GM/Merc 250 I6 Rebuild/Purchase

Hello all;
Just bought my first boat with the old GM/Merc 250ci Inline 6 engine. Compression results show three bad cylinders and there is water in the oil, so motor is shot.
Question is, whether or not to rebuild or get a new one.
I'm looking for recommendations for sources for the engine, and opinions on rebuilding vs. just getting a new one. I am working on a budget, but time is also important...
Thanks for the help,

David M. Sexton
USCG

Oh, boat is a '74 Starcraft Chieftain I/O with Alpha One Drive.

Re: GM/Merc 250 I6 Rebuild/Purchase

Easy engine to rebuild. Depends upon your skills and time. Certainly cheaper to do it yourself. You could look for a good used engine... but the chances are good that any used engine will be on its last legs as well. I've rebuilt a few of them - great engines!

Re: GM/Merc 250 I6 Rebuild/Purchase

sextondm said:

there is water in the oil.

Click to expand...

I think you need to find out why there is "Water in the Oil". Could be a cracked Head, Block or something else. If the Head's cracked you can get another, If the Block's cracked you can get a short block. Don't waste your money trying to fix something that's ruined.

Re: GM/Merc 250 I6 Rebuild/Purchase

Well, I'll be pulling the engine this week to see what I can find out. Results from the comp. test are: #1 75psi. #2 0psi. #3 150psi. #4 150psi. #5 141psi. #6 97psi.
Engine hasn't been run for a few years. I converted it to Pertronix ignition and replaced all the fuel lines/filters all the way to the tanks, cleaned the tanks out and checked basic timing and carb settings.
I'm going to possibly attempt a valve adjustment and see if that changes anything before I pull it.
Looks like rebuild kits can be had for around $300 on ebay, and a reman. engine is anywhere from $1200-$1700 on the web.
But, I'll know after I get it out!
Thanks!

Re: GM/Merc 250 I6 Rebuild/Purchase

Be sure to check your manifold and riser for the water intrusion problem. And as mentioned above, don't buy any parts till you know what needs to be replaced. Depending upon what you end up doing, you may just need a complete gasket set. Never know till you tear it down. If you do end up needing parts and machine work, be sure to ask your local machine shop about parts and gaskets as well. They can usually get them at a good price, and will often pass along some of the savings to you.

Re: GM/Merc 250 I6 Rebuild/Purchase

if your anywhere close to pittsburgh ,pa send me mail i may have a motor to rebuild or parts

Re: GM/Merc 250 I6 Rebuild/Purchase

Thanks jim372. I'm actually in Alaska, but when I get the motor out if there are parts I need I'll let you know. I'm going to try to get it pulled tomorrow night.

Re: GM/Merc 250 I6 Rebuild/Purchase

Well, engine is out!
The 0psi in the #2 cylinder was from the intake valve being stuck about 3/8" open. I tapped once on it, and it seated all the way. Other than that, I can't find much as far as obvious problems. There is a little corrosion in the #1 intake port, or the part all the way at the front of the head. It's not bad, but has more than all the other ports on the cyl. head. All six pistons and cylinders look ok to me, no bad corrosion, and can still faintly see the cross hatch from the honing. Not much of a lip either. The boat has been run in salt water here. The inside of the exhaust manifold looks pretty rusty from what I can see. How does one go about checking the exhaust/intake manifold for leaks??
I'm taking the whole thing to the local machine shop tomorrow to have him check the block and head to see if I can rebuild it.
If it's bad, I'm looking at sandjengines.com as a possible supplier for a long block. Anyone heard of them or have a recommended supplier??
Thanks again,

Dave Sexton

Re: GM/Merc 250 I6 Rebuild/Purchase

With the manifold off, you can fill the water passage with acetone and look carefully for any leaks inside the ports. (Acetone is thinner than water and leaks are easier to see).

You also want to visually inspect the area where the riser and manifold bolts together. There should be a good quarter of an inch of clean shiny metal - not black corrosion. Here's what a new one looks like - and the old one should have a similar amount of metal left.

Re: GM/Merc 250 I6 Rebuild/Purchase

Thanks for the good picture. My manifold does have lots of metal left there, but is a lot more rusty than the new one! I am going to try the acetone trick in the next few days.
Took the block to the machine shop today and it's cracked, so it's done. I'm going to be shipping an engine up here, probably from sandjengines.com out of WA unless someone has another recommendation.
Then it will just be a matter of swapping all the accessories onto it and getting it all cleaned/painted before installation.
In the meantime, I'll go through all the wiring and everything in the bilge area while the engines out. The boat hasn't been in the water in a few years, so it will be good to go through everything.
Oh, anyone have a parts supplier they would recommend for Mercruiser stuff?
Thanks again!

Re: GM/Merc 250 I6 Rebuild/Purchase

I ended up sourcing a used engine block in Anchorage. It just need to be cleaned up and bored .030 over. I've had the cylinder head checked out, and it only needed regrinding the seat, and a few other things and is done alrteady.
I capped and filled the exhaust manifold with acetone and let it sit overnight, and the exhaust ports were all dry in the morning, so I'm assuming it is not cracked internally! Good news!
As soon as I get the engine block and my rebuild kit, it will be time to set to work getting everything assembled.
Thanks to all for the great info. and support as always.

Dave Sexton