Is it worth purchasing a rebuilt 351M or should I buy a rebuilt 351W instead? A 351M came in my 1979 Ford 2wd F100, it has 98,000 miles and needs to be replaced. All I am looking for is a good reliable engine.

 

The 351M is a bolt in. The 351W is not.

Besides being two completely different engines, the 351W was not used in an F100/350 till 1980. So, trying to use a 351W will result in all sorts of problems.

Different motor/trans mounts, different C6 transmission, different everything.

 

I do understand I would need different mounts and another C6. Most transmissions cost less then 1,500 dollars so that isn't really an issue. I don't want to spend 3,500 dollars on a rebuilt 351M if later on I'd only wish I had spent the extra money for a rebuilt transmission and got a 351 windsor. With a truck this old, I would replace the mounts anyways. I really just want to know is the 351M a decent engine. My other Ford F100 is a 1973 with a 302. I spent 3,000 dollars on a rebuild and only get 14 MPG, needless to say I am looking for a more reliable mode of transportation. The 79 has air-conditioning and is an automatic. So putting in less then 5,000 dollars would be worth it in my opinion. I just want to do this once and be happy.

 

Well, the 351W will get about the same mileage as the 302 you rebuilt. If I were you, I'd build the 351M into a 400. If you correct the oiling problems the 400 is a reliable engine and can be built for HP. The 351M and 400 were hampered with smog crap and got a bad reputation from the beginning. I'd look in the 335 engine forum and I'm sure they will help you out more.

 

If you've got a rebuilt 351M sitting there, I'd put it in. They are a good, reliable engine, even if they're not the best that Ford ever built.

Mileage won't be much better, tho. These "brick" trucks just don't go far on a gallon of gas...

 

Personally, i would invest in a 429 or 460. The 460 is not much more heavier then the 351m, has more power,more torque for pulling,more parts support,will bolt up to the 351m's bellhousing,gets decent gas mileage if mildly built,and is a more dependable, longer lasting engine.All u need to make it work is the motor mounts n headers. for the price you are going to pay for trying to swap a 351w, u could go all the way and have a sweet big block.The 351m and 400m were gas crunch era, detuned,under powered, smog controled boat anchors....ditch it and get a 351w or 460..u will be glad and satisfied you did!The parts options for these engines are endless, where as hi performance parts for the 351m are slim to none.

 

Oldhawkins, take a gander at the 335 series forum...you'll learn fast that a 351M/400 will give a 429/460 a run of their money if they were built with performance.

Disregard your inaccurate myths, thank you.

 

If this engine is done like you say it is, but the block isn't cracked, a rebuild through TMI(335 series forum sponsor) would be your best bet (make it a 400). Buying a rebuilt engine from a private party can be a crapshoot.

 

lets see a 351M/400 is gonna give a 429/460 a run for it's money? What? in what fantasy world? Sorry I have run all of them, built all of them and the only way a 351M/400 is gonna give a 429/460 a run for the money is new rebuilt 400 with performance parts VS a 429 with 100k miles in totally stock trim. Now if it makes you feel better great, if it's what you have wonderful build it but stock for stock just the added 50 cid is more hp to start with lets say you have a 400 and oh for aurguements sake a 429 you build both motors for .5hp per cid (really bad engine if htats all you can do) ok now the 429 already has 15 more hp and the corrosponding torque required to get there, now actually go for a performance engine and figure you build for 1.5hp per cid streetable power (actually pretty radical for street but can be done) now the 429 has 45 more hp. Wanna get series build a stroker ok 400 can go what 434cid ok at equal hp per cid you have just barely matched a 429 stock and are still short of a 460 and haven't even started stroking that engine yet like how about 545 on a mild .030 overbore and the ability to make an easy 500+hp on pump gas pulling vacuum for your brakes and cool running.
the 335 series engines are good and can be built but reality check here they don't compare with a big block, like I wouldn't try to compare a 460 with a 427 SOHC it's not gonna work, yes I can get a 460 to beat the factory 600+hp of the sohc but thats assuming you don't touch the sohc at all.

 

The 429(more performance oriented in stock form to start with)/460 wasn't exactly a powerhouse during the 351M/400's time, either.....but if the extra 25HP gets you going, it's your cup o' joe.

What's a big block?

