crf 450 head explosion

i bought a crf 450 , while i was driving the engine siezed , after opening it up i found 3 broken valves inside the piston chamber . 3 of them broke of but 1 of them was still in tack . the crank play was perfect the timing chain was undamaged . what caused the valves to break off ? because i dont want to rebuild it for the same to happen trhanks :)

 

Good chance that the cam chain tensioner was the cause in the abrupt stop of your fun. Check the ratchet mechanism of the tensioner these usually fail from factory. what were you doing when it quit? How do you ride? Did you let it warm up?   

Did the cam chain tensioner fail?

i was doing a wheelie and the back wheel locked , the bike was well warmed up , how would i check the cam chain tensioner ? the chain was perfect thanks for the reply 

pease help :) 

Valve could have broken and taken out the other valves, who knows really.

 

For a rebuild, you're going to need to replace it all anyways. New crank, timing chain, tensioner, head, piston and possibly cylinder.

You need a service manual

You need to read all the 'pinned' posts at the top of the forum

You are going to need to replace everything in the head, the cam chain, tensioner, gaskets, and visually inspect the rod  for play.

If any piece of metal is un accounted for, your are going to have to split the cases and verify all metal pieces.

I would NOT go stroker crank, but stay as close to OEM as you can, unless you are an expert with these motors.

Just buy a replacement head with the SS valves already installed from one of a dozen  US companies.

Pull the tensioner out and see if the plunger can be pushed back in. And how do you know the timing chain is ok? Just cause its not broke does not mean its ok. They can stretch and become to long to the point that the cam chain tensioner cant take up all the slack. They where out just like a drive chain. 

Did it damage the cylinder at all?


Are they stock valves?

no the head was rebuided by a motorbike shop , they could of been cheap valves there is a chip out of the cylender the piston is ruined and the head is quite bad too ! i was thinking of getting the cylender re honned and get a new pistin and getting the head repaired . thanks for the replys :)

Consider using the MCCT

Mechanical cam chain tensioner

What he said^ a mcct should be a must for you at this point and the cam chain will look fine... Until you compare it to a new one. a service manual and torque wrench is the best investment you can make doing a rebuild. Goodluck!

Edited by Diggla117

no the head was rebuided by a motorbike shop , they could of been cheap valves there is a chip out of the cylender the piston is ruined and the head is quite bad too ! i was thinking of getting the cylender re honned and get a new pistin and getting the head repaired . thanks for the replys :)

You can't just bore these cylinders out and hone them. You need a new cylinder, a new piston and a new crankshaft with a connecting rod. If you don't replace the crank now, it's going to fail. The cranks on these bikes cannot take abuse.

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