What does "Blueprinting an Engine" mean?

I see this term from time to time, but have no idea what it means. Anybody know?

Basically an engine is built to very tight tolerances, tighter than factory specs. That s my understanding, although my answer is over simplified.

It means machining all the engine parts to the blueprint dimensions. But it is still a vague term because there will always be some error so unless the "blueprinting is defined by the allowable tolerances, then the term is useless.

If you use the blueprint tolerances, then you are simply returning the engine to brand new factory condition. Now if you specify tolerances that are only 10% of the original blueprint tolerances, then you are building a tighter engine. I.e. if the original tolerance for a part is 0.01’ and you specify 0.001", you are building a tighter engine.

Hard to justify calling it blueprinting if you bore the cylinders out and regrind the crankshaft and camshafts to undersized dimensions, unless you have made a new blueprint.

Bearing and other clearances are carefully set, piston and rod weights are matched, cylinders bored to strict roundness, combustion chamber volumes are matched, valve seats and heads ground as precisely as possible, intake and exhaust ports carefully shaped and matched, crankshaft chosen/ground for best possible straightness.
The result is an engine that has the best possible balance under high speed and load.
I have a book by Smokey Yunick that talks about this.

“returning the engine to brand new factory condition.”

Some factory tolerances were pretty sloppy to begin with

Relatively speaking, of course . . .

I was under the impression that it meant when building the engine everything (al parts) was measured and recorded.

ok, check this out.


To get truly accurate timing, the cylinders have to be aligned with the main bearings and crank journals. Without that and a properly aligned crankshaft, your top dead centers (8 of them) could all be off and the ignition timing won’t be perfect either. That is one important step toward getting high engine speeds that don’t tear the engine apart.

All the pistons have to weigh the same. All the rods and wrist pins have to weigh the same. The rods all have to be the same length. The piston pin holes must be in the same places. The lifter bores have to be straight and the camshaft bumps have to be in the right spot. The part of the head that is part of the combustion chamber must be the same volume, all eight of them.

The valves and pushrods must be the same size and weight.

The pistons have to be the same height at top dead center and bottom dead center. There are many factors that have to be accounted for in blueprinting. Standard manufacturing tolerances are OK for stock engines but sometimes can’t give the best performance for racing applications.

I have a completely different take on the term. I think it is commonly used to indicate that the components are within the tolerance - BUT - they are selected or machined to the side of the tolerance that results in some benefit - like selecting pistons on the lighter side of the tolerance (higher rev limit), or boring the block to the higher side of the tolerance (more cubic inches)

Engineers design a motor with a certain level of precision. It’s up to the manufacturing group to deliver a finished product. Production has to assemble XX motors in XX time. Most of the time it results in a good motor but not great. A hand built motor with perfectly machined components and extreme assembly care will result in a smoother running motor

Generally speaking, it’s a cool-sounding term thrown out at automotive B.S. sessions…

Back in the day, automotive engines were mass-produced by hand and assembled mostly by hand…Some of them could be pretty ratty. Enthusiasts of high-performance cars would tear down their engines and “blueprint” them…Build them to very carefully controlled tolerances of fit, timing and balance in hopes of gaining improved reliability, performance and engine life…

It originated at a time when automotive tolerances were very loose, and you could produce a “better then new” engine by getting everything to fit perfectly. It did cost some money but it allowed you to win races with a legal engine.

When GM firts started exporting finished cars to a prospering Japan, they were so roughly assembled that they had dealer body shops take the cars apart and reassemble them! In this case not the engine was involved but rather the rest of the car.

I sort of understand blueprinting from the point of view of racing engines. (Although I don’t exactly know what the end result is, other than presumably a set of static measurements.) But other than wanting to build an engine intended for racing use, is there any benefit to blue print an engine you are rebuilding for your every-day driver? Do people blue print their Toyota corolla and Honda civic engines?

Nope, they would typically just check that everything is in spec, not that they’re to identical and optimal spec.

But would a ‘balanced and blueprinted’ Civic engine be noticeably smoother? I bet you could notice the difference, back to back.

I’ll mention it again: I think the use of the term implies that there is some benefit - and the benefit is carefully machining and carefully selecting components such that they are on the positive benefit side of the tolerance, rather then just within tolerance…

Caddyman is right in that when engines were mainly assembled by hand…there was a wide range of variances between engines. Today those variances are much much smaller. Most engines (if not all) would be considered blue-printed engines as by the definition of 30 years ago.

I’ll agree with CapriRacer. Back in the day, it meant trying to hit the specified value on the design prints, not just being inside the tolerance limits.

Funny story (at least to me)- years ago I had a friend that decided to pay someone to blueprint his 327. We thought he was wasting his money. A friend who was an engine guy offered up a challenge. He would take another beater 327 and just rebuild it, paying no special attention to detail. One step up from a spray can rebuild :wink: When done, they would attempt to balance coins on edge sitting on the intake manifold. The winner would be the last coin standing. Both engines ran smoothly but the slapped together engine won the challenge when the dime fell over on the blueprinted engine. Naturally, there were other factors that didn’t make it exactly apples to apples (carb, cam etc). But in the end, no one could tell the difference between the two by the seat of your pants.

@CapriRacer has the right definition. All the parts are within spec, as equal from cylinder to cylinder as possible and skewed towards the “good” dimension. I used to race in a class called Showroom Stock in the SCCA. You were supposed to run it like you bought it. Yeah, Right! I know of one engine builder for V8 Chevies that charged $18,000 (in the 90’s) to “blueprint” the engine. Longer rods, short blocks, “good” cylinder head castings, looser pistons, longer stroke (0.013 inches more) and more to boost the 215 factory HP to about 300. Re-flash the computer for higher revs, more spark timing and richer mixture under full throttle. He had contacts for raw castings that he machined “his way” to make more HP. Worth every penny to win a $10 trophy

Assembly line cars are not getting a balanced and blueprinted engine in the truest sense of the words. They’re getting an assembly line close enough spec.

It seems like there would indeed be an advantage to hand-select the fast moving items such as pistons, rods, etc, so each weighed as close to identical as possible. It wouldn’t matter so much whether a rod weighed 2.00 pounds or 2.05 pounds, if the spec was 1.9 to 2.1 (just making these numbers up for example); but I’d guess there would be an advantage to the long term life and performance of the engine if one of the rods weighed 2.05 pounds, all the other ones weighed 2.05 pounds too. I can see that.

There’s something similar here in Silicon Valley. Transistors are made sort of the same way you make cookies. Mix the ingredients and bake them. Just like cookies, you get slightly different results no matter how hard you try to do everything exactly the same as the last time. But within a batch, the cookies are pretty much the same. Same with making a batch of transistors, if they are integrated on a single chip. So electronics guys – the ones here who design the circuits – they decided to take advantage of that effect. If all the transistors being used for a DVD player circuit for example are baked from the same batch, the transistors may deviate from their ideal spec, but even so, from the same batch, they will have close to identical specs for that DVD player’s circuit. They’ll be matched to each other in other words. So with integrated circuits, it’s possible to build a circuit that performs much better than if the transistors were from different batches. Sort of like engine blueprinting in a way.

Thanks for everybody’s comments. Interesting discussion.