Olds 403 Engine Info

The 1979 Pontiac Trans Am:

2 "Big Inch" engines were available, the 400 and the 403:
The Oldsmobile 403's were hooked up to automatics. 
The L78/W72 Pontiac 400's were matched up with a four-speed.
The 403/auto combination was the most popular by far selling 94,773 units. The coding for these engines were QE, QJ, QK, QL, Q6, TD, TE . Most 403's left with either 2.41, 2.56, or 2.73 gear ratios for fuel economy reasons which hindered the launch considerably, although the big-inch torque did offer good midrange acceleration.
The 403 V8 is easily identified by the oil fill spout growing out of the front of the engine. 

General Information:

  • Bore: 4.350"
  • Stroke: 3.385"
  • Rod size: 2.50"
  • Main Size: 2.125"
  • Compression: Most 403's: 4 bbl 8.5-1
  • Horsepower: 175
  • Torque: 310 ft/lbs
  • Head ID: 4A Matches the 4A or 4B block. '77 - '79
  • Part #: 554717
  • CC's: 83
  • Piston Compression height: 1.615
  • Wrist pin diameter: .980
  • Easy way to ID a 403:
    • The Block casting number is found behind the timing cover between the oil fill tube and the oil sending unit.
    • All 403's have 1/2" Head bolt holes.
    • Also all 403's have the 403 in large numbers cast into the side of the block.

Torque Specs.

  • Mains:
    • (#1-4) 80 ft/lbs.
    • (#5) 120 ft/lbs.
  • Rods: 42 ft/lbs
  • Heads: 95 ft/lbs.
  • Rocker Arm bolts: 28 ft/lbs.

The 403 was made from 1977 to 1979 and used across the Buick, Olds, Pontiac and Cadillac vehicle lines. It was also used in other applications like motor homes, boats and some industrial applications. It was 'sort of ' designed to replace the previous use of big block engines in the BOP line.

Building a 403 Pontiac T/A Street Machine:

  • If you are looking for a super reliable street engine that will squeeze the absolute maximum torque out of 87 octane. cheap to buy, pollution legal till 1979, minimum weight package that is a direct bolt in (including weight) for any Olds small block, try a 403.
  • Try this -  Free flowing 2.5" dual exhaust, Headers if available. Nice aftermarket aluminum intake & carb combo. Hotter cam with stiffer valve springs, the usual tricks. Improve the oiling system. Re-curve the HEI distributor along with hotter coil package.
  • All 403's have windowed main webs. It is rumored that a number of blocks have full main webs (no holes). Never verified and I have never seen one. In reality, however, you're not likely to blow out the bottom end of this motor on the street below 5800/6000 RPM.
  • The 403 has Siamese bores (making overheating in this area an issue). All the other Olds blocks have water jackets between the bores.

403's used ½" head bolts; all others used 7/16" head bolts. Thus the use of any other head requires reaming out the holes. No big deal.

Quantity of 403's Produced: 2500
Block Casting Numbers: 557265-4A, 553990-4A, 554990-4A

Blocks with these casting numbers were used in certain 1977 Buick Station Wagons built in 1976, the Pontiac Bonneville with tow package, the 1979 Pontiac Trans Am with tow package (w/block casting #554990-4A), and in 1977 Olds 98's. Might also be found in 1977 Tornados. The Block casting number is found behind the timing cover between the oil fill tube and the oil sending unit. 
All "4A" blocks don't necessary have solid main webs. Part #554990 has been found to have open webs. It's possible that 1977 models built early in the model year, or fall of 1976, are likely to have solid main webs. But never confirmed to me at this point.
Big pistons and a short stroke are an excellent combination for a high-RPM engine, but the other inherent 403 weakness comes into play. Windowed main webs, and to a lesser extent, Siamese cylinders. The bottom end of this block is not suitable for extended use at high rpm with very high cylinder pressure.

Other sources for 403 Engines:
1977 Olds Cutlass, Vista Cruiser, Delta 88, Custom Cruiser, 98, Tornado, Electra 225, Pontiac Bonneville
1978 Olds Delta 88, Custom Cruiser, 98, Tornado
1979 Olds Custom Cruiser, Olds 98, Pontiac Trans Am.

