Can this be done(Ryan?)? How much can a 273 block be bored and/or stroked? Thanks for replies.

The 273 I had tested would have gone .060 but it looked like a nice block with little core shift. Without sonic testing I would not go over .030. As for useing those heads you are gonna have to notch the bores to get valve clearance. At .060 over a 1.88 valve barely clears, I do not think it would have cleared a stock bore, that is how close it was and a notch would still be good for airflow. Those aluminum magnum heads with a 1.92 valve are gonna need a notch no matter what you bore the 273 out to. Now if you went with a stock iron mag, the valve job is terible in stock form for a 1.92 valve however you that same bad valve job will allow you to machine the valve head down to a 1.88 diameter and with a .060 over bore you should get enough clearance to run but again a notch is still gonna help air flow tremendously. The only other option I can think of to use the aluminum magnum is to have a smaller seat installed and machine the valves down to that diameter.

As for strokeing it I know a 4 inch will fit with SCAT I beam rods but pistons are gonna be custom only. I mocked one up with a stock piston and just had to clearance the skirt to clear and it would rotate, check very closely at the rod to wall for clearance, not as much clearance there as a 318 even. With notches at the bottom of the bore you may get a little more stroke out of it and it can get around 340 cubes the really hard way and with very shrouded valves so it will not make the power a real 340 will.

Thanks Dave! Sounds like an aftermarket head just wont work too well on this block. Wonder if I should just look for a 340 block and be done with it.

I use a 1.9 int valve at 030 overbore.

Thanks guys, just thought it would be cool to use the original 273 if i could. Now I think I will find a 340 or 360 and some commando heads(MP Aluminum, not the original little iron ones)...that way I could still call it a "commando" engine...

Build a 360, it is 20 more cubes and you will not
have to fight with the restoration guys over 340
parts and costs.

Just my $0.02

.040 differance in bore.

The original 273 heads can be made to flow plenty to make 400 hp with the stock small valves.

wow, Dave, thats impressive! And that is with stock displacement? Pretty big cam, I would imagine.

You would need the heads profesionally ported and yes you would need basically the same cam to make 400 HP in a 360 or a 273 but the 273 is just gonna want to spin a lot more RPM to make the same HP, you would need pretty close to 7500 RPM to make 400 hp with a 273. It would be a very fun motor in a light car.

That does sound like fun..but..I want this to be a highway cruiser, too, so cant go crazy on cam and gearing. You've given me alot to think about, thanks again.

if i could get the 273 heads ported really well and then fit a 4." crank like Hot Rod Dave mentioned..THAT might get me to where i want to be for a good cruiser...what do ya think?

ps, custom pistons would be called for, i'm sure..

I did a search and found this article...kinda makes this whole thread moot. Now, I think the best thing to do is NOT build the little 273 commando...just preserve it for posterity(or someone that wants an all original Commnado 273.) If I do build a SB 340 or 360, I will re-use those cool valve covers and air cleaner, tho.

Anyway here's the entire story about what one guy did with his early B'Cuda.


Mopar 273
My Barracuda originally came with a Mopar 273. The 273 was the first engine in Mopar's LA engine family. Introduced in 1964, it was Mopar's attempt at modernizing the earlier A series Polyshphere engines. The new LA series shared the A engines' crankshaft, bearings, connecting rods, timing chain, and vibration damper. The block and heads were redesigned with thin-wall castings, which made the LA engine about 50 lbs lighter than the A engine. The cylinder heads were also changed to an inline valve design that was easier and less expensive to produce than the earlier Polysphere design. Because the 273 was the only Mopar V8 that would fit in the early (1964-1966) A bodies, it was a fairly common option in these cars. It was also offered as a lower end V8 option in some of Mopar's other cars, even in large body styles like the Satelite. Its production spanned from 1964 through 1969.

The Mopar 273 had a stroke of 3.31" and a bore of 3.63". When it was introduced in 1964, it was equipped with a 2 barrel carburetor and produced 180 hp at 4200 rpm and 260 ft-lbs of torque at 1600 rpm. Because of the small size, the 273 needs to rev fairly high to get any decent kind of power. The standard 4 barrel version of the 273 produced 235 hp at 5200 rpm and 260 ft-lbs at 4000 rpm. (For comparison, the lowly Mopar 318 2 barrel engine made 230 hp at 4400 rpm and 340 ft-lbs at 2400 rpm) A special "D Stock" version of the 273 made 275 hp at 6000 rpm and 295 ft-lbs at 4400 rpm. It was the only Mopar engine other than the 426 Hemi to achieve 1 hp/cubic inch.

