and such and went through everything, also found the gasket is 47X.  So with the 2ma gasket and skimmed head the compression must be ridiculous!! I'll probably get a new head rather than start messing with different base gaskets and such. Do you know the reasoning behind the different head gasket?

[problemchild]

Anyone know the thickness of the 2ma head so I could see how much was skimmed and maybe I can get a copper gasket to suit?

Cheers

around 33-33,2mm, measured thru the Plughole on a flat surface.
the "F3-Head" is 3mm thinner but stil has this small lip around with 0,3mm

[David141]

Anyone know the thickness of the 2ma head so I could see how much was skimmed and maybe I can get a copper gasket to suit?

Cheers

around 33-33,2mm, measured thru the Plughole on a flat surface.
the "F3-Head" is 3mm thinner but stil has this small lip around with 0,3mm

Cheers for that problemchild, going to measure mine when I can.

Oh dont even go there with problems starting/running, when I write up my project you'll see why!

As for why the difference in gaskets, no idea. Guess are: Instead of producing a new casting for a KIT head, why not just use a thicker gasket, Both of which were still in production at that time? But id also say because the 2xt uses different pistons. (Which are still different according to the part list vs KIT manual)

^aka I have no idea 

David

[problemchild]

i´m not experienced in 2XT but i would use the usual 2MA/1KT-Gaskets and just check Squishgap to be correct.
Or you use a 3MA Headgasket(but you have to cut it in half). or a 2MA Head on 3MA with 2MA Gasket,or RD500...it does not matter wich way round, they are all the same at least. Piece of Metal in around 0,3mm thick...
just check the squishgap in operating configuration and your compression.

[Robbi347]

Thanks for the input guys 

Measured my head and it's 31.9 mm, so about 1mm skimmed, seems a lot? The head surface is factory smooth so never picked up on this till started checking everything.

The previous owner has obviously had notions to go F3 at some stage, on the cheap by the looks,  as there was a F3 photocopy in the paperwork I got. Think I'll go back to factory base and start there, 1kt head, 47X gasket , the japs knew what they were doing

[problemchild]

Thanks for the input guys 

Measured my head and it's 31.9 mm, so about 1mm skimmed, seems a lot? The head surface is factory smooth so never picked up on this till started checking everything.

The previous owner has obviously had notions to go F3 at some stage, on the cheap by the looks,  as there was a F3 photocopy in the paperwork I got. Think I'll go back to factory base and start there, 1kt head, 47X gasket , the japs knew what they were doing

I guess they knew...ask some guys from Pearl Harbor...
I ride some heads also with that height,not a real problem if your squish and compression is ok. and i got also an head with no lip around the combustion chamber,so it was an usual tuning-mod i think... but dont like it in that way. i machine this lip in every head with the right measurement

[mboddy]

Isn't the 47X gasket thicker?

[problemchild]

Isn't the 47X gasket thicker?

It could be a doublelayer, so 0,6mm maybe. Is usable but keep an eye on the Squish. Its a good option for Heads that are skimmed too much.

[Robbi347]

Isn't the 47X gasket thicker?

It could be a doublelayer, so 0,6mm maybe. Is usable but keep an eye on the Squish. Its a good option for Heads that are skimmed too much.

47X gasket is measuring at 0.7mm, so with the skimmed head on I'm getting about 0.7 squish. I've rebored and plated the cylinder to take standard 2ma pistons which are slightly taller than 2xt ones so I think I'll need to get a 1kt head and see what squish that gives me

[James P]

If it is of any use here, I think the 1KT and 2XT cylinder heads are the same - I would suggest that the part numbers are different because the 2XT head was painted a different colour.

As far as I have been able to tell, the 2XT was restricted by way of 'reducers' in the exhaust header sections and also by retarded ignition timing. It would therefore not be surprising that the secondary compression ratio was also reduced by the fitting of a thicker head gasket. If you (Robbi) want to derestrict your 2XT, you could possibly start by fitting standard 1KT exhausts, converting 'back' to a 1KT ignition system and using a 1KT head gasket with a standard head! Alternatively, you could use a 47X head gasket with your skimmed head (did you say that this resulted in 0.7mm squish clearance?).

I think the dome profiles are the same for 1KT and 2XT pistons (Mark B; can you confirm?), the only real difference being the half-size reliefs around the pin on the 2XT pistons. I wasn't aware that the compression height of a 2XT piston was/is any different to a 1KT piston, but will defer to anyone who has actually compared them. In the absence of any definite information, I daresay that the squish clearance will end up rather large if using any combination of standard parts, but it depends how much work one wants to do to reduce it yet still keep the compression ratio sensible.

The squish clearance increases towards the centre of the bore when using standard pistons and head because the squish band is cast steeper than the piston dome. I have never personally inspected a Formula 3 head, but I know that special pistons were supplied with/for the kit. It could be that the piston domes and squish bands of the F3 parts were more closely matched and using these parts together may have resulted in a smaller squish clearance, along with a compression ratio suitable/safe for racing while using special high-octane fuel (there is also some ignition adjustment available when using the F3 flywheel-magneto assembly).

I (and perhaps others) would be glad to know more about 1KT-2XT differences, as well as related features of F3 kit parts - please let us know!

Thanks & regards,
James