Solid or Windowed Main Bearing Webs:
All small block Olds engines produced after 1977 (with a few exceptions), have windowed main webs, with exception of the diesel 350. Oldsmobile re-tooled the engine foundries in early 1977. This is the reason that you see so many 350 Chevy engines in 1977 Delta 88's. There was a shortage of 350 Olds engines, and the Cutlass line was selling like crazy, so those engines were reserved for that line.

403's are usually the engine mentioned when windowed webs are discussed because there are rumors and some sightings of solid main web 403's. Solid main webs would mean a much stronger bottom end. As far as 403's go, some people doubt that a solid web engine was ever made. According to Mondello, 2500 solid web 403's were produced in 1977 and they were used in big Buick and Olds wagons.

You can determine if you have a solid web by looking at the earlier serial number.

Supposedly 403 solid main web blocks are found in the Pontiac Bonneville with the towing package, and in some 1979 Pontiac Trans Am's with a towing package. Then again, for all but 600HP race motors, it's not necessary; the metallurgy of the 403's is quite well done.

While assembling a few of these 403's, I have noticed that the oil holes in the aftermarket bearing shells don't line up with the angled feed hole in the block. Some were actually covered up with less then 030 opening!!!!!!! Seems the factory didn't center the feed hole in the housing to the block or bearing centerline. I recommend either elongating the hole in the bearing shell or block.

The best option is to take a die grinder and elongate the feed hole in the block to line up with the bearing.

Siamese Cylinders:
Horsepower and compression limitations are due to the Siamese cylinders and windowed webs. You must insure that the cooling system is large enough and in good order. I would not recommend one for an ultimate dragstrip build up and I would avoid doing anything that greatly raised the compression especially if the block was over bored. Head gaskets blowing would be the concern here.

There are a few different ways you can boost up the compression, which would help with a hotter cam. You can shave the heads, which makes the combustion chamber closer to the piston head and might require a manifold or intake side of the head cut also. Then you have the Earlier 350 Olds heads. See Chart below. You can buy pistons that can kick up the compression. Most pistons have two relief cut into them, which eliminates the possibility of having a piston slamming into a valve. You can buy pistons with smaller cuts in them, which raises the compression. Just make sure you check piston to valve clearance. Otherwise your piston and your exhaust valve will get married, and you'll be buying them one expensive present! 

RPM Maximum:
The 403 is a mediocre all out performance engine. I would work a combo in the 5200/5500 max rpm range. The main bearing webs are the weak link and the engines can usually accept only a .040 overbore. They are thin. Danny Lattimore has a stock class 403 in a Trans Am that turns 7500 RPM. So things can be done but at a major cost.

But First of all, you would need a forged crank.

Add in a wicked cam, large valves, strengthened valve-train and pushrods, and a set of "REALLY" stiff valve springs.

Theoretically speaking I suppose it could be done, since the basic geometry of the 403 is pretty decent.

You could probably get 8000rpm out of a 403 if you:
Start with a solid-main web block (if they in fact exist), completely blueprinted, and filled the lower water passages with block hardener and use a full block girdle. O-rings would be a definite "MUST".

Use "Ultra" lightweight forged pistons, and Aluminum Rods. A billet crankshaft, but if you like living on the edge, a factory 330 steel crank could be used, though I'd want to magnaflux it about every 10-15 passes. Batten heads, "heavily" ported, with big valves, might just get you to 8000rpm. Sheet-metal intake, with at least 1050 cfm. Big tube custom headers. Crank-triggered ignition. A .700+ lift roller cam, shaft rocker assembly, titanium valves, External oil pump, perhaps dry sump, or at least a stock system with the standard modifications and external drain-back lines. It would be good to have more than 8 quarts of oil circulating in there. The big problem would be finding a solid-main block, without which this buildup would simply be a kamikaze run (i.e. you'd see 8000 rpm with this motor... once LOL).

And If you could find a suitable block, you could probably put together the above motor for $20,000 or so. It might produce 800-900hp, and have an effective power band of 5500-8500rpm. Wouldn't be that much fun to drive a car with that much engine on the street, though.

