Crack Head

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Nigel888

Nigel888

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34
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Kent
:helpsmilie:

Just stripped the motor to find it has cracked from carb port to the outside near head bolt.

Has anyone experienced this?
The head is a CY27 bored to 29cc (36mm piston) and ported, running a 2mm stroker stuffed crank.

What now?

I have in my collection for the rebuild the standard 26cc Zenoah head, a 23 with piston, standard crank.

Advice and opinions much appreciated. :scooter:
 

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I'm thinking the ring may have run off the assister and impacted the port (you can see the wear on the edge), stressing the head and eventually cracking it, as I know it ran with stacked gaskets.
 
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...

figured it was dead ... going to copy the porting onto the original 35mm ... see how it goes and maybe grab another 36mm head
 
junk it. a few things to note. the 35mm bore cy cylinders are cast, then milled, so welding it would be a trick at best, if it could be done at all. Cast alum don't weld for poop.

2nd you say its a 27cc thats been milled to a 29cc. so am i getting this right that the 35mm head was bored out 1mm to fit the 36mm piston you posted was installed? if so what did you do to re-plate the cylinder? or do you mean it was a 27cc (35mm bore) that has a 2mm stroker installed? this is how i expect it would of been done, as boreing even 1mm out of the cylinder will certainly weaken the cylinder and possibly cause the head to break off exactly as it has.

imho, i hate the 35mm bore cys. they are a cheap and weak part prone to early failure. if not from the cylinder plating flaking off early, from the high compression that tends to break cylinders off.


put the Zen 26 top end on. but put new crank bearings in the case.
 
Nigel's head was an O'Neils 30.5cc kit so I assume it was based on the GP290 head (36mm) and had the +2mm stroker crank fitted. :)
 
Rebuilding ...

Ok .. so all that snow outside and no car :no: had to do something quickly

Original 26cc Zenoah head, piston, pin etc refitted with stacked gaskets to give correct clearance, retaining 2mm stroker.

Just need to fit carb, replace upper rear wishbones and bleed up rear brakes.

I'm leaving my tall gearing but I may have to swap back if it bogs down.

Nige
 
oh thats a bummer. was it cut for one gasket and proper compression ratio? that head in stock form is around 18:1 compression, which is pretty high. 14:`
 
oh thats a bummer. was it cut for one gasket and proper compression ratio? that head in stock form is around 18:1 compression, which is pretty high. 14:1 is about where it should be?

just watch your coil height too. you may need to oval the coil mounts to get it to gap correctly. sometimes you can get away with it.
 
Timmahh said:
oh thats a bummer. was it cut for one gasket and proper compression ratio? that head in stock form is around 18:1 compression, which is pretty high. 14:1 is about where it should be?

just watch your coil height too. you may need to oval the coil mounts to get it to gap correctly. sometimes you can get away with it.
Click to expand...

We stacked the gaskets to give a squish of .3mm ...

I think it may be where I had to helicoil new treads into the carb mount ... maybe the insert grew with heat ???

Think I'll get a proper .4mm copper gasket with the new head/piston.
:oops:
 
Timmahh said:
oh thats a bummer. was it cut for one gasket and proper compression ratio? that head in stock form is around 18:1 compression, which is pretty high. 14:`
Click to expand...

Is that compression ratio only from the billet head or do all of the gp heads have it that high? Or am I misunderstanding something?

Thanks Timm,
Trip..
 
depends really on what works been done

from my understanding the stock GP290 motor is pushing 17.5:1.
the cy29rc (non headkit) is around 18:1 and the cy29 (headkit) is around 15:1.


some of the modified topends will get a corrected squish band, and the better builders also correct the compression ratio around 14.5:1. on the GP290s, and if done on the cy29rcs and the cy29s (if its been modified at all, this should be a number one modification even befor porting imho) they would only have the poor cylinder plateing to contend with. not to mention, lower compression should lead to to less broken heads like we see above.

and speaking of lower compression, this would also lower the running temps of those motors, and probably lead to less plateing issues as well.
 
With the 29CC billet head kit..you can get different alloy head domes...from DDM
I use a 14:1 alloy heard dome my self...With all other things being done to bottom end
with Zanaon bearings...only real issue would be cylinder for the plating...But I have had on issues as of yet with that...Maybe due to lower compression...Only real issue I have is getting the colour for spark plug..being brown.....I suspect carb is worn some and sucking in air form the shaft....AKASlawhammer
 
What color is your plug? I have never had a "Brown" plug tip in any of my 2-strokes... Mine are always a little black.. lol I figure if it's running well and isn't running lean, I am a happy boy..

Trip..

P.S. Try the ole WD trick around the carb and gaskets...
 
lean it out to much and your black plug electrode will go white. lol

black just says your on the rich side. medium brown (like a hersey bar when broken open ) is a sign of a good tune. light brown/dark tan (carmell in color) is a sign of a performance tuned motor, and grey is lean, and white is too lean, near siezeing.
 
Sean Oniell race ported head 30.5cc

I just had a new head from Oniell crack on the exhaust side. He must have some bad head's! He is sending a new one over night that was on Monday and as of today 1week later NO Head. I guess you get what pay for!! I had one other order with them and it too was very slow over 2weeks!! I think DDM has much better service.
 
i am very very happy with my o neills 27.2 if a little hard to tune though at times,otherwise a ripper.....

and it has ran a lot of vids for me so far without any bothers!!
 
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