mercruiser 350 with 305 heads?

[jsmith3102]

I have a 1994 Mercruiser with a strange setup??
Is it ok to have a mercruiser 350 block with 305 heads? what will result from this.. HP difference?

I have a blown head gasket and am wondering if this caused it.

 

[Bondo]

Re: mercruiser 350 with 305 heads?

I have a blown head gasket and am wondering if this caused it.

Ayuh,...

Just the fact they are 305 heads will Not cause a blown headgasket...
But,...
Due to the smaller combustion chamber, the compression ratio Will be higher, which should translate to More HP...
But it Doesn't, due to the Smaller Valves that are required for the smaller combustion chambers...

The Best heads availible today for a 350 in a boat are the Vortec heads from a post-96 350....

 

[FreeBeeTony]

Re: mercruiser 350 with 305 heads?

Just curious...........where did this engine come from?
Is it a original Mercruiser or where did it come from?

 

[jsmith3102]

Re: mercruiser 350 with 305 heads?

It was originally a 305 but the owner before me replace the block with a 350. I am trying to replace the blown head gasket. I am new to this and it has been an experience so far... This forum has been a great help though. Still wondering if I should put the gaskets in dry or add rtv to them..

 

[chiefalen]

Re: mercruiser 350 with 305 heads?

Supposed to be dry sometimes they come with a sticky substance on them already.

But i have used aviation form a gasket, wont melt.

The mating surfaces are they true?

 

[Bondo]

Re: mercruiser 350 with 305 heads?

Still wondering if I should put the gaskets in dry

Ayuh,...

It should be Clean,+ Dry surfaces,+ the gaskets....

 

[turborich]

Re: mercruiser 350 with 305 heads?

You shouldn't ever use RTV on a head gasket. You can use the spray on gasket sealant. I forgot the name of it. Anyways you just lightly paint the gaskets. This is up to you & isn't necessary. Like the others said, make sure that the surfaces are 100% clean. If you pulled both heads you may want to get them checked & resurfaced at a machine shop.

 

[jaxnjil]

Re: mercruiser 350 with 305 heads?

You shouldn't ever use RTV on a head gasket. You can use the spray on gasket sealant. I forgot the name of it. Anyways you just lightly paint the gaskets. This is up to you & isn't necessary. Like the others said, make sure that the surfaces are 100% clean. If you pulled both heads you may want to get them checked & resurfaced at a machine shop.

spray copper coat

 

[Mkos1980]

Re: mercruiser 350 with 305 heads?

Ditto to all above. CLEAN and Dry. Any spec of old gasket residue could cost you a new engine down the line. This is where you should spend the extra time. Using new GM Headbolts for 25 dollars is a good investment as well!

 

[chiefalen]

Re: mercruiser 350 with 305 heads?

Mkos makes a very, very important point here.

I posted very, 3 times already cause, i had a problem cost me almost the whole summer on the water.

Do not over torque the bolts, and start from the inside and work your way out to the sides, or corners.

And i believe 90-100 no more.

But please verify with your manual, correct torque.

USE A TORQUE WRENCH.

If you over torque like my buddy did with my head kiss your block goodbye, if you pull up the bolt hole.

Good luck.

 

[45Auto]

Re: mercruiser 350 with 305 heads?

One thing I never see mentioned much is to use the correct tap and run it through the threads in the block to clean them up BEFORE you install the cylinder head. Usually amazing how much rust and crud comes out of the threads. Makes a big difference in getting accurate torque readings when you reinstall the heads.

 

[chiefalen]

Re: mercruiser 350 with 305 heads?

See why all these minds together are better than a pm.

45 auto makes another great point.

Write them all down before you start the work.

 

[mtnrat]

Re: mercruiser 350 with 305 heads?

Correct torque for head bolts is 65 ft lbs. Do it in three increments, like 25 then 45 then finally at 65. The proper torque sequence is very important.

 

[dan t.]

Re: mercruiser 350 with 305 heads?

as far as valve size is concerned I ran 305 heads with 1.84 intake,1.50 exhaust on a .030 over 350 in an oval track car .it pulled to 6500 rpm with no problem . this combination will give you better than 10:1 compression ratio so be carful with your timing and listen for pinging. use permatex liquid teflon
thread sealer on the threads, oil under the bolt heads and torque to 65 ft.lb
good luck

 

[chiefalen]

Re: mercruiser 350 with 305 heads?

Just looked it up and it's 60-70 for the head bolts.

I also use a little oil under the bolt head, to get true torque.

Should do it in 3 increments.

20 all around.

40 all around.

60-70 all around.

This is critical.

Do not re-torque when motor is hot.

Do it once leave it alone.