What set up do you think this is?????

[wes350x]

See what you guys think. I was curious of how my motor was bulit so I pulled off the head. It has a ton of compression and wont run on pump gas. Weird thing is I have a 22cc dome in my cool head. I tried to get the best pics I can. Tell me what you know from these pics cause Im still learning on these weird set ups. I know I shoulda done a compression test but I will when its back together.

[wes350x]

SIDE VIEW

[wes350x]

SPACER PLATE

[312R1]

Looks like 265cc with a cr head gasket to me.

[LONG-ROD]

looks like that piston comes up a little far, there is like 0 squish. maybe thats how it should be. i would say that spacer is a little thin, no telling how many times that cylinder has been decked.

[wes350x]

The piston actually comes up a little higher, just couldnt hold it at tdc. Is this a weird set up? Can I re-use the head gasket?

[LONG-ROD]

deffintly do not reuse the head gasket, put a new one on every time that head comes off, and I would use some copper spray on both sides to make a nice seal. That black stuff looks like dirt on the piston are you keeping that filter clean.

[wes350x]

Thought the gunk on top was carbon. Yea, I keep my filter clean (so I think). What kinda gasket should I order? Any cr 250 gasket. Man I dont know much about this thing yet. When you say decking, what exactly is that.

[LONG-ROD]

From what I have been told over the years, black stuff on a piston is dirt, not carbon. I could be wrong. I run the stock gasket, I would think that would lower your compression a bit. decking a cylinder is when they machine it so it is perfectly flat. does your bike run good? It looks like it has a few rides on that piston. I would just ride it if everything is is good then when its time for a rebuild do reseach to find out what kinda build you want to do. I am sending my next top end to Neil at C -Leigh raceing. I have heard really good things about him, and he gave me some advise when I rebuilt my lower end, that with out out would have resulted in instant chaous.

[wes350x]

It ran great, Im still playing with the jetting though. I have a stock head gasket now, would you think Id lose much power if I use it. Correct me if im wrong but isnt it on its last bore? The guy I bought it from last year said it had a YZ piston.

[LONG-ROD]

Ok, time for Neil to chime in. It looks like it is on it's last, I would no go any farther. I am not 100% but a yz piston rather connects to the rod at a diffrent point, or is shorter. I beleive you can get more displacement out of a yz piston set up. I wish you did a compression test. the more I think on it, the piston comeing up like that dose'nt seem to odd. I take it you are running a long rod crank. hum this just dont add up to me why you would have such high compression with a 22 cc dome, if my thoughts on the yz set up are correct. some one will chime in and peice the puzzle together.

[wes350x]

Thanks for all the replys LONG ROD. Hope Neil checks this post.

[rustyATV]

The black stuff us carbon buildup, nothing more. Dirt will show up on the face of the cylinder in the form of scratches. Looking at the top of the piston, the cylinder might have a decent port job on it; it's balanced and I don't see any "hooks"

Yeah, that's a CR head gasket. The ATC and TRX head gaskets have a thin metal spacer between two metal films that are like the CR gasket.

I forget the details of the YZ setup, like what year piston and what displacement it takes you up to (it's close to 265cc's, if not right on), but it required the thickness base plate you have. The early TRX piston on the late TRX crank uses a 5mm base plate, which is much thicker than that.

Can't decide if you've got OEM base gaskets. They're green like OEM's, but they look thinner.

You've got a few options to reduce your compression.

If your base gaskets are thinner aftermarket, then you can switch to OEM's, which are about 0.020in thick (I've got 0.015in gaskets in mine).

Also, you can use either an ATC or TRX head gasket. Both are MUCH thicker than the CR250, with the ATC being slightly thinner. You'll need to slightly enlarge the inside diameter of the gasket to clear the piston, though.

[wes350x]

Originally posted by rustyATV

You've got a few options to reduce your compression.

If your base gaskets are thinner aftermarket, then you can switch to OEM's, which are about 0.020in thick (I've got 0.015in gaskets in mine).

Also, you can use either an ATC or TRX head gasket. Both are MUCH thicker than the CR250, with the ATC being slightly thinner. You'll need to slightly enlarge the inside diameter of the gasket to clear the piston, though.

Im really not too concerned about lowering compression, I can get race gas pretty easy. It seems to be very reliable how it was. I dont want to lose power. But I opened it up today just cause my curiosity got the best of me. Now it make me think of the future to do for a rebuild.

[rustyATV]

Well, realize that you also have a little tuning opportunity in the base gaskets. Changing the gasket thickness raises and lowers the cylinder also raises and lowers the port height, which in turn (slightly) raises and lowers the engine's power characteristics.