The 273 had small port heads that it shared with the 318. Intake valves are 1.78" and exhaust valves are 1.50". Because of the small bore, swept volume of the cylinders, and valve shrouding, installing larger valves is not worth the expense on a 273. Another consideration when building a 273 is that heads on the earlier engines (1964-1965) used a different angle for the intake manifold bolts; later intake manifolds won't work with the early heads so if your engine has them you'll probably have to run the stock intake. (Offenhauser used to make several high performance manifolds that fit the early heads, but they are almost impossible to find now)

Another problem with building a Mopar 273 is piston availability. Because the Mopar 273 was never as popular as othe small block Mopars, piston selection is limited. Basically there are only 2 choices: a stock replacement style cast piston from Federal Mogul or custom forgings. The cast pistons are low compression (8.8:1) and availability is spotty. The custom forgings can be had in any compression ratio (within reason) you desire, but they're expensive (especially when you consider the limited performance potential compared to other small block Mopars). Because of this and the other factors mentioned above, I think in most cases building a 273 as a high performance engine is a waste of time and money. Even a well built 318 will give you a lot more power for a lot less money.

There are, however, other good reasons to build a 273. One reason is originality. If you have a Mopar 273, chances are it's because it's the original engine in your car. I think there's a lot to be said for keeping vintage cars as original as possible, especially if you don't want or need the extra performance of a larger engine. Another reason to consider building a 273 is fuel efficiency. Since the 273 is 14% smaller than the 318, in theory you should be able to get 14% better gas mileage (all else being equal). I realize it doesn't work exactly that way in the real world, but the 273 at least has some potential gas mileage advantage over the larger small blocks.

The 273 in my Barracuda was the 2 barrel version. Even though my car is a '66, the heads had the earlier style intake bolt holes in the head. For the Barracuda I wanted a little more power (and at a lower rpm range) than the 273 could provide, so I decided to go with a 318. My original plan was to strip the parts I wanted (the forged steel crank and adjustable rocker arms) out of the 273 and junk the rest. After thinking about it though, I decided to keep it and rebuild it. I'll replace the cylinder heads with "302" castings from a 318 so I can run an Edelbrock Performer intake manifold and save the stock heads and intake for another project I have in mind. With the 273, I'm aiming for "performance with economy" with a heavy emphasis on economy.

At this point, I don't even know if the engine is rebuildable. It didn't look bad on the outside, a little grungy but not too bad for a 43 year old engine that had maybe never been out of the car. When I pulled the intake manifold though I found a bunch (and I mean a LOT) of burned oil and crud in the lifter valley. I don't know if the engine was ran hot too many times, if the oil was rarely (if ever) changed, or some combination of the two. Whatever, it's not a pretty site, and it makes me question the internal condition of the engine. The crankshaft is no big deal because I can substitute a 318 crank (but it will kind of suck replacing a forged steel crank with a cast iron one). If the block won't clean up with a .030" over bore, I won't be building a 273.

Time will tell though, and I won't know until I get a chance to tear it down and inspect it. I hope its rebuildable though. I think it would be a lot of fun to build one of these and see just what kind of performance and economy I can get out of it compared to my Subaru wagon. I know the performance will be better and I'm guessing the gas mileage will be just as good. I think it would be perfect in something like a '64 Valiant station wagon.

More pics:


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Dont do the 273 because of what everyone else says.
Stock there is no kind of power what so ever.
Even for a daily driver its not good, no torque at all.
Aldough i did manage to get my 273 heads with 1,9 int and 1,6 ex flow better than a set of x heads at 145cc. 228 cfm intake and 190 ex.
A 2.02 int valve wouls increase flow a little more but wont fit a 273.

"what are the plans for the car? how much power do you want to make? what kind of mileage are you hoping for?"

since I want it for highway use, I'd like a motor that has the power down lower..so prob a stroked 318 or 360 would be best(?). I'd like milege to be reasonable, at least 15 mpg.
Thanks for the info on the magnum roller motor I hadnt considered the newer small blocks...but that makes sense.