I threw #1 rod on my Merc 1500 - Why?

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Chris1956

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I was out cruising along near 5000RPM, when I heard a noise, and the motor slowed down dramatically. Turns out the bottom of connecting rod # one made it's exit thru the crancase cover, above # 1 carb. The question is why?

I guess I always thought the piston would seize first, as it has in the past.

BTW this motor spent a whole week at the bottom of the saltwater bay, two years ago. I have run it many times at this RPM since then. I did install a new (used) distributor. Is it possible I fouled up the timing? Would that cause it?

The motor still got me home
 
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Laddies

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Re: I threw #1 rod on my Merc 1500 - Why?

Chris, chances are the engine may have had some etching from the time it was sunk. When you tear it down if it has a broken bolt it may have been over tightened or if the engine has been down some one may have reused a rod nut or bolt but I don't think the trigger caused the problem
 
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Chris1956

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Re: I threw #1 rod on my Merc 1500 - Why?

Laddies, I did notice that one of the bolts that hold the crank side of the conecting rod bearing cap was broken. That piece was in the lower cowl.

I did reuse them, after the teardown (after it sunk) BTW. OK so it was a worn crank bearing that had too much play to stay together? Sounds like a bad country-western song!
 
j_martin

j_martin

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Re: I threw #1 rod on my Merc 1500 - Why?

The rod bolts are stretched upon install. They are a one use item. Re-using it caused the failure.

Hope it helps
John
 
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starcar331

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Re: I threw #1 rod on my Merc 1500 - Why?

i have heard a lot of people talking about a bolt streaching
that is not the case, if you examine the bolt very close you will find the threads are very thin and they bend. the manufacturers of bolts started making these bolts and went up on the cost, so people would have to replace them. i myself find older engines and use the bolts from them so i don't have to pay the extra money to replace them.
yall have a great day!
 
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emckelvy

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Re: I threw #1 rod on my Merc 1500 - Why?

On later-model motors, the conn rod big end is a "cracked-cap" type and the rod is threaded for rod bolts; these rod bolts are "stretch" type and shouldn't be reused.

For older rods with a machined mating surface, with thru-bolts and nuts, you can re-use them. Althought it's probably not a bad idea to renew the rod nuts. Use a drop of red Locktite on the nutted threads.

Cheers........ed
 
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Chris1956

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Re: I threw #1 rod on my Merc 1500 - Why?

Ed, Ironically my '77 1500 had the bolt/nut combination, which I reused w/o incident on a couple rebuilds. I ordered a rebuilt block from Iboats a couple of years ago which had a newer crank and rods with the bolts that thread into the rod. I was therefore "done in" by a newer design.
 
redjmp

redjmp

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Re: I threw #1 rod on my Merc 1500 - Why?

Just curious who rebuilt that newer block that you ordered from iBoats?
Maybe they overtorqued the bolts or stripped the threads somehow?
Were you able to determine if the bolts threads let go or the rod threads were to blame or did the bolt snap?
In any case, it sounds like its their fault and you should name names so that we can steer clear of them.
Now I'm worried because I rebuilt my 86 115 and reused those bolts.
I never thought that those con rod bolts had to work so hard anyways as the force is always on the con rod and not so much on the end cap unlike a main bearing which is the opposite.
 
Clams Canino

Clams Canino

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Re: I threw #1 rod on my Merc 1500 - Why?

redjmp said:
Just curious who rebuilt that newer block that you ordered from iBoats?
Click to expand...


Well... It wasn't me. :D

Yes.... you should not re-use the new style rod bolts. You can re-use the old not/bolt type with no worries. Could have been a bad rod-bolt, could have been re-used, (or mis tightened) etc etc.

Running at 5000 as an SOP doesn't help any... they cruise best at 4000.

-W
 
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Chris1956

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Re: I threw #1 rod on my Merc 1500 - Why?

All, I reused the rod bolts when I repaired the motor after a week in saltwater. The iBoats rebuilder was not to blame on this. I was the ignorant SOB in this case.

Clams, just so you know, I had not even reached 5000RPM yet. I was still accelerating from a no wake area. It was good that it blew near home. Actually it ran well enough to get me home and restarted yesterday when I drove it to the ramp 1/2 mile away.
 
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