Dropped exhaust valve in number 8 mcm 260

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chris0061

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OK, I bought early 80s Mercruiser 260 and found out it has a dropped exhaust valve. I took the head off and the piston will definitely need replaced. I'm taking the other side off tomorrow to get a valve job done on the heads. If the other 4 pistons look as good as the three on the starboard side would it be safe to just replace the one piston and is there anything else I should look for before I dump any money into this engine? By the serial number on the valve covers it is indicating a 80 or 82 350 but by the exhaust manifolds it is indicating after 82 as this does not have the log type manifolds but the elbow comes off the center of the manifold. What ya all think?
 
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meesh

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Re: Dropped exhaust valve in number 8 mcm 260

If you have a junk piston its time to rebuild or replace the motor.
 
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chris0061

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Re: Dropped exhaust valve in number 8 mcm 260

This is a spare engine I picked up for 100 bucks complete. I have it on a engine stand and rebuilding it would be cool with me if it needed a complete rebuild, what I need to know would it do any harm to just replace the one piston if all the others look good.
 
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meesh

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Re: Dropped exhaust valve in number 8 mcm 260

Chances are the rod bearing is beat to death. Replacing one piston would throw the assembly out of balance anyway. You really only have 2 options, rebuild it or replace it.
 
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chris0061

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Re: Dropped exhaust valve in number 8 mcm 260

So we are talking about taking the block and having it board out to .30 over then getting the .30 over pistons with new rods and bearings or does the cam and crank need replaced also?
 
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meesh

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Re: Dropped exhaust valve in number 8 mcm 260

your not going to know until you tear it down. Your probably going to end up boring, and if you want to do it right check the decks, the alignment through the main saddles new pistons and rings, rod and main bearings rebuild the rods and balance the whole assembly. Oh yea and polish or grind the crank depending on the damage or wear to it. and rebuild the heads. Its more than just replacing a few parts if you want it to live very long. All that is going to add up fast it will be cheaper to by a long new long block.
 
Big Boat Bay Buster

Big Boat Bay Buster

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Re: Dropped exhaust valve in number 8 mcm 260

The machine shop will decide to bore or just hone what you got depending on his inspection of the block. My guess is since it looks like you are getting a new set of pistons, a bore might be the way to go.
I would just automatically get a new cam.
The rods and crank will again depend on the inpection at the shop. My guy acually resized the holes on the rods and I re-used them.
If the crank is scored, he can cut it down undersized. Chevy cranks are tuff-more than likely it will just get a polish!!
You can avoid all this if you want by getting a mail order engine--cost about the same.
Chris
 
Silvertip

Silvertip

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Re: Dropped exhaust valve in number 8 mcm 260

A good machine shop knows that "high limit" pistons are available from GM. These are standard bore pistons but they measure at the high end of the diameter limit. Provided the bores look good and are within tolerance for taper and out of round, but may be a little on the loose side of the standard bore limit, you can install these high limit pistons in those marginal bores and you don't need to bore the engine. Everything else regarding the bores does need to be in-spec however. Can you get away with what you want to do? It all depends on what I said about the bore specifications. If this engine has a ton of hours on it, you are well advised to bore it. If it measures in tolerance, replace what you need to. The cam, crank and bearing condition will tell you what needs to be done. Lifter bases will tell you whether they need to be replaced. Rocker arm balls and arm pivot points will also tell a story.
 
mkast

mkast

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Re: Dropped exhaust valve in number 8 mcm 260

meesh said:
Chances are the rod bearing is beat to death. Replacing one piston would throw the assembly out of balance anyway. You really only have 2 options, rebuild it or replace it.
Click to expand...

How can a valve hitting a piston damage a rod bearing?
Considering the factory doesn't balance the engines anyway, replacing one piston/rod assembly won't change anything.
 
Silvertip

Silvertip

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Re: Dropped exhaust valve in number 8 mcm 260

When the valve hits the piston the resulting "bang" is like a direct strike on a bearing with a hammer, only in this case the shock is transferred from the piston, to the rod, to the bearing. Remember, this happens at anywhere from idle to 5000 rpm. The faster the engine is running at the time. the more force is exerted on the bearing. The factory doesn't balance engines because the parts going into it fall into a specific weight range. Auto engines are also not required to run 4500 - 5000 rpm for extended periods like a boat. So for best longevity and performance, it is a good idea to balance a boat engine but it is not essential -- just a recommendation, especially when using aftermarket parts.
 
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chris0061

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Re: Dropped exhaust valve in number 8 mcm 260

OK sounds like allot, as I said I paid 100 bucks for the engine, It has good manifolds, y pipe starter, alternator, everything on it. It's got the thunderbolt ignition also. I have the 24 foot Mark twain with a older 350 in it that I have only fired not ran so maybe my best bet would be to strip it and store the parts. I did look at the cyl wall today on the bad piston and there is one tinny scratch on it that may be able to be honed out. I am going to take the other side off tomorrow, flip it and pull the piston and look things over. But then again I have my pickup with a 305 in it If I have it rebuilt, cam it, bore it and raise the hp I could use it in a boat or truck right? By the way, the valve head looked good but the piston has the piece that broke off the stem embedded in it and a couple cuts in the piston must be from the edge of the valve. That had to have been a pleasant surprise when that went out. Thanks guys for the input
 
Silvertip

Silvertip

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Re: Dropped exhaust valve in number 8 mcm 260

You are wasting money on the 305 primarily due to the restricted breathing of the small valve heads. It would cost you the same money to rebuild the 350 as the 305. If you are talking rebuilding and have both engines available, the 350 is a no brainer.
 
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chris0061

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Re: Dropped exhaust valve in number 8 mcm 260

No the 305 in my truck is fine, I was thinking rebuild the 350 and if the Twain egine is bad I'd have it for that or if my truck engine went bad I'd have it for that.
 
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meesh

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Re: Dropped exhaust valve in number 8 mcm 260

Thank you silver tip!
 
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chris0061

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Re: Dropped exhaust valve in number 8 mcm 260

OK decided to put it up for sale as parts on Craigs list. Thanks for all the input.
 
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