Merc 302 vs 351

[pmddouglas]

Re: Merc 302 vs 351

Hey I have finally gotten the motor pulled out and now I can't determine if I have a 302 or a 351 engine. One valve cover say 188 hp and tag with sn#4490968 the other valve cover says 4456651 and the tag on the block says 4490968 and the heads have 351 casting #'s. <br /><br />How do I know if I have a 302 or 351 or 302 with 351 heads.<br /><br />Thanks,

 

[pmddouglas]

Re: Merc 302 vs 351

Hopefully the block is not cracked etc. because 2 freeze plugs fell out and water was still in one of the exhaust manifolds. Piston tops and valves rusted. Have pan off and heads etc and still can't get the crank to rotate with a socket wrench. Rusted together or something.

 

[Boomyal]

Re: Merc 302 vs 351

The intake manifold for a 302 measures just under 9" across the top just forward of the two front carb studs. If your heads are original they are "351" heads drilled for 7/16" headbolts.<br />I'm not sure why they bothered with 351 heads. They are probably D5AE heads. They were no different than 302 heads after 1974.<br /><br />Your rings are probably rusted to the cylinder walls. Liberally apply liquid wrench and let sit over night. Then try to turn using the crank pulley nut. Look carefully across the top of the cylinder deck for any cracks. That is a likely spot but you may just have to have the machine shop check the whole block for cracks.<br /><br />If you,ve got the cheddar, put it back together as a 331.

 

[JasonB]

Re: Merc 302 vs 351

You likely have a 302 with 351 (as Boomyal said above) heads which was common. If so your intake is also 351 and the cam should use the 351 firing order.<br /><br />Don't feel too bad about the pistons being stuck. I had to pull the crank and drive all 8 pistons out with a piece of 2x4 cut down and a hand-help sledge hammer. Ended up boring the block 30 over. Nice effect of ingesting salt water and sitting that way for a few months....

 

[Boomyal]

Re: Merc 302 vs 351

Originally posted by JasonB:<br /> If so your intake is also 351 and the cam should use the 351 firing order.

Jason, although the heads are interchangable, atop a 302 block, the intake is not. The taller deck of a 351 requires a wider manifold to span the increased gap. You are correct re: the 351 cam and firing order.

 

[pmddouglas]

Re: Merc 302 vs 351

I am a little confused. I am pretty sure I have the 351 heads so I would imagine I have 351 manifolds and distributer. Why would the cams have a different firing order? I mentioned it to the machine shop doing the work, and he told me that I should use a 302 cam or something like that. Am I headed for a disaster?

 

[Boomyal]

Re: Merc 302 vs 351

The OMC 190 and Merc 188 were 302's with higher output than the plain 302. In OMC the plain jane version was rated at 175 hp. I don't know if Merc used the plain jane version. They accomplished this by using the 351 heads and a higher lift camshaft that used the 351 firing order. It's that simple. If you still have doubts as to which you have, read my post above re, the width of a 302 intake manifold. They are not the same as a 351 intake manifold.

 

[f_inscreenname]

Re: Merc 302 vs 351

Boomyal is right. You have a 302 with 351 heads. The 351 block is bigger (taller) then the 302 and when the heads are bolted on to the block they will be further apart because of the taller block. 302 and 351 head casting are the same size. That is why they are interchangeable.

 

[tryan]

Re: Merc 302 vs 351

pandj, i have to throw in my 0.02. if you have the money, go 351. all of your EXTERNAL 302 parts will work except for the oil pan and intake manifold when converting to a 351 winsor. <br /><br />your cheapest route is to source a good 302 block and pray the machining will clean up the crank. magnaflux the block. you may have gotten lucky and just blown the casting plugs.<br />be sure to use stock low tension chrome rings and the proper pistons if yours are out of spec.<br /><br />make a plate and pressure test the manifolds before they are returned to service.