On a side note, Dave Smith built a 403-based engine to try out a trick main girdle to strengthen the bottom end on those lightweight blocks. He cross-bolted the custom main caps, supplemented with studs at the original 2 bolt locations. He brought it out to Orange County on a test day and had Tom Chelbana run it a time or two. That's all it got -- the girdle worked, but the webs inside the block broke. I don't think Dave ever tried another 403 that way, the diesel block's are much better and his main caps for it are the only way to fly. 

Block Boring:
An Olds 403 can only be bored out 0.040" (.020 recommended, see footnote below) before problems with cooling can occur. The Siamese cylinder layout (no cooling jacket between cylinders) and the minimal iron between cylinders is the cause of this limitation.
A 403 should not, at least for a car that sees any kind of traffic or distance driving, be bored .060 over. With a bored 403, or even stock 403, I would keep the cooling system in A-1 condition. Drain cooling system, radiator, block drain plugs, etc, at least every 2-3 years. I recommend using a cooling system additive that promotes the thermal transfer of heat from the engine to the coolant, such as **Water Wetter**. Use a 6 or 7 blade fan, radiator shroud, 3 row or more radiator, and keep the lower air dam in place. If you can get away with it in your climate, use a lower 20/30% ratio of coolant to water to enhance the performance of the cooling system additive. A thermostat rated at lower than the stock 195° unit might help as well. But the real problem is the ability and capacity to transfer heat than overall temp. The ability and capacity to transfer heat directly affects overall temp.

**** Hastings Corporation Footnote **** 
Re-boring the Oldsmobile Built 1977 - 1979 403 Engine. The General Motors Corporate Engine 403 CID built by Oldsmobile cannot safely be re-bored beyond .020 oversize. There are several reasons that attribute to this condition.

1. The walls between the cylinder are very narrow, resulting in a minimum load area for cylinder head gasket sealing.
2. Cooling holes in the cylinder head are very close to the combustion flange of a gasket with a standard gasket bore diameter.
3. If the block is re-bored more than .020", the standard cylinder head gasket may fall into the cylinder bore chamber. This condition may cause premature gasket failure. GMPD Hastings does not offer pistons or rings above .020" oversize.

Head Gaskets:
It seems that the 403's need a constant flow of coolant to keep everything nice and square; when you let it sit, the nice cool (relatively speaking) radiator fluid keeps that thin bit of iron between the Siamese bores somewhat straight, but when it's shut down, the heat from the engine evens out, and the cylinder walls perhaps distort. When you give it full-throttle when it's hot, the higher cylinder pressure can blow out the gasket between the cylinders.
I recommend O-ringing any major over bored 403's with small-chambered high-compression heads when you're doing other machine work, unless you don't mind an annual head-gasket change.
I recommend replacing the factory used head bolts with new ARP units. Anything will help. The big bore/ big piston are part of the 403's weakness. The large bores leave little room for the head gasket, which limits the amount of cylinder pressure you can develop before blowing a head gasket. I also recommend Copper Coating the head gaskets.

Short Block:
O-Ring (option), but make sure the deck is true!!!
Deburr and bottom fill the block for strength
330 early forged crank (if you can find one) for operation over 5,500 RPM. Balance the assembly.

Other Olds Connecting Rod Center to Center Lengths:
260 6.0"
307 6.0"
330 6.0"
350 6.0"
403 6.0"

A stock 403 Olds engine has tons of torque. Intake and Carburetors aren't the problem. The problem is the heads. Their combustion chambers are too large, around 83ccs. This results in a compression ratio around 8.0 to 1. Try to find a set of 350 heads from 1973 or earlier. This will raise compression enough for premium gas. Before you think about a carb I would think about an intake manifold change to an aluminum one. Big weight difference!!!, if yours is stock.

If you add 72 or earlier 350 cylinder heads, your compression will be abound 9.5:1-10:1. It will increase your HP considerably, though you'll probably want to add a cam that can take advantage of that higher compression. If matched with an Edelbrock intake, I'm guessing around 300 hp, though that's a really rough estimate and depends on the parts that you use. Most 350 heads will boost compression.

Calculations say that un-machined 64 cc heads such as '68 to '72 350 heads would boost the stock 403 to 9.5-1 compression. Mill the heads 0.030" and you have just over 10:1 Compression Ratio. This is with a 0.040" head gasket.

