My buddy wants to buy a 400ex and he found one a guy is selling for 800 bucks. He says the bike is in good condition accept that it blows head gaskets. The guy says a mechanic bored it .030 over and ever since its been doing it. He claims he can ride it fine for a day. Then if he lets it sit and tries to start it again the head gasket will be blown. Any ideas for why this is happening. Is it possible the top machine is actually making too much compression and its blowing the head gasket? What if its a high comp. piston and hes running the wrong fuel? Thanks for any help!

 

The cylinder head is probably out of tolerance on flatness of the mating surface.

 

800 bucks would be a steal. If you think the head gasket is blown, take a compression test and if the results are out of spec; heres how to fix the problem and NEVER have it come back.

Tear down the entire top end
Have the head and cylinder milled
REPLACE THE STUDS (you wont save any money by skipping this step because your head gasket will probably blow again)
Use a brand new OEM Honda head gasket. Why use an OEM gasket? They're solid metal and the aftermarkets are made of several thin plies which blow out extremely easily

Take what I say to heart as I replaced the head gasket 4 times on my old 440EX before i figured out how to do it right. The main thing i have to stress is that you dont skip any steps. It's a little more money up front but it pays off in the long run. Feel free to message me if you have any questions. I built 3 440ex's and I know these machines like the back of my hand.

 

well, worse case scenario is that you'd need a stock piston and cylinder. I'm sure you could find parts for it pretty easy. Just have the head and cylinder checked for warping and go from there.

 

Whats been said is good advice.

1) The first thing I would think about is octane since its the easiest. If he had it bored over, did he get a new piston? And if so, what compression? A lot of guys will go out and get a high compression piston and then dont provide enough octane to keep her together - there is pre-ignition and it blows the head gasket.

2) if the bore is out of spec (as already mentioned) that too can be a factor.

The EX is pretty prone to blowing headgaskets if everything isnt right. Yes, bigger stay bolts are nice to have, but not nessesary in my opinion if everything is right. Then again, a nice safety margin is a good thing too.

I've had my 440 EX with stock stay bolts for nearly 2 years. However, when I went with a stroker and NOS, I did end up getting the larger stay bolts just incase I didnt give her enough octane. I currently have a JE 12:1 piston and I run VP C12 for fuel.

Marky

 

What’s your most recommended way to build a 440 on a budget? I bought the quad with one on there but the threads were all stripped out and bolts snapped, I wanna build it back to 440 just don’t know the best way for a budget