If you do that, then you'll want to change your head gaskets to raise or lower your compression accordingly.

Something to think about.

[matt250r21]

87 YZ250 was the piston used and the spacer plate should be 2mm unless the cylinder was decked. Your cylinder head should also be cut to allow for the piston's position at TDC. On my setup, I run a 01 CR250 piston with a 3mm spacer plate and the top of the piston sticks out of the cylinder at TDC just like yours dose.

[1promodfan]

If I'm not mistaken.....(which I probably am).....I think the YZ piston is as far as can go on a stock cylinder. I think it makes it a 74mm bore.....something like 73.95....so like I said a 74mm. I was at Neil house a few months ago, and thats what he was telling me. It may be time for a re-sleeve. Well, time for me to shut-up and Neil to chime in.

[C-LEIGH RACING]

I'll see if I can help, but please, dont nobody body slam me cause I'm gitting old & cant take much abuse like I youst to could.

The pic of the piston, is that as far as it will come up to TDC ??.
Looks like it is down in the cylinder just a bit which would be normal because of the ignition magnets pulling on the crank at TDC.

The piston has 100 stamped on it & no other numbers,, right,, other than a pointer on the exhaust side.
By it having 100 stamped on it, that would indicate that it is a cast piston & 1mm oversize. Wiseco would have the part number etched into the top.

Now, looking at the spacer plate, it being thin as it is, would indicate the piston to be a YZ Yamaha piston 86/87 model because of where the wrist pin hole is located in it & 1mm oversize for that year model would be a 69mm.
The YZ piston on a long rod 250R crank, would need a 2.5mm spacer plate & 2 base gaskets to make the pistons timming edge come flat with the cylinder deck.
Could very well be though, a 2mm spacer plate if the deck of the cylinder has been machined down & by the looks of the scratches, it has been touched, but no idea how much has been removed.

Far as the compression, even though the head dome is 22cc, kinda indicating to me the port work is low, say like a bottom end port job & your trapping a lot of volume from the roof of the exhaust port up to TDC which would explain the high compression & + you have a CR head gasket.
An engine build like you have, is normal to have high compression because its was built like that for a special type of racing.

I can tell you this, that engine you have, if it was in rustyatvs bike & he was running a GNCC, it would trip his trigger but he wouldnt like it because of having to use race gas.

When you get ready to put it back together, find somewhere to get you a tube of Three Bond, #1211 sealant, clean the thin head gasket good & mop on a coat & use it over again. I've done it many times on the metal ones & unless you have bent it real bad, it can be reused.

The piston you have now, if its a Pro-x brand, it is a part number 2645, .40 oversize, which you might not be able to find anymore from LA Sleeve, but maybe at some supplier on their shelfs & still instock. Because of the 100 marking on top, it might could be an OEM Yamaha piston, but doughtfull because builders dont normally use them.
The Wiseco number, if you wanted to get one of those, 560MO6900 for the same size you have right now.
Pistons for the 86/87 model YZ250 Yamaha go up to the 70.00mm size, so you have 4 more piston sizes you can bore to before having to replace the sleeve.

Give you a little tip, on an engine with high compression, real high compression & using cast pistons, be aware of piston cracking from the wrist pin hole up to the dome.
I'm not saying they cant be used, but that it can happen.
Neil

[wes350x]

WOW, thanks Neil. When Its time for a rebuild, what piston do you suggest?

[C-LEIGH RACING]

Wiseco or either a Wossnor, if they have a Wossnor in that size.
Neil

[rustyATV]

Originally posted by C-LEIGH RACING
I can tell you this, that engine you have, if it was in rustyatvs bike & he was running a GNCC, it would trip his trigger but he wouldnt like it because of having to use race gas.
Neil

But......I use 110 now......

Of course, I decided not to go back to racing after I built it. :rolleyes:

[wes350x]

Originally posted by C-LEIGH RACING
Wiseco or either a Wossnor, if they have a Wossnor in that size.
Neil

Would the pistons have a dome like the one now? or would I have to do some new configurations to make the proper compression?

[C-LEIGH RACING]

Yes, it will be domed just like the one you have in the engine now.
Most all 2 stroke pistons will have a domed crown.
There are a few though that are flat top, like the 97/98 model CR250R.
Neil

[superevil]

wossner or however you spell it makes good stuff!!

[wes350x]

I finally bought a caliper so I took some measurements. I thins these are as accurate as I can get.

Cylinder (inside wall to inside wall)= 68.50mm

Bottom space plate= 2.5mm

[wes350x]

Here is some pics of the port work, What can you guys see?

[wes350x]

another

[C-LEIGH RACING]

:eek: :D :D :p I can see it looks fair to partly clowdy :cool:
Neil