Get '72 350 heads [hard exhaust seats], install the 403's bigger 2.000 valves or even larger W30 2.072" valves, and limit your expenses. That motor will kick ass with the appropriate cam, intake and exhaust. As for those who doubt the ability of the 403 to handle power with its open main webs, you might call a racer who uses one. Such as; Don Dagget 517-886-9295 (might not be correct). Mark or Dan Van Koevering 616-453-0497 or 956-3378 [extensive experience]

The stock 403 heads have 4 additional coolant holes drilled for additional cooling, because of the Siamese bores. You must drill these additional holes in any 350 heads for the additional cooling potential and additional protection to keep from blowing head gaskets. This advice comes from a number of experienced Engine Rebuilders and Oldsmoholics.

A high compression 403 will not shatter a stock piston, unless you are running nitrous. The "7a" heads only give you about 9.5:1 compression, and the factory high-compression 350's came with 10.5:1 and cast pistons, so you're pretty safe. Just have it balanced, and keep her under 6000rpm.

Any 350 head will raise compression a little to a lot. Depends on combustion chamber size.

A set of 1968 to 1972 heads will raise compression, and allow the engine to make more power.

Use the 2.00" intakes from the 403.

Oversize the head bolt holes for the 403's 1/2" head bolts. You'll have to use premium gas, and perhaps re-curve the distributor.
As far as heads are concerned, my personal choice would be a set of big block heads with big valves. Given that the 403 small block obviously moves as much air as the 400/455 big block, why strangle it with small block heads?

Adapting the big block heads to the 403 requires a couple of things:
You'll need to open up the 7/16" head bolt holes in the heads to fit the 1/2" head bolts used on the 403. The Edelbrock Performer small block intake has enough meat to allow the runners to be ported to match the larger big block head intake ports. You'll either need to do this yourself or pay someone to do it.

Edelbrock aluminum heads would be great, but that's an additional $1500. Stock big block heads with 2.07/1.625 valves will be more than adequate for this motor, especially if you can do a simple cleanup on the exhaust ports to remove the A.I.R. bumps. Also, have the exhaust flanges milled or welded and milled (as required) to ensure that the divider between the center two exhaust ports goes all the way to the flange surface. You will also need to run the numbers on true compression ratio with your pistons, head gaskets, and head chamber volume. Most stock big block heads run about 80 cc in the combustion chamber, which will be a little large for your 403. Of course, milling the heads will require you to check pushrod height and possibly running a non-stock pushrod length. If you're planning to do some work to the heads (milling and exhaust port cleanup), the smog-motor J heads will be fine. These have hardened valve seats and if the exhausts are ported will flow the same as the harder to find (and priced accordingly) C heads.

Oldsmobile Head Identification Info
ID/CodeYear(s)CIDValve SizeCCsCasting NumbersNotes
4A77/79403***835547171/2" head bolt holes
3A77/80350***755547161/2" head bolt holes

403 With a 425 Crank:
Tool up for some 403 castings with a diesel bottom-end to take the 425's crank. A 425 crank with cut-down journals being dropped into a 403 block. A 500 cu.in.+ SMALL BLOCK!

The 403 engine has the late style crankshaft bolt pattern.

403 Engine Buildup:
Actually, it seems to me that the 403 should be a kick-@$$ motor. The bore/stroke ratio of this thing is incredible. The thing has a bigger bore than a big block Chebby. If you relocated the valve centerlines in the heads, you could run some impressive valve diameters, with the result that you should be able to flow an incredible amount of air. Think about it: many on this list sing the praises of the short-stroke 400 motors - here is essentially the same motor with a better bore/stroke and a smaller/lighter block. Why aren't there more of these out there kicking butt? The ultimate Olds motor might just be a 403 (yeah, good luck finding a SMW block - anybody thought about welding up the windows and doing an align bore?) with a forged 330 crank and Edelbrock heads with relocated enormous valves. The smaller bearing diameters would also reduce bearing speed and thus friction.

The 403 responds well to a bit more camshaft and compression. Using earlier 350 heads ('68-'72 will do it, but the '72's are the best) to raise the compression, and an Edelbrock Performer-grind cam (these cams are available from other sources than Edelbrock, who I suspect purchases them from another supplier).

A true-dual exhaust really livens up the motor; the factory cross-member from a pre-'75 (year of mandatory catalytic converters) allows true duals without excessive bending, though you'll have to check with emissions laws regarding keeping or discarding your catalytic converter.

