Engine Machining 101: Getting Started With Your Engine Build
From a practical perspective, going to a machine shop is like going to a marriage counselor. Seriously, and we’re not trying to be funny here, but there is a relationship between what your goals are and what your pocketbook and equipment can produce. Like any good counselor, the machinist should help you navigate through the tough problematic areas between wants, needs and capabilities. There is a most definitely a chance to save the marriage between man and machine, and that road leads through a good engine machinist.
Finding the Right Machine Shop for You
Not all machine shops are the same just as not all engine builds are the same. When selecting a machine shop, the engine builder should consider the things that are most important. If you are looking for a quick turn around time, a close machine shop that has less of a back log is probably a better selection or deserves more consideration in the selection process. A machine shop that has less of a back log in work deserves a little more investigation. Don’t automatically assume that a shop that is less busy than others in the local area does bad work. The shop may be newer and has not built it’s reputation yet. Most machine shops do very little advertising and rely on word of mouth advertising between racers to help their business.
Find a machine shop that is familiar with the equipment that you are working work. Checking specs and knowing the tolerances is extremely important. This is what you are paying for, expertise.
Find out about the machine shops in your area. Experience says a lot about machinists, and those that have good reputations have very loyal customers that are only too happy to tell you how great their machine shop is. Ask around about the type of engine builds that the shop normally performs. A shop that is dedicated to doing machine work on daily driver 4 cylinder street cars may not be on the cutting edge of top fuel dragster engine technology. Likewise, a machine shop a couple of hours drive away may not be aware of the rules at your local track. Through lack of familiarity or experience, they may machine your engine outside of the rules and cause you a great deal of grief when you get disqualified from an event. This can be prevented by asking if the machine shop has experience in your type engine application.
Take a Look
Before committing to a machine shop, it would be prudent to visit the shop and take a look around. A shop that is in disarray is a warning sign, especially if you are bringing your own parts for the build. Parts can be misplaced or lost causing delays in your engine build and damaging the trust between you and your machinist. Organization in a machine shop is a big deal. A dirty shop can also alert you to problems. Ideally, a machine shop will have separate areas for disassembly, cleaning, machining and assembly. In the assembly area, the shop should be neat and clean. Dirt is an engines worst enemy. A machine shop that has a dirty assembly area is asking for shortened life span on your internal engine components.
The Next Step
Once you have found a machine shop that has the experience, the right equipment and is within your comfort level, you should talk to the machinist about your engine build. The more that you understand about the machining process, the more intelligent questions you will be able to ask. Asking the right questions and getting the best answers will assure that you will be happier with the end results. A good machinist will help guide you through the process of selecting the right parts and making the best machining decisions to meet the goal that you are trying to achieve with the build.
Understanding What the Machine Shop Needs to Know
We talked with Keith Clark of Rancho Performance Machine in Temecula, California. Over a twenty four year career of building race engines for the inland empire region of Southern California, Keith’s shop has built a solid reputation in the offroad, circle track and drag race communities. We asked Keith how he has been successful. “When a customer comes in and wants machining work done, we try to find out certain things about his project to ensure that he gets what he wants out of the final product.” Keith went on to explain “there are three basic areas that we want to discuss before any work is done. What is the application, how much horsepower does the customer want to make, and what kind of budget are we working with?” According to Keith, “we use different machining techniques and procedures for different types of applications. For an engine that is being built for the circle track, we tend to machine the components more for endurance than we would for a ultra high horsepower drag strip engine that requires taking the tolerances to the edge of machining.”
Keith explained that a good machine shop will sit down with a customer and go over the entire engine build and cover all the options as well as the benefits of each option. Based on the budget a customer has to work with and the target goal of the rebuild, the machinist will suggest a combination of parts and procedures that will get to the desired horsepower for the application that the engine is intended for.
Our Project Build – a 355 Chevy
In the case of our project engine build, we had a modest budget and a set of track rules to work within. Beginning with a common small block Chevy 350 engine block with four bolt mains, and a set of stock Vortec 061 heads. Keith recommended that we start with some basics. Opening up the oil drain back holes at the front and back of the block in the lifter valley was pretty much standard. Because the block was an experienced (used) block, Keith recommended a good cleaning and degreasing followed by thorough inspection and magnaflux process to check for cracks. A good inspection is essential early in the machining process to find any show stopping problems before any money is spent on parts. If a block is cracked and unusable, the parts that you have already bought may not work in a new block. The cleaning process is also a must. Even more than aesthetics, the cleaning removes buildup of gunk and crud from oil galleys and water passages.
