305 heads

Clark in Michigan

Clark in Michigan

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I have a 1979 GM 305 Mercurser with a cracked head.
The question I have is.. Will the heads off a 88 GM 305 OMC have the same bolt pattern?
If not will automotive heads work?
Thank you
 
Scott Danforth

Scott Danforth

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Re: 305 heads

the head has a casting number. replace it with another of the exact same casting number. you will most likely find the head on a truck in a salvage yard.

the 88 heads will have the same bolt pattern, however different combustion chamber and will be center mount valve covers. you should stick to the 78-85 castings or swap both.
 
F

Failproof

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Re: 305 heads

Yes, you could swap both heads. Depending on CC's of combustion chamber of the new heads vs the old heads, you may be lowering your combustion pressure. Eg: going from 9:1 to 8.5:1. Looking at casting #'sto should tell you. Plus runner lenght/size/shape may be different, changing torque/hp curve. But yes they will bolt on. While they are of look to see if your block needs drilled for the "extra" coolant passage. The head may have an extra passage that matches with the gasket, but have a blank on the block. It is often recommended to drill to prevent steam pocket.
 
RogersJetboat454

RogersJetboat454

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Re: 305 heads

It must be mentioned that the angles of the intake bolt holes for the center 4 bolts on the '88 heads are different then older heads, and would require the use of a matching intake manifold for a guaranteed fit.

If you don't have the intake with the heads, you will need to find one. If you want a direct fit for your heads, look at the casting numbers for your heads, and match them to something out there. Your heads are no different then auto heads (no marine specific heads), and the casting number would most likely match something that was offered on a pre '87 truck. You may be able to score a pair of reman heads for short money.
 
RogersJetboat454

RogersJetboat454

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Re: 305 heads

Failproof said:
While they are of look to see if your block needs drilled for the "extra" coolant passage. The head may have an extra passage that matches with the gasket, but have a blank on the block. It is often recommended to drill to prevent steam pocket.
Click to expand...

Extra cooling passages, steam pockets, and drilling things is only a concern for the 400, due to it's siamesed cylinders. The 305 has the same standard cooling ports and passages as most other small blocks.
 
Clark in Michigan

Clark in Michigan

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Messages
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Re: 305 heads

Thank you all for your help.
I think I will start looking for some old truck heads.
Thanks again
 
RogersJetboat454

RogersJetboat454

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Re: 305 heads

Clark in Michigan said:
Thank you all for your help.
I think I will start looking for some old truck heads.
Thanks again
Click to expand...

Like it has been said, match up your casting numbers. You want to make sure you get something compatible. Heaven forbid someone has a pair of heads they thought were off a 305, but turned out to be 350 heads. Performance would take a nose dive due to the compression loss.

Good luck!
 
Scott Danforth

Scott Danforth

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Re: 305 heads

not to mention the shrouded valves.
 
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