 

Jermafenser it's simple actually dollar for dollar, modification for modification a 385 series engine wins everytime over any of hte 335 series engines. if you have a 335 series engine to start with then build it but if your looking for an engine and don't have a base to start with suggesting a 400 over anything is foolish, the aftermarket still isn't there on them, yes it's improving, yes it can be done but they don't give any engine a "run for the money" to quote. stock for stock the 400 still losses even against a smogged up 460 from the late 70s or early 80s
The times a 351M/400 wins is this and only this, when you have one in your truck and swapping to another engine would require buying the core, building it, paying for all the parts for the swap, the extra work etc then yes with a $1000-$1500 head start in the money dept towards the build they will win and are a good choice.

 

Whoever made the comment about the oiling problem in 335 series, you should read a little more into wherever you heard that from...

Oiling MIGHT become an issue at 8000 continuous RPM on a circle track, which I don't think anyone here will be putting their truck into the NASCAR Truck series...

And the one who said the 460 is a 'more dependable, longer lasting engine' than a 351M/400; Where do you get this load of crap from? For Pete's sake guys, post some real-world information here, or stop posting your fluff!
There are fans of all different Ford engines on this site; Just look at the engine specific forums to see that. I am tired of reading people who put an engine down based on a bad vehicle they bought from someone that abused it, or heard it from their cousin's friends' uncle.
I own both engines, and both have their good points, but you will never find any evidence about the longevity of X vs. Y engine. I have a 1978 351M with 540,000 on the odometer, and yes, that is the ORIGINAL ENGINE. That means stock cam, crank, bearings, seals, pistons, rings, etc, etc. NO OILING PROBLEMS. Still runs great to this day, and yes it got abused now and then.

For the real answer to the posted question:
Someone mentioned the 400. If you can find one, it is preferable over the 351M because of the extra power/torque from the larger displacement, and will bolt in, as it is the same engine as 351M, just like 429 and 460 are the same. TMI would build you a killer 400 based on what I have read here and at fordmuscle.com , and can build from mild to wild, as you desire.
There is nothing wrong with the 351M/400. Many people rebuild them and run them just like any other engine, and in many cases get better reliability than anything they have owned, as in my case. Take care of your stuff and it will last. (oil changes!) There is nothing wrong with the 351W either, but why bother with the extra work of mounts, hoses, accessories brackets, etc. when you don't have to?

 

im not ryin to rain on anyones day,but if 335's dont have an oiling problem why do 75%of them have a main knock at start up by the time they have 80,000 miles? shure there are exeptions to every thing like 540,000 miles is "nontypical" results some have good luck but most dont. the M family has low power and bad mpg in stock origanal form and thats just the way its.heck i have a reman M in a parts truck and cant even give it away

 

I've ran & built quite a few 400's over the years and the main problem I've seen isn't the oiling system, it's the machining / or core shifting.
Finding 400-351M blocks with good straight round main bearing bores are few and far between. Same goes for the rods. If those problems aren't corrected they won't last. The problem is too many back yard engine builders don't know to check those kind of things = bad rap on 400's. You can put rods and main bearings in all day, but if bores are crooked, cocked, and out of round you will be right back to where you started in no time.

 

mark A: Do you recommend every rebuild of a 351M or 400 be started off with align boring of the mains? Sounds like good insurance on a seasoned block...

As for the 'other guy's reply, I never heard of any 'knock' at startup on a 351M or 400 at 80,000 miles. I have yet to hear one of these engines with 100,000 with the condition you describe...
And you wanna talk low power stock engines? Do you think the late '70's 460 was a powerhouse? C'mon, man... The 351M/400 ONLY came with a 2V carb... Imagine how close those HP numbers would be with a 4V??? My '76 F150 has a reman 460, and it sucks gas BAD and won't even chirp the tires when I floor it. (or one tire, for that matter). I installed a roller timing set at straight up and didn't make a difference. Took off the 4350 carb and installed an Edelbrock, calibrated it to perfection, and still won't chirp the tires. 3.25:1 gears. gained a little mpg, but still not near the 351M.
My 351M can STILL chirp the tires in my '78 F150, even with those miles on it, and a 3.00 Trac Lok gear!

Now you will say "oh, depends who built it", or, "Gee, they must have put Chicago Deep Dish pistons in it" etc, etc.
Like I have said many times before, I am a fan of all the Ford engines, and have owned FE's (including 428), 335 series, 385 series, 300 I-6, and Windsor small blocks.
There are just too many variables to say what engine is better or worse. But to say, without any actual scientific numbers that the 335 series are no good compared to the others, is just rubbish.
Boss 302? Boss 351? Very successful in racing, very dependable. And, they are 335 series.
Just ask guys like Parnelli Jones or Jack Roush. They know.