Starting the Work
Our block checked out fine during the magnafluxing and inspection process, so we met with Keith to discuss the next steps. Tapping the oil galley plug holes in the front of the block is another standard procedure. It’s unlikely that the press in plug will blow out in a circle track engine, but why take the chance?
SBC Blocks have press in oil galley plugs on the front of the engine. Even in a "hot" street engine they typically won't blow out, but why take the chance?
Pipe plugs are fitted into the oil galleys. The length of the middle plug is critical because it could block the oil port for the #1 camshaft bearing.
Running a thread chaser through the head bolt threads is another one of those common procedures that should not be forgotten. Keith reminded us that “yes, there is a difference between a thread chaser and a tap. A thread chaser will re-form the threads while a tap will cut threads”. Make sure to ask your machine shop to use a thread chaser in the head bolt threads.
Keith’s shop removes the oil galley plug at the back of the block near the oil pressure galley with wax and a torch. These plugs must be heated up for removal because they are installed at the foundry when the block is still hot. As the block cools down, it secures the plug into the block. The only safe way to remove the plug is with a torch.
This is Boring
By this point the machinist will have inspected the block enough to tell how much your cylinders need to be bored in order to clean up the cylinder walls. This is critical when ordering pistons for the build. We were shooting for 0.030 over boring on the cylinders and oversized pistons to match. Your machinist will need the new pistons to check the fit and decking of the block, so ordering these in advance will speed up the turn around time. Talking with your machinist before ordering any parts will prevent ordering the wrong piston size which will cause the process to come to a grinding (or boring) halt.
There are several types of cylinder boring machines on the market, and all of them will work successfully provided the machinist does a good job during set up. Our machinist Keith explained that “ many people don’t realize how much time the machinist spends setting up the equipment to get the machining work done in these precise measurements.”
Keith went on to show us what he considers one of the more precise boring machines. “Our Rottler unit is designed for a long life of heavy use. They mount the block off of the crankshaft bores which make it a very accurate machine. Getting the cylinders bored exactly 90 degrees perpendicular to the crankshaft is extremely important. The boring machines that mount to the top of the block can sometimes angle the cylinder bore to the front or back of the block. This puts unwanted load on the pistons and can cause early failure”. Keith even goes a step further when setting up the boring machine. Using a wet stone, he hand stones the oil pan rails at the bottom of the block and the top surface of the block to ensure that there are no burrs or gouges in the metal that may prevent the block from mounting completely flat in the Rottler machine. Keith reminded us that “there’s no such thing as being too accurate”.
Using a boring machine that registers off of the main caps is probably the most accurate for straight bores.
Cutting cylinder bore in the middle and then jumping one bore to the outside of the block helps keep the temperature down between bores.
Once the boring tool tool has passed the entire length of the cylinder bore, the new bore is shiny new and about 0.005" away from the final bore size.
Surfacing the Block
If you are trying to build a dependable leak free engine, you will probably want to resurface the block. This helps on a number of levels. Primarily, you can prep the surface for whatever head gasket you are going to use. For many years gasket manufacturers claimed that a roughness average of 55 to 110 micro inches (50 to 125 RMS) was acceptable. But that was during the age of cast iron blocks with cast iron heads. As the block castings have become less rigid, flatter and smoother mating surfaces have become more important. The current wisdom is a surface finish of 30 to 110 roughness average for cast iron and recommendations for aluminum heads on bimetal engines range between 30 to 60 roughness average finish. These smoother finishes help the gasket’s ability to achieve a positive cold seal and maintain a long lasting leak free seal.
