Can 2.5L GM block be bored for 3.0L?

[fixb52s]

I am currently have 1985 3.0L Mercruiser engine that has a trashed block. It appears to have been overheated and cracked a cylinder and the deck.

I have searched for a replacement block and found one from a 1983 Sea Ray. Even though it was marked 3.0L and the block casting number checked out, it turned out to be a 2.5L. The block though is in perfect shape. It has the same casting number as the 3.0L (2770087) but has a 3.877" bore instead of the 3.99" required for the 3.0L pistons. I want to reuse my 3.0L crank, rods and camshaft along with some new 3.0L pistons. Can this 2.5 engine block be safely bored to the 3.99, or should I just put it back together as a 2.5L? It appears that GM used the same engine blocks for the two, but my local machinist is not sure it can be overbored .110. Any ideas?

 

[bomar76]

Re: Can 2.5L GM block be bored for 3.0L?

xxx

 

[Bondo]

Re: Can 2.5L GM block be bored for 3.0L?

It appears that GM used the same engine blocks for the two, but my local machinist is not sure it can be overbored .110.

Ayuh,... I find it hard to believe that Chevy would Waste tons,+ tons of cast iron like that,...
They Don't on their V-motors...
But,...
Have your Machinist guy sonic test it for wall thickness...

I think I'd just be Happy I found a usable block....

 

[smokeonthewater]

Re: Can 2.5L GM block be bored for 3.0L?

wellllllll my gut says that the same casting number means it is the same casting...... In that case it would be yes BUT I do not know this for absolutely certain..... If it were me I would be calling LOTS of machine shops and maybe a call to a company that sells engines and components such as jasper or similar

 

[cedarjunki]

Re: Can 2.5L GM block be bored for 3.0L?

im no expert by any means here....... but if the machinist isnt sure, i think id be finding a different shop... to my knowledge the 3.0 is simply a bored out 2.5 chevy block...since there were no actuall 3.0 liters in any auto application.... but like i said im no expert



check this out


http://forums.iboats.com/showthread.php?t=40985

 

[fixb52s]

Re: Can 2.5L GM block be bored for 3.0L?

I would also think the same blocks would be used for both, but I had to wonder about what the concerns of the machinist are. He said he bored a 2.5 block before and it came back to him later due to a cylinder wall collaspe. He did admit it was a different casting number (consistant with an OMC/Mercruiser 1963-67 block) He claims that even though the casting number is the same for the mid 80s blocks, the walls might be poured thicker for a 3.0 block. Doesn't sound right to me, so that is why I asked here.

I called 2 other shops after posting and they think all should be fine, and one did say they can check the wall thickness first. I should be in good shape since this is a freshwater engine and the water jackets look great.

I will contact some marine rebuilder today and see what they say.

 

[Bondo]

Re: Can 2.5L GM block be bored for 3.0L?

the walls might be poured thicker for a 3.0 block. Doesn't sound right to me, so that is why I asked here.

Ayuh,... That's My point,...
The 305,+ the 350 are also the same blocks, but you'll hit Water, long before a 305 can be punched to 4"s...
Muchless 4.060", which is the standard outer limits of a 350...

to my knowledge the 3.0 is simply a bored out 2.5 chevy block...since there were no actuall 3.0 liters in any auto application....

Apples & Oranges...
The 2.5l motors used in autos since the late-60s are of No relation to the 2.5l/ 3.0l Marine/ industrial motors...

 

[fixb52s]

Re: Can 2.5L GM block be bored for 3.0L?

Ayuh,... That's My point,...
The 305,+ the 350 are also the same blocks, but you'll hit Water, long before a 305 can be punched to 4"s...
Muchless 4.060", which is the standard outer limits of a 350...

Yes the 305 and 350 blocks are the same blocks, but cast different. They carry different casting numbers, where in this case, the casting number is the same for both the 2.5 and 3.0.

 

[flockshot]

Re: Can 2.5L GM block be bored for 3.0L?

They are the same. The casting number is the indicator. The question you have to ask is whether or not all the drilling and taping for the 'ad on' stuff matches your aplication.

 

[zbnutcase]

Re: Can 2.5L GM block be bored for 3.0L?

No you CAN'T bore a 2.5 out to 4 inches. End of conversation. 'nutcase

 

[fixb52s]

Re: Can 2.5L GM block be bored for 3.0L?

I got two answers so far. The first is a major marine engine rebuilder. They said GM cast all these blocks the exactly same on the same line, (casting number 2770087) and machine the bore for the different application. As long as the walls are good (check via sonic testing) it will be no problem to bore it up to 4.0300.

Then I had another guy say that it cannot. Oh yea, He also just has another block he will sell to me for $900, so that makes me wonder.

As far as everything else like bolt patterns, main bearing sizes and such, they are identical. I hope to use the 3.0 crank, rods, pistons, camshaft and head. I also know I would have to use the 3.0 harmanic balancer and flywheel.

And yes, an older standard 2.5 Chevy II block cannot be bored to 4, that is a definate fact.

 

[fixb52s]

Re: Can 2.5L GM block be bored for 3.0L?

Update: I found a machinist that is taking the block next week. He is very familiar with this issue and said that it is VERY possible to do the machine work. Where he has run into problems in the past was some blocks could not be done because they were salt water units that were internally rusted. He has seen many 3.0 factory blocks that have failed due this. He is going to sonic test the block first to see if is a good candidate. If so, then he will do the machine work, matching the bore to new pistons. He did a visual tonight and said it does look like a good core, but I will find out for sure next week.