• entries
    15
  • comments
    44
  • views
    12,560

So the engine threw a rod, broke the crank, a bearing went out . . . Why?

pflaming

1,447 views

If an engine has run for many an rpm, it will change the insides, most notably the top of the cylinders, for a ring of 'carbon' will build up. This ring reduces the length of the piston's travel. To compensate for that loss several things must happen, the rods must torque a bit, or the bearings must give a bit, or the crankshaft or the piston or some of all of these must compensate for a shorter available stroke.

Eventually, especially when the engine must pull a heavy load, or climb a long hill, or is suddenly called upon to run at significantly higher rpms, or is run hot or on less oil something will give.

Thus the reason to pull the head on an unfamiliar engine and check if there is a ring and remove whatever is there for with that removed, the engine now is able to run it's rpms with full freedom. By assuring that the piston has full range of motion the engine's life is extended.

In like manner the valves, cleaned and seated provides the air flow required thus a cooler temperature and thus less fatigue. Now these improvements do not a new engine make, but they may provide the amount of extended time one needs until a complete build is required or desired.

I find the basics of the mechanical arena as interesting as the infinite details, important as they may be and by knowing and improving the basics I can enjoy this hobby without spending lots of money.

Now if I can refrain from burning down the shop, I may be able to play a bit longer.



  Report Entry


4 Comments


You do get a ridge at the top of a cylinder but that is from wear in the cylinder walls caused by the rings. The ridge is at the top of the stroke & does not shorten the stroke. If it did it would break the top ring. The only way to shorten the stroke is with a different crank or a shorter rod.

66nykr and BigDaddyO like this

Share this comment


Link to comment

You'll notice a black ring around the top of the cylinder. That is carbon buildup. that will extend down as far as the top ring comes up during it's travel. Underneath the carbon, once it's removed, you may still find a ridge. That is wear from the rings, riding iron to iron or moly to iron, what ever the rings are made of. The rings rock just a little in the ring grooves and will wear the sharp edges of the rings down so they and the ridge have a small amount of radius to them. The ridge is removed because the new rings will once again have sharp edges, should fit tighter in the ring grooves and will want to travel those few thousandths that it didn't when they were worn out, thus the potential for broken rings, ring lands on the pistons, scoring of the cylinder walls. Trying to remove pistons with a cylinder ridge is like running the rings up against a wall. They may pass by the ridge with no ill effect or you can break things.

Stuck engines may have a coating of rust on the cylinder wall. When the cylinder travels up, it will scrape some of the rust off. With a ridge, it may trap some of that rust under the ridge, effectively lowering the distance the rings can travel without damage, again, potential for piston and ring damage.

Share this comment


Link to comment
Guest Dumprat

Posted · Report

You will not break a rod from the ridge on the cylinder wall! You break the rod from the bearing seizing on the crank and stuffing the rod into the cylinder wall. If if it is really spectacular it stuffs the rod out through the side of the block!

Share this comment


Link to comment

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now