The 2400rpm stall converter in front of a TH-400 improves the launches of the car.

Here are some results:
A Stock 403 from 1978 Tornado, new timing chain, Jacobs ignition, diesel sized 4" tall K & N air filter (extra low Tornado bottom half, tall diesel top half), dual 2 1/2" pipes with crossover and no cats, switch pitch transmission with electronic controller, 2.41:1 axle in a 4100 lb RWD car. Measured many times with a VC2000 computer: 0-60 7.49sec; 1/4 mi at 91.2 mph in 15.86 sec. The slow time reflects the axle ratio; the car has gobs of power between 70 and 100 mph. Estimated 270 horsepower by modern measurements by comparing it to the published times and weight of many modern cars It is a very practical street car which has seen 18/20mpg.

Buildup Examples:

  • '87 Cutlass: Olds 403 in a 87 Cutlass: 
    Bored .030 over
    72 350 heads
    Edelbrock performer RPM intake
    Crane cam/lifters/springs; (284/292 advertised duration, with .480/.496 lift)
    True roller timing chain
    Cast pistons 9.5:1 
    ***With not-so-expensive upgrades, this combo ran mid 13's.***
  • 1979 Pontiac T/A:
    403 block bored .030 over = 409 cu.in.
    403 crank ground .010-.010
    Stock resized rods w/ARP bolts
    Arias forged aluminum pistons, 10.5:1 
    Crane Cam .480/.496; (280-290 advertised) lifters, pushrods and springs
    '72 350 (7a) heads
    Edelbrock #7111 performer RPM Intake
    Carter 750cfm AFB 
    Mallory Unilite distibutor w/ Accel supercoil
    TH-350, stock converter, shift kit.
    3.08 posi
    *** Runs low 13's ***
  • 1979 H/O (Hurst / Olds):
    Built Racing Head Service 403
    Mondello JM 22-25 cam
    W-31 heads 
    Edelbrock Performer intake 
    Carb Shop rebuilt quadrajet 
    Holley electric fuel pump 
    Turbo 350 & B&M 11" converter 
    3.23 Auburn posi unit 
    Hellwig swaybars
    new dual exhaust
    solid body mounts
    fiberglass cowl induction hood
    Hurst dual gate shifter
    original Hurst wheels w/ good BFG tires
    *** Runs VERY strong at 13.10@105 mph in the 1/4. ***

403 CID on a $500.00 Rebuild:
Well, for that price, you'll have to stick with your stock shortblock, and the stock intake, carb and ignition will have to serve. Under that budget, however, you might be able to address the two main weaknesses of the 403: the cam and the compression. A set of 7a heads from any '72 350, with your 403's stock 2.00" intake valves, will raise the engine's compression and improve its exhaust flow. The purchase price and machine work will cost you around $400.

A decent cam like the Mondello JM18-20, the Edelbrock Performer or even the '68 Hurst/Olds cam with new lifters and springs with a new timing chain, will make a noticeable difference as well.

In all the above things, the details (such as degreeing the cam, port-matching the heads and manifolds, etc) are the most important thing, and will maximize the gains you realize from these modifications. Needless to say, the above price estimates will not include labor, and neither of these mods are "bolt-ons", but they will be the most effective.

Any other mods above and beyond the heads and cam would be geared toward improving the durability of the engine, but wouldn't necessarily contribute toward power output in any dramatic way. Those kinds of mods will go way beyond $500, though. (things such as forged pistons, oiling system mods, rod and crank preparation, balancing, etc.)

1977 Engines:
Engine Type Car Series Description Code Vin code
V8-403 (Oldsmobile) A, B, C High altitude stds U2 K 
V8-403 (Oldsmobile) A, B, C High altitude stds U3 K 
V8-403 (Oldsmobile) B, C Fed. stds UA K 
V8-403 (Oldsmobile) B, C Fed. stds 
V8-403 (Oldsmobile) B, C Calif. stds UB K 
V8-403 (Oldsmobile) B, C Calif. stds VA K 
V8-403 (Oldsmobile) B, C Calif. stds VB K 
V8-403 (Oldsmobile) B, C Calif. stds VJ K 
V8-403 (Oldsmobile) B, C Calif. stds VK K



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