273 Commando Rebuild Or Not?

-
Bossanova5

Bossanova5

Well-Known Member
Joined
Dec 27, 2020
Messages
144
Reaction score
60
Location
Los Angeles
Hello All,

I picked up 67 273 Commando. Did a compression test on it today.
Here were the results from the cylinders:
1) 155 psi
2) 150 "''
3) 155 ''''
4) 140 ''
5) 150 ""
6) 140 ""
7) 155 ""
8) 140 ""

Shot oil in the low cylinders and the compression rose indicating ring sealing issues.
The engine supposedly sat for a long time. Back three cylinders are obviously low. The manual says 130-165 psi and + or - 25 psi between cylinders.

The engine is in the back of my truck so I can't hear it run. The prior owner mentioned seeing white smoke out the tail pipe when standing on it.

I was hoping to get away with just a deep cleaning and seal and gasket replacement.

It seems like I will probably have to tear it apart and rebuild it if I want a good street runner.

All advice excepted.
Thanks
 
Last edited:
It depends on what you want. I would reseal it, change out the valve seals, and run it. The compression numbers aren't that bad and fairly even. It will probably get better with some miles. It may last for years the way it is.
 
toolmanmike said:
It will probably get better with some miles.
Click to expand...
I agree with Mike. Got a unknown 318 for free. Guy said it was from a Dodge Diplomat that was rusty beyond repair.
Was sitting in his backyard. Did just what Mike said. Sealed it and did the heads.Ran it in my truck. Used a quart every 250 miles and blue smoke when cold. Drove it daily and it actually got better with use. That was over 10 years ago. Still driving it. Uses about 1 quart 1500 miles.No smoke at all.Worth a shot if you don't want to spend allot.
 
I agree, you'll know more once you get into it. But those compression numbers aren't bad. 65
 
I would run it like that. They are tough little motors. My 65 Belvedere wagon had the original 273 that looked like it was never out of the car. It ran like a champ. It just leaked every fluid because of the old gaskets. I finally replaced it with a 360 about a year and a half ago just for more power.
 
I agree #'s are good. If you're concerned about rings. Fill cylinders up with something to free them up??Marvel Mystery oil? Pull plugs and rotate by hand. Let sit a few days and rotate again. Lather rinse repeat and then give oil change and prime, then run it.
 
Hang on a sec, are you sure that's a Commando short block?

That should be a 10.5 engine with domed pistons and a 248/248 cam. (I think)
My FSM does not list the LSA.
If I assume it's a 112 and in at plus 4*, then the Ica comes to 52*. When I plug that into the Wallace calculator, it spits out
191psi at 300ft elevation.
If that's true, and I do not know that it is, then your "commando" is some 45/50 psi down on pressure.
Maybe I got the wrong cam info.
For instance, if the 235hp cam was a 276/112cam, then the Ica could be 64 degrees, and the proper pressure would be 172 psi, which is still a Loooong way from 145average
But, even if it was a 9/1 engine with a 2bbl cam, the pressure should still be 161psi@ sealevel.
To get that 9/1 engine down to 145psi, with an Ica of 48*, the elevation would have to be ~3000ft.

IMO, and if it was mine; I'd be doing some more investigating.
and
If you are at 3000ft, not a chance would I rebuild that 273. You'd be way ahead with more cubes.,
 
Last edited:
AJ/FormS said:
Hang on a sec, are you sure that's a Commando short block?

That should be a 10.5 engine with domed pistons and a 248/248 cam.
My FSM does not list the LSA.
If I assume it's a 112 and in at plus 4*, then the Ica comes to 52*. When I plug that into the Wallace calculator, it spits out
191psi at 300ft elevation.
If that's true, and I do not know that it is, then your "commando" is some 50 psi down on pressure.

IMO, and if it was mine; I'd be doing some more investigating. Maybe I got the wrong cam info. For instance, if the 235hp cam was a 276/112cam, then the Ica could be 64 degrees, and the proper pressure would be 172 psi, which is still a Loooong way from 145average
Click to expand...
It's rated 10.5:1 but probably no where's near that cr.
 
Clean, reseal, change valve stem seals, paint and run it


If it needs rebuilding later rebuild
 
I have not used a scope in the cylinders to verify that the pistons are in fact domed. However, the engine has the proper valve covers, double points distributor and a cast iron four-barrel manifold. The 67 Plymouth Factory Service Manual Spec depicts the compression pressure for the four-barrel (Commando) engine between 130 & 165 psi, See picture. Being that the compression of 5 of the cylinders are 150 psi or above, that leads me to believe that it is the high compression 10.5:1 compression engine. As the factory service manual indicates the compression for the 9:1 is in the 120-150 psi range. (The double asterisks next to the numbers in the service manual indicates four-barrel) The compression test was taken in Santa Monica which is about 105' above sea level.

20241103_192428.jpg
 
toolmanmike said:
It depends on what you want. I would reseal it, change out the valve seals, and run it. The compression numbers aren't that bad and fairly even. It will probably get better with some miles. It may last for years the way it is.
Click to expand...

