CR to ATC cylinder

[KASEY]

today i picked up a 1985 CR250 cylinder and head,, its a direct bolt on for a ATC ,, it had a ATAC chamber that bolts on the front of it,,, so you can use it without,,, i hope it has a few more hp than a stock cylinder,,, i will see,, its still stock bore too and in nice shape,,, i think i will put it on the 85 later ,,,

[tecat-z]

Yep, i heard a few times they were a direct fit, and make a huge difference. Improved cooling and more agressive timings. Should have a big hit in the middle and pull very strong til max engine speed. No compromise design, unlike atv detuned cylinders of all makes. Nows the time to think about a lightened fly. It will let it rev like it's suppost to. FAST. Since the atc engine already has more mass to get moving. Please keep us posted.

[Billy Golightly]

Thats what Mike Coe said they ran on their Rs before Paul Turner started working on them and they hauled ass.

[MTS]

Anyone know if the pro-x jugs fit on say a 90 cr250?

[jeffatc250r]

Will an atc exhaust flange bolt up to the cr jug? Might also be worth the effort to try and mod an old beat up pipe and run the valve? Im very interested to hear your test results.

[EAT_A_450]

Ok just ran across this thread and I', thinking o trying the same thing but I have an 84' cr jug. Have any luck? Does the exhaust bolt right on? I have a cool head and if I do this setup I want to keep my dome piston and head combo.

[3WW Star-CoeShow]

I'm replying to this post but my memory may have faded some so don't take this as gospel..... The ATC and CR250 exhaust flange use the same bolt pattern, but the flange for the CR250 is much larger than the ATC flange.

A larger spigot would have to be welded to your existing pipe to use the CR flange on your ATC. This was a common modification regardless of cylinder choice. If ATC jugs were used, the CR flange was installed and the exhaust port was modified to blend into the larger flange.

Good luck and ride safely.

[KASEY]

since the atac valve is removed the bolt patern is way different ,,, its not even close . there are 4 bolt holes in the cr cylinder, and one is to close to the edge to use too,,, i redrilled a billet flange and got 2 bolts to work , the inside diameter of the flange is within thousandths of being the same size as the port,, which is larger than a atc cylinder by quite a bit,, its even larger than a pro-x 310 i have too,,,,,,,,,

[EAT_A_450]

ok, how does the cylinder compare to a good mx ported 250r cylinder? From what I was told by a guy which I lost contact with is that it was way faster than any ported jug that he ever came accross in the dunes but how does it perform on bottom end- woods? Also I know the atac on the 84& 85 are a little different but they should bolt up the same- will either work?

[EAT_A_450]

ok, how does the cylinder compare to a good mx ported 250r cylinder? From what I was told by a guy which I lost contact with is that it was way faster than any ported jug that he ever came accross in the dunes but how does it perform on bottom end- woods? Also I know the atac on the 84& 85 are a little different but they should bolt up the same- will either work?

[dansvan]

Here's a few pics of the exhaust ports side by sides. This is the ATAC cylinder minus the powervalve.




ATC flange gasket on ATAC cylinder.

[Andrew_250R]

I might have missed it but what does the CR250 cylinder have over the ATC 250R cylinder that gives the engine more power? And I am sure this is a total noob question but what is the power valve that you guys are talking about?

[Louis Mielke]

No. The power comes from the different porting. Maybe port timing as well but I don't know this for sure.

[cr480r]

Has anyone tried an '83 cylinder? does it share the same case stud location? The exhaust flange matches Coes description alot better than the others do..