Full Tilt CR250 Build - MXA Two Stroke Hall of Fame

Olson
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Olson 12/26/2015 2:59pm
12/26/2015 2:59pm
I hate my own CR250 now. It looks like trash compared to yours. Sad
Awesome Johnson 12/26/2015 3:12pm
12/26/2015 3:12pm
Reefer, how was the watts built 02 cr in regards to low-mid-high end power. Was it gobs of power all around or mild to warp speed type thing?

Black, I can't wait to see this finished but at the same time totally digging every post. I got a 2007 cr250 that is due for a tear down refresh and a lot of what I've seen so far will make its way on my bike. Thanks for this killer build thread!
40
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40 12/26/2015 3:38pm
12/26/2015 3:38pm
Reefer is correct about the durations: Honda opened the main and auxiliary transfers and the boost port simultaneously. The main transfers have a somewhat steep roof angle of approx 23 degrees, with the aux transfer roofs approx 15 degrees.
The (arguably) most poorly engineered aspect on the '05-'07 variant is the head design. The squish measures a very large 0.084", but worse, has a negative taper of approx 0.0015" at the cylinder wall. This might have been an attempt at better ring sealing since they went to a single ring, dome top piston design on the '05. But, more realistically the negative taper trapped some end gasses at the cylinder wall which promoted detonation. Correcting the squish gap, width, and taper angle are important.
The crank is a different story. Honda went to a solid crank design on the later years which was a good improvement in durability over the older "tin can" cranks.
2
DrSweden
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DrSweden 12/26/2015 4:05pm
12/26/2015 4:05pm
HOLY BATMAN!!! This thread delivers. Amazing build!!!

The Shop

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blackdiamond
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blackdiamond 12/26/2015 7:36pm
12/26/2015 7:36pm
Here are some of the pics of the bottom end assembly process. For some reason the cases were mismatched by a few thousandths. I really haven't had this problem on Hondas but this one needed the cases blueprinted before we could move forward. You can see the case matching work in this pic as well.



After the cases were anodized I epoxied the holes for the serial plate rivets just to be safe.





Billet clutch basket assembled and ready to go.









The crank has very few hours on it but had been sitting for years and had rusted a bit so it was sent to Crankworks for a rebuild and balance using a Wossner connecting rod. The rod is about 10 grams lighter than OEM.



JBernard_401
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JBernard_401 12/26/2015 7:38pm
12/26/2015 7:38pm
howd you anodize the cases with all the steel bearing races pressed in? Did you mask off the gasket surfaces as well? or machine it a hair after getting it back?
blackdiamond
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blackdiamond 12/26/2015 8:24pm
12/26/2015 8:24pm
The cases were bare and all of the bearing surfaces were masked when they were anodized. The center cases were lapped before assembly as well.
JBernard_401
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JBernard_401 12/26/2015 8:26pm
12/26/2015 8:26pm
mind me asking how much they charged to hard ano those 2 pieces. and how much of the cost was added masking labor.
thats surprising, i didnt know they could mask that precisely. They normally use polyamaride tape which will bleed under the edge a little. The cost of it bleeding under and ruining a bearing race would be pretty risky.
DPR250R
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DPR250R 12/26/2015 8:29pm
12/26/2015 8:29pm
I think I need to sell my tools.
blackdiamond
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blackdiamond 12/26/2015 8:36pm
12/26/2015 8:36pm
I used Temecula Quality Plating, they are very reasonable. If I remember correctly it was $75 minimum batch charge for the anno and $25 to mask the bearing races. They did the nickel on the brake tip and footpeg mounts as well. Duke is really good to work with over there.
smezmx
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smezmx 12/27/2015 4:45am
12/27/2015 4:45am Edited Date/Time 12/27/2015 4:46am
Think You better get her one of these.Laughing



354racing
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354racing 12/27/2015 7:37am
12/27/2015 7:37am

This is an AWSOME build! When that OCD kicks in detail attention goes out the roof!
blackdiamond
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blackdiamond 12/27/2015 9:41am
12/27/2015 9:41am
smezmx wrote:
Think You better get her one of these.:laugh: [img]https://p.vitalmx.com/photos/forums/2015/12/27/117006/s1200_image.jpg[/img]
Think You better get her one of these.Laughing



They should have invented these things years ago, could have saved thousands of relationships worldwide!
Moto520
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Moto520 12/27/2015 11:41am
12/27/2015 11:41am
Very cool build. I love following these.
Mit12
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Mit12 12/27/2015 12:11pm
12/27/2015 12:11pm
Your attention to detail is amazing. Great build. I was looking at your bell crank and was wondering if you considered putting a hole in the web? Would not take much weight off but would look cool.
blackdiamond
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blackdiamond 12/27/2015 12:37pm
12/27/2015 12:37pm
Mit12 wrote:
Your attention to detail is amazing. Great build. I was looking at your bell crank and was wondering if you considered putting a hole in the...
Your attention to detail is amazing. Great build. I was looking at your bell crank and was wondering if you considered putting a hole in the web? Would not take much weight off but would look cool.
I actually considered it, I had done that on another build a few years ago and it did look pretty cool.
philG
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philG 12/27/2015 1:01pm
12/27/2015 1:01pm
We started making a billet rocker for our MXV SM bike to alter the centres , wasnt going to be pretty, just functional until we found what we were after, and then we would have tidied it up a bit.. my sponsor was a hole freak.. nothing loses weight quicker than holes.

