Crack Head

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No not with this head. This was a new head and i was still breaking it in. This head was for 30.5cc so is was machened for 2mm strorker. the next head i put on has run all last year till the chrome started to come off. The temp killed it, it went over 425F the head got full of grass so I was at fault.
 
Oh ok, I saw the second photo. Popped it right off. Ya, I've seen that before. Think it was from too much compression? Do you do your own porting?
 
The second head had no porting it was a stock 36mm. How much compression is too much. I was told the head only had 14 to 1.
 
the GP290 heads stock are around 17.5:1 compression. unless it has stacked gaskets to make it a 30.5, then it could be anywhere from say 10:1 to 20:1 if its to tight and the squish wasnt proplerly measured and set.
 
This was a cut head and i used solder to check the squish it was .035 with a .020 gasket. This was a stroker kit so I don't think they messed up when they cut the head but you never know. I have had 3 heads form O'neill bro's 2 have cracked and the new one he just sent me. I have one from DDM and that is the one i over heated so i needed to replace it. the DDM head was running good even with the chrome comming off the wall's. I think it was bad casting.
 

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Here is a head that lost the electrode off the spark plug. This head still works with a new ring and sand the high spots down.
 

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seems .035 squish (if measured at the cylinder wall) is a bit large. im under the impression compression ratio on a well built motor should be around 14.5:1 corrected, and the squish band should be about .018. im not a motor builder and cant say for sure but these numbers seem to be the average area most the top builders shoot for.

interesting stuff.
 
Well I got 8 tanks of gas on this head before it broke. I talked to Oneill
and they are going to send a new head after they find out why they keep cracking. Well i'll just have to put the old head back on for now.
 

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Bigger the Better said:
Are you sure when you over heated the bottom end didn't warp and the heads are not fitting flush?
Click to expand...

Did not over heat! Where did you get that from? I have a head from DDM that is stock 36mm no porting it is just worn but it the best running so far and it has not broke! I'll be putting that one back on till I get a new one!
 
Hurl said:
No not with this head. This was a new head and i was still breaking it in. This head was for 30.5cc so is was machened for 2mm strorker. the next head i put on has run all last year till the chrome started to come off. The temp killed it, it went over 425F the head got full of grass so I was at fault.
Click to expand...

I think BTB was thinking this...
 
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