broken ring 79 rude 100 hp

[beach]

Purchased motor found cracked ring, minimal scoring. What causes a ring to crack and is a complete rebuild necessary?

 

[Hooty]

Re: broken ring 79 rude 100 hp

Most probably, the engine was not decarboned and the rings got coked up. Carbon builds up in the ring groves behind the rings which pushes 'em out, causing them to snag the exhaust port. A complete engine rebuild is not necessary. Just repair the damaged cylinder. Bore and new piston if needed. Check the rest of the cylinders and if they're in spec, hone 'em and rering all.<br /><br />c/6<br />Hooty

 

[Dhadley]

Re: broken ring 79 rude 100 hp

Coking is mostly caused by excess compustion heat. Most of the time we find broken rings on motors that are lugged (not capeable of getting to 5800-5900 with an average load). With the added combustion heat its not unusual to find the other cylinders out of spec once you measure them. <br /><br />The broken ring is an effect. Find and fix the cause.<br /><br />Bet it happened on the starboard side.

 

[Chinewalker]

Re: broken ring 79 rude 100 hp

Wasn't '79 the last year of the high ring pistons? Might have been a contributing factor as well... It was a design used in the late 1970s that proved to have inherent flaws...<br />- Scott

 

[Dhadley]

Re: broken ring 79 rude 100 hp

Yes, it probably was built with the high ring pistons. However, if thats whats still in there it shows that the ring placement was probably not a design flaw. Pistons were built for many years before like that with no problems. <br /><br />What changed? The lead was taken out of the fuel. OMC motors were typicaly propped out in the low 5000 range at that time. With the removal of the lead the combustion temps came up. The engineers came up with the "low lead" kits and updates. They lowered the ring placement. <br /><br />About that time we started to find out that setting up the motors to run "freer" let the combustion temps came down. We ran tests on both style pistons and found out that without lead as the "cushion" rpm played a greater role than was believed before. Lug either style and the rings will coke up and eventually break. <br /><br />Those high ring pistons were at a premium for a while. All the race guys wanted them.

 

[beach]

Re: broken ring 79 rude 100 hp

What should I look at to determine the cause of this?

 

[Dhadley]

Re: broken ring 79 rude 100 hp

Tell us more about the set up and how its run. Whats the top rpm with an average load? If it hits 5500 with one person and a light fuel load then when we add the family and coolers and fuel and.....<br /><br />What grade of fuel? Is it always fresh or does it sit in the tank for a month (or more)? What oil? What spark plugs?<br /><br />Whats the water pressure? How are the rubber deflectors on the side that broke the ring? Has the motor ever been hot? How were the head gaskets?<br /><br />I know, I know. Too many questions. Just tell us what you can and we'll help as you disassemble the motor. If you're going to do it yourself, get an OE manual. You'll thank yourself later. This board is great but never let it take the place of a factory service manual.<br /><br />When you get the pistons out, let us know. There are a couple of things to look at there too. It would help if we stick with this thread -- just add to it.<br /><br />Good luck and keep us posted!

 

[beach]

Re: broken ring 79 rude 100 hp

I didnt want to run it with a bad ring so I have no data as far as rpms and such. I did get all 3 manuals for this specific motor, owners, repair and parts. I will be doing the work myself mostly.