Topic: Is a 550 a "clearance" or "interference" valve-piston design

[Kevin400F]

Is a 550 a "clearance" design engine, such that when it has a cam chain failure (as did the engine I'm currently working on), the engine can still turn through without pistons hitting valves?  I've got the head off and don't see any evidence of contact, which really surprises me.   I've heard some engines are designed to allow this, but find it unlikely in this case with flat-top pistons. 

[mack808]

i don't believe you will find a 4-cycle m/c engine that is not a interference fit. I have seen a young "newbie" disconnect the cam chain and while being told not to turn over engine (on the bench), bend a exhaust valve with nothing but a wrench on the stator bolt..

[bryanj]

If the chain broke at least two valves are bent. Pour kerosene into the ports and see what happens

[Kevin400F]

All the valves are out on the bench and #3 intake is bent for sure.  From the lobe orientations, it looks like #1 and/or #2 exhaust would be the next most likely to hit, but they look OK while turning them against their seats (with the springs out).  I'll clean them up and check them with a dab of machinists blue.

A couple of the lobes and rockers look a little rough, so when I dive into another "donor" engine here for a valve, I'll evaluate the cam and rockers.  Hopefully I can swap all the necessary parts from the other lump (which has a bad third gear in the trans).

Kevin

[Tim.]

The 500/550 pistons have indentations on the tops for the valves, so I would summise that a thrown cam chain = big time carnage.

[Kevin400F]

Looking at the pistons & valves, I have pretty much reached the conclusion that the disintegrating front cam chain guide somehow bound the chain and caused the breakage when the engine was being kicked over or turned by a wrench.   The chain was sitting neatly in place on the cam, and was broken at the crank sprocket.  Only the one valve was bent.  If the engine had been running at the time, there would have been more carnage for sure. 

A couple of cam lobes and rocker followers have some rough spots, so I'll be evaluating parts from the donor engine when I tear it down to rob the valve.

Kevin

[bryanj]

Its unusual for that guide to break up, if i was you i would replace both that and the tensioner blade with new--its cheaper than doing the job again with a new gasket set!