If you are trying to squeeze as much horsepower out the engine as you can and maintain durability, you will probably want to “zero deck” the block. Building a competition race engine requires a machinist to use precise measurements on block deck height, crankshaft stroke, connecting rod length and compression dimensions that will work in perfect harmony together to produce the most power for the application. One of the cornerstones of this process is controlling the distance that the piston is above or below the block deck surface. The further the piston is below the deck of the block, the move volume it adds which reduces compression. Figuring in the head gasket thickness as added volume also, this compression reduction can add up to a considerable amount. Most high performance engine machinists will try to bring the piston as close to the top of the surface as possible. This is zero decking, and if done properly the only clearance is the head gasket thickness in it’s compressed state. According to our race engine machinist Keith, “increased compression can help all the way around. From low end torque, coming off the corner’s faster to higher rpms. Compression can be a good thing.”
When "zero decking", the machinist will measure piston height to the block deck and shave the deck to the point where the piston top is flush with the block deck.
Most machinists will bore the cylinders to within 0.004 or 0.005 of the final bore size to allow for the material that honing will remove from the cylinder walls. The actual method and honing procedure varies from shop to shop, but Keith explained that “your machine shop will need to know what type of rings you are going to use so that the proper bore surface finish to apply so that the rings seat properly”. Most ring packages that come with race pistons are manufactured with materials that require none or very little time to seat.
Honing with torque plates used to be reserved for high horsepower blocks only. Studies by several gasket companies and aftermarket engine block manufacturers have proven that even a street engine benefits from torque plate honing. Using a thick plate that is torqued down on the block with head bolts simulates the distortion created when the cylinder heads are torqued on the block. Torque plate honing allows the machinist to get the surface as close to perfect cylindrical perfection as possible.
Using a torque plate will allow the machinist to duplicate the torque load on the block that creates distortion in the cylinders.
Torque plates are manufactured to allow the hone to pass through the plate while still providing the torque pressure against the block.
Align Boring and Honing
If you plan on using an aftermarket crankshaft or a nice trick set of engine bearings, you probably want to consider getting the block align bored and honed. If the goal is to build a high horsepower motor, align boring and honing is a must. Keith tells us that “a lot of customers believe that align boring and honing is a step that you can skip to save a few bucks in the machining budget, but I disagree. Tolerances are very tight in these areas and if you are out of specs far enough where you have metal on metal contact, the engine will eat itself in no time”.
Basically, align boring is measuring the inside diameter of the main journals and machining them to the correct measurement. Then a long boring tool is used to bore the journals in relation to each other and finished with a honing tool. This procedure will ensure that the crankshaft will rotate freely and with less parasitic friction.
To sum this up in a couple of words; A Must. Even a mild street engine with stock replacement parts will benefit from balancing the rotating assembly. Balancing will ensure that dynamic components are compatible with each other. This is cheap insurance if you are mixing aftermarket components from different manufacturers. The basic idea behind balancing the crankshaft is checking the weight of the pistons, rings, rod and wrist pin against the counterweight of the crankshaft. Our machinist Keith explained that “an internally balanced crankshaft can be balanced without the flywheel/flexplate or balancer but an externally balanced crankshaft must include these in the balance process”.
Summing it up – Top 10 Things You Need to Know about Dealing with a Machine Shop
There are some clear advantages to understanding what services you are paying your machinist for and it is obvious to us that talking with your machine shop operator will help you avoid some common pitfalls that can be real show stoppers.
- Find a reputable machine shop that has experience in the type of engine application that you are working with.
- Work with the machinist on the engine build plan, the parts that will be used and the budget that you have to work with. You’ll be surprised how much time and money a good machinist can save you in wrong parts or parts that won’t work well together.
- Always get the block cleaned and inspected before ordering any parts.
- Bring the new parts in for the machinist to measure. Tolerances are very close on internal engine components, and a machinist will need to know the specs on your new aftermarket engine components to make sure they fit properly with the machined block.
- Understand that your machinist spends a lot of time measuring and checking clearances. Rushing your machine shop operator will limit the checks and double checks that machinists normally do which can end up costing you horsepower or engine life.
- Get the block surfaced and bored. It’s never wise to try and get by with a power honing. It’s best if you get the cylinders bored with a machine that references off of the main journals.
- Have the bores honed with a torque plate. This will ensure a more cylindrical bore with the heads bolted on the block.
- Don’t skip on the align boring and honing, especially when you are trying to make a dependable engine.
- Never bypass the rotating assembly balancing. Getting the crankshaft balanced is a must if you are using aftermarket parts.
- Build a good relationship with your machinist. A machine shop that earns your trust is worth being loyal to.