I'll second this, but add use the Fel-Pro Viton positive valve seals, Replace the valve springs with 340 valve springs, brass freeze plugs, and a double roller timing set. Re-torque the head bolts, do not replace the head gaskets. If you take the heads off replace the exhaust valves with stainless steel ones.
 
66fs said:
340 valve springs,
Click to expand...
Or Comp 901's are another good choice. I might try Beehives next time (if there is a next time) I don't think I would even replace the head gaskets unless the heads come off for a valve job. Where do you stop if you know what I mean.
 
Bossanova5 said:
Shot oil in the low cylinders and the compression rose indicating ring sealing issues.
The engine supposedly sat for a long time. Back three cylinders are obviously low. The manual says 130-165 psi and + or - 25 psi between cylinders.
Click to expand...
actually
the passenger side 2-4-6-8 is all low, and
the driver side 1-3-5-7 is all a little higher; My guess is that one head or headgasket is different

The point remains, to get a "stock" ;
>"high compression 273", DOWN to 145psi will take a really late Ica, or
>"lo-compression 273", with a stock Ica of 48*, DOWN to 145 will take an Scr of about 8.3

I'm not arguing what your engine is or is not.
After all these years, there is no telling what's inside it.
I'm suggesting that the truth will require more investigation.

Furthermore;
As regards the FSM pressure guidelines;
That factory range which you quoted only reflects the quality of the factory builds, in the environment of the real world, and reflects the normal-wear changing values. Numbers like that are NOT a good target. Numbers like that just keep the warranty claims down.
At 130psi, the 273 will be gutless.
At 165psi, it will be as strong as it can ever be and still run on pumpgas.
At +/- 25psi from 145, that translates to a total of 34%; which IMO indicates a serious problem. In my world, if an engine has a pressure spread of more than 10%, something is wrong. My target is 4%.

Furthermore, no matter which engine you're working with, the low-rpm performance, at 145psi average, will be sadly lacking in anything but a lightweight early-A.
For comparison, 140psi is the pressure typical of an 8.0- 318 at 100ft elevation. In the heavier A's of the mid 70s, those were the most lackluster 318s of all time.

My point is this;
if that was my engine, and it was not a numbers matching engine, I would replace it with something bigger.
But if I just had to have a 273, then first I'd check why the one head is down on pressure. The factory 020 gaskets are no longer available and so, if it's just a fat gasket, then I would have a decision to make. Cuz if I put a matching gasket on the other side, it too will have the low numbers, and so the question becomes, can I live with ~140psi?
But if it's NOT a gasket mismatch, then I gotta go hunting, and depending on what I find, that will determine in which direction I gotta go.

Happy HotRodding
 
Last edited:
I was thinking perhaps it could be a leaking head gasket on the passenger side bank. Why not replace the head gaskets? From what I understand, the Commando came with longer valve springs then the 2 barrel version. What will putting 340 valve springs on the engine do? Isn't anything related to 340's in the unobtanium price range?
Thanks for the ongoing help
 
Bossanova5 said:
I was thinking perhaps it could be a leaking head gasket on the passenger side bank. Why not replace the head gaskets? From what I understand, the Commando came with longer valve springs then the 2 barrel version. What will putting 340 valve springs on the engine do? Isn't anything related to 340's in the unobtanium price range?
Thanks for the ongoing help
Click to expand...
Leaking on all cylinders on one bank, to cause lower compression, is next to impossible. Leaking head gaskets are usually one cylinder or possibly 2 adjoining cylinders and rarely are they just 10 or so pounds lower than the rest.
 
Bossanova5 said:
I was thinking perhaps it could be a leaking head gasket on the passenger side bank. Why not replace the head gaskets? From what I understand, the Commando came with longer valve springs then the 2 barrel version. What will putting 340 valve springs on the engine do? Isn't anything related to 340's in the unobtanium price range?
Thanks for the ongoing help
Click to expand...
The stock 273 valve springs are all the same....2bbl and 4bbl. 318's used the same springs too. When installing replacement springs and retainers, check the diameters too, as some will rub the stock adjustable rockers. That's where using some bee hives can be beneficial. As for spring tension, that will depend on the cam and the RPM range you plan on using. If you don't expect to ever go over 6000 RPM, I'd use the stock or weaker springs as they will be a lot easier on the cam and lifters.
 
I noticed in the 67 Plymouth FSM that the specifications on the free valve spring length for the commando engine is 2.00" vs 1.92" for the two barrels. See the picture. Don't know how much of a difference it makes?

20241104_144203.jpg
 
I stuck a scope down a few of the cylinder heads. From what I could see, the pistons had two indents for the valves confirming it is a commando engine. The back three cylinders with the low compression had huge carbon deposits on top of the pistons. Might have to tear it apart....
 
Those numbers are extremely even and strong for the age of that engine. There's no way in hades I'd rebuild it. I'd reseal it like @toolmanmike said and drive it.
 
Last edited:
You must log in or register to reply here.
-

Back
Top