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When an off-the-shelf slug won’t do, you need to turn to the custom boys.
by Mike Mavrigian
photos by author
Need a set of custom slugs to work with your new heads and your bore/stroke combo? A ton of info is required before a piston manufacturer can begin to execute your order. By the very nature of a custom order, the piston manufacturer is usually able to provide an engine builder with exactly what they need to achieve sizing, compression ratio and clearance goals, but they can’t work in the dark. Detailed pre-planning is critical.
Today’s custom forged aluminum pistons are CNC machined from precision-forged 2618 or 4032 aluminum alloy slugs. Due to the precision and consistent duplication of computer numeric control processing, piston dimensions (within a set or for future replacements that reference the original order) should be exact matches in every detail, including weight (usually +/- 1-2g).
All custom-piston manufacturers require that you fill out a custom-piston data sheet when placing an order. You can call and obtain a custom piston sheet directly from the piston manufacturer, or you can download a sheet from their Web site. Once this sheet is completed, the piston manufacturer will have all the information required to begin your order.
Basically, fill out as much of the piston data sheet as possible, using real (measured) dimensions. After you send the sheet back to the piston manufacturer, make a point of talking to the company’s sales/tech guys to discuss all your data. At that point, any additional information can be obtained, dimensions can be updated, changes can be made, and the tech guy can help you “fill in the blanks.” This helps to ensure that your new pistons will be made correctly, and for your application. Remember, even the best piston manufacturer can only base the dimensions of the finished piston using the information that you provide.
MEASURE YOUR STUFF
Making assumptions will usually come back to bite you. When filling out your data sheet, DO NOT use “nominal” published dimensions for things such as block deck height, crank stroke, rod length, rod pin bore diameter, etc. Since actual dimensions can often vary (if even by a tiny bit) from published specs, having pistons made using theoretical measurements can create potential disasters on assembly day.
Don’t assume anything. Eliminate the variables by measuring/checking/verifying each component. This can’t be stressed enough. For instance: let’s say that a certain OE block is listed as featuring a 9.8″ deck height. NEVER assume that your block meets that spec. You need to actually measure the distance from the main bore centerline to the block deck (on each bank, and front-to-rear), since your block might feature a plus or minus as compared to “factory spec.” Plus, your block may have been previously resurfaced for cleanup or it may have been cut for block squaring to a specific dimension for a previous build. The piston manufacturer needs actual dimensions for your specific application, as it pertains to a custom order. Shelf pistons have pre-established specs, and they usually leave room for an engine builder to machine a block to the desired stack-up, but customs are a different story. Make the piston fit the custom application, rather than have something that doesn’t work properly because no one measured the actual stack up of the parts beforehand.
Today’s leading custom-piston manufacturers are very intuitive, but they aren’t psychic. If you provide the wrong info, the piston will be made incorrectly, in terms of working with your application, so don’t blame them if you have a fitment problem on assembly day. Yes, mistakes do happen sometimes, and when they do, most manufacturers will work with an engine builder to correct a problem, as they have tractability built into their product via an engraved job number, so errors can be traced all the way back to the point of order, if necessary.
When specifying your bore diameter, don’t assume you can have any piston diameter you want. Yes, the piston manufacturer can produce whatever piston diameter you request, to fit almost any bore size, but if you can’t obtain the proper ring set for your new bore size, you’re screwed. Always have your rings selected (and in-hand, if possible) before you order the pistons, to make sure you’re covered. You can always have custom pistons made, but you can’t have custom ring sizes made (unless you’re on a very healthy corporate budget and don’t mind spending thousands for custom rings).
Once you know that you have the correct rings, you can then specify your target finished bore size. Some piston manufacturers can supply the needed rings (in which case they machine the ring grooves to match the style and radial depth of those rings on to your new pistons), but again, only certain specific ring diameters are going to be available. Regardless of where you obtain your rings, make sure that the correct rings are actually available. This will dictate your choices in terms of desired piston size and finished bore diameter.
Diamond Racing’s Eric Simone noted that one of the most difficult challenges for any piston manufacturer is to produce a custom piston with the correct valve pocket depths, without giving up compression ratio, but creating valve reliefs deep enough to clear the valves. Although most of the info the piston manufacturer needs relates more to your block and cylinder head combo, simply providing them with your cam lift figures just won’t cut it. Here’s what they need:
As Simone explained, “We have our pre-established standards, and they work in most instances, but there is no rule of thumb, so getting as close as possible without any previous job to reference, or sample piston to refer to can be tough. We have layouts for almost any cylinder head, but every engine combination is different, so valve pocket depths will vary. Required valve pocket depth directly relates to an engine’s valve timing events, and actual valve position on the seat.
The depth of the valve pockets will absolutely dictate top ring placement, and also the total effective volume of the piston. Flat top and dished pistons are not quite as sensitive to ratio calculations as domed pistons are, as it pertains to achieving the proper ratio for a given engine combination. Domed pistons for some engines can be a real challenge to get perfect the first time. The valves must clear the piston first, and then we try to achieve the customers’ desired compression ratio second.”
He further explained that to get as close as possible to the correct pocket depths, most manufacturers need to know lifter rise at 10 degrees BTDC overlap on the exhaust side, and for the intake side, they need to know lifter rise at 10 degrees ATDC overlap. This usually requires pre-assembly mock-up of the valvetrain. They also need to know the rocker arm ratio, valve angles (for both intake and exhaust) and valve free-drop (how far below or above the leading edge of the valve is in relationship to the deck of the head). With that info, the piston manufacturer can then attempt to calculate more accurately the required valve pocket depth for a specific combination. If you’re in doubt, call the manufacturer. They can assist/advise you based on their experience on what you may need and how to obtain the required data so they can process your order. Any good custom piston manufacturer will be willing to work hand-in-hand with you to achieve your goals.
By the way, if the piston manufacturers’ data sheet asks for cylinder bore length, don’t freak out. This is exactly what is implied…it’s the actual length of the cylinder, from bottom to top. This length can easily be obtained by using a simple tape measure. Knowing the cylinder bore length is important in order to make sure the piston will remain properly supported at BDC. The manufacturer will use stroke, rod length and cylinder length to determine if the piston can be made with the proper ovality and skirt taper to operate properly-at the top and the bottom of the cylinder. It will also tell them if the cylinder length is too short to provide proper support for a specific combination. According to Simone, this can be a problem on some LS1 combinations with custom stroker configurations, where the entire piston skirt might fall too far out of the bottom of the cylinder.
Combinations with too long of a stroke and too short of a rod/cylinder length are the cause of this problem. This results in annoying piston slap, which, in a very short time, leads to damaged skirts and severely deformed cylinders.
The key is to make careful measurements of your components, and performing pre-assembly mockups. The more accurate information provided to the piston manufacturer, the better chance they have of providing accurately sized/manufactured pistons that run properly, and give thousands of hours/miles of trouble-free operation.
Tags: CNC, CUSTOM PISTONS, DIAMOND, J&E, PISTON DATA SHEET