Sadly while the bike was epic from a performance point of view, and weighed in at 112kg , it didnt have the level of perfection you have . If i could actually use the stuff we have at work to get stuff done, id have a killer bike .
1
bonseff
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bonseff 12/27/2015 1:22pm
12/27/2015 1:22pm
Olson wrote:
I hate my own CR250 now. It looks like trash compared to yours. Sad
yea.

this is level 18,000, dude

we may need to get this dude a hooker when ts all said and done.

Pit Row

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MANNERS226 12/27/2015 1:46pm
12/27/2015 1:46pm
Very impressive build. Keep the photos coming!
sesker15
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sesker15 12/27/2015 2:44pm
12/27/2015 2:44pm
sesker15
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sesker15 12/27/2015 2:45pm
12/27/2015 2:45pm
Very Nice. Factory two stroke!
RMRider1
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RMRider1 12/27/2015 4:32pm
12/27/2015 4:32pm Edited Date/Time 12/27/2015 4:33pm
40 wrote:
Reefer is correct about the durations: Honda opened the main and auxiliary transfers and the boost port simultaneously. The main transfers have a somewhat steep roof...
Reefer is correct about the durations: Honda opened the main and auxiliary transfers and the boost port simultaneously. The main transfers have a somewhat steep roof angle of approx 23 degrees, with the aux transfer roofs approx 15 degrees.
The (arguably) most poorly engineered aspect on the '05-'07 variant is the head design. The squish measures a very large 0.084", but worse, has a negative taper of approx 0.0015" at the cylinder wall. This might have been an attempt at better ring sealing since they went to a single ring, dome top piston design on the '05. But, more realistically the negative taper trapped some end gasses at the cylinder wall which promoted detonation. Correcting the squish gap, width, and taper angle are important.
The crank is a different story. Honda went to a solid crank design on the later years which was a good improvement in durability over the older "tin can" cranks.
Interesting 40.... I sent my cylinder and head to Eric Gorr last year specifying I will run race gas and this is what he ended up doing to my head... Does this correct what you are referring to? Bike is an '06 CR 250...

Excellent work Blackdiamond!


40
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40 12/27/2015 7:02pm
12/27/2015 7:02pm
That has corrected the negative taper. It would be interesting to hear what squish clearance Eric sets up with a race fuel head.
As previous, there are a lot of variables that go into a head design. The width of the squish band as a percentage of combustion chamber area, the squish clearance, squish taper angle, dome shape and volume are all important factors.
As an example, RB Designs sets up his squish clearance in the 0.045" neighborhood for a CR250 and uses a wider squish band than Eric. While Pro Circuit does so much machining to the upper dome area that a spark plug spacer is required while using a larger squish clearance than RB. Lots of variables and different approaches.
blackdiamond
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blackdiamond 12/27/2015 7:18pm
12/27/2015 7:18pm
This is what Varner's spec looks like on this bike.





Mit12
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Mit12 12/27/2015 7:43pm
12/27/2015 7:43pm Edited Date/Time 12/27/2015 7:43pm
I do not understand why they would have a flat spot on the out side of the head and the squish inside with a domed piston. The picture of the Varner head is a way more conventional design. Yamaha did testing on heads and squish clearance. What they found is they will keep making more power when you tighten up the squish clearance all the way until the piston hits the head. Squish area, angle and radious all make a big difference in power and RPM. It all comes down to proper squish velocity for the RPM you are ported for.
offthechizzain 12/27/2015 7:57pm
12/27/2015 7:57pm
Chad, this bike is out of bounds, puts mine and Moto120's to shame!!!! man you really did a nice job, looks like Pat over at RCR transmission took care of you on the tranny! Nice job!
TxT0RKiT
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TxT0RKiT 12/27/2015 7:57pm
12/27/2015 7:57pm
Best build thread I have seen. Quality every step of the way.
blackdiamond
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blackdiamond 12/27/2015 8:25pm
12/27/2015 8:25pm
I appreciate all the compliments fellas, I have a lot of blood sweat and tears in this one so I am glad all the 2 smoke die-hards dig the thread. It's too bad Honda couldn't just step up and keep developing the smokers so we could all just buy one like this off the showroom floor (or at lest something close to it)
slipdog
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slipdog 12/27/2015 8:50pm
12/27/2015 8:50pm
Mit12 wrote:
I do not understand why they would have a flat spot on the out side of the head and the squish inside with a domed piston...
I do not understand why they would have a flat spot on the out side of the head and the squish inside with a domed piston. The picture of the Varner head is a way more conventional design. Yamaha did testing on heads and squish clearance. What they found is they will keep making more power when you tighten up the squish clearance all the way until the piston hits the head. Squish area, angle and radious all make a big difference in power and RPM. It all comes down to proper squish velocity for the RPM you are ported for.
Assuming you're talking the Eric Gorr head, the flat spot on the outside is the squish band. The second cut is the angle he used blend the new squish band into the dome and get volume(cc's) back after decking the cylinder
philG
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philG 12/28/2015 5:20am
12/28/2015 5:20am
I know my sponsor is hoping to get Eric Gorr to do a cylinder for our TM , to mate with the VHM heads we have , be good to see what he can do for us.

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