I'm considering buying a prepped Chevy cylinder block to piece together an engine build. This will be for a hot street/strip motor. I've seen some machine shops that sell blocks that are already hot tanked, decked square to the crank centerline and bored .030 over with a plate. My question is, would you be concerned that the block is already bored .030 without the machine shop having the pistons in hand to check the clearances with? Is it OK to assume that if a shop bored and honed a block .030 I can just buy a set of .030 pistons and i'm good to go? I was figureing on buying a balanced rotating assembly once I had the block this way I can build it piece meal and spread out the cost of the build. Any constructive help is appreciated.
Well....you might get lucky.... One way to look at this is that you might be able to select a different piston type if necessary, to get the clearance where it should be. Some alloys and/or skirt designs expand more than others when up to temperature. So, say, if you had planned to use TRWs, you might be able to use another brand piston instead, even with larger-than-optimum clearances, and make it work OK...just have more piston slap when it's cold. You might end up having to return a set of pistons, or sell a set on eBay, too, to get rid of an extra set. Depends on the vendor and their return policies, especially with time involved (see below). This would be a complete crap-shoot...you might also get a block that is sloppy enough to be a problem, depending on the machinist. If you are on the three-year engine-building plan, like a lot of folks, you can bet the block seller isn't going to make things right after that amount of time. On the other hand, you might get a block that needs further honing to make things work, so you will pay twice for the same service. Also, you'll need to make sure that the rings you end up purchasing are suitable for the block's bore finish. Personally I would not do this (buy the block separately), but others have done it with success.
Best practice is to bore and finish hone to the actual piston that will be used. This takes the variability of different measuring tools out of the equation. You also need to pay attention to the bore surface finish depending on your ring material.
I'm not sure what your knowledge base with engines is, but you seem relatively well educated, so I'll make this pretty to the point. The piston to wall clearance is dependant on engine applications, piston type, fuel, Naturally aspirated, forced inducted, or nitrous applications. ALWAYS follow the piston manafacturers rec's. For instance, a hypereutectic piston you can run pretty tight, like, .0015-.002. Thats on a street/strip motor that won't see nitrous or anything more than natural aspiration. A forged piston typically wants .003 or so for the same application. I did all the machine work on my motor. 306 ford out of a 289. I ran KB hyper's. I bored it to .025 over, then final honed it to an exact .0305, leaving the final bore size at 4.0305. KB manufactures their pistons .0015 under stated size, so my piston to wall clearance is .002. I would find what slugs you want to run and find out what they call for for your application. Then call the machine shop, tell them what you have, and see what they say. If they say "aaahh, we just hone em' to 30"- then they're probably not concerned with anything performance. You said they did it with a plate?-I'm assuming you mean torque plate honing, which is performance based, so I'm guessing the machine shop will help you out. They may have machined the block for a specific set of pistons that you may have to buy seperately-so call them before you buy anything. Thats what I would do. Oh, and ALWAYS double check every measurement before you begin final assembly. If you don't know how to read a micrometer-its easy, learn how to do it. Even if the machine shop is top class and very reputable, never fully rely on them. In the end your the one putting it together so if it fails its your fault. Hope that helps!!! good luck
Thanks for the input guys. I'll wait until I have pistons first. I kinda figured it would be a bit of a crapshoot but it's been quite a few years since I assembled a short block so I'm glad I asked before I bought anything.
Current piston production standards call for extremely tight tolerances to maintain a QS9000 quality rating. The clearance is built into the piston. The block if properly bored to .030 will be fine for new .030 pistons. Of course it is up to the assembler to verify everything. If you are using some old TRW's or something then none of this applies.
Almost every part today is made on a CNC ( that includes China) machine. However if quality control is not adequate you can get pistons out of tolerance. I have built many motors and found only one piston out of tolerance and unfinished(yes a TRW). I have had blocks bored before Ive ordered pistons. Many machine shops want your pistons first before they bore a block. Do you ever wonder, if they screw up a cylinder bore by a thousandth or two but are lucky enough to find a piston in your box that is oversize.
I think these guys covered the block issue, as far as "balanced" rotating assemblies . Can't be done, flex plate and balncer need be present as well as rings and set of rod bearings . so don't get caught up in that BS either . you can sometimes save money when buying a rotating ass. , our warehouse gave us breaks and we passed savings to customer .
GASSER...boring an engine isn't rocket science. if you trust an automotive machine shop for your block, don't you think they can bore it correctly? believe me they should no how. the worry's you should have, are you quilifed to build the engine. i'am not being snotey noised eather. but it takes a real mechanic to build a good mill...POP.
We do engines for a living, and the info above here is "dead-on". Thanks in most part to the machining techniques available today. There's a "flip-side" to all machining operations, if you order the pistons first, you have no guarantee the bores will make the size?? By boring the block only first, you know what size they will finish-hone up at! If we aren't very familiar with the components, then we wait for the pistons! Thanks, Gary in N.Y. P.S. Either way, if any problem does come up, it's up to the assembler to "catch" it before the pieces go together!
There are some engines that are internally balanced and don't require the damper and flexplate/flywheel be balanced to be right.
I would agree with the above, ie avoid finish hone until the pistons are obtained,as above the hone finish instructions as well as size instruction do vary through piston manufacturers. I would also like to say that from my point of view (even here in the uk) that JE pistons are really helpful and if necessary may be able to machine a set of blanks to your bore size (still at a reasonable cost), you could then finish hone if necessary to recover your bores to suit the piston. Definately worth the decking true to mains, the deck heights do vary from front to back significantly. Oh almost forgot, as said before never trust any measurement from the machine shop no matter how reputable, we are all human and a micrometer turn is easy to do. I carry out a final check which has many benefits. I rig a temporary oil pressure system, connect a filtered oil supply from a container such as an old fire extinguisher and pressurise this to 15 or more psi. I then connect to the engine oil gallery and with the assembled sumpless engine inverted you can observe the oil feed. If there is an undersize journal, oversize bearing or similar the result is a very pronounced oil flow. You can also observe cam and any aux oil feed, rotating the engine will show any oil jet delivery provision to the bore if it is applicable. Even a long length of oil feed pipe connected via a filter and gently pressurised by a foot pump can show a lot.
Oh your right ,a 350 chev being one of them , but to go into which ones do and don't and really make things confusing , I just tell people to have all their shit together from the start .
so lets get this right you go out and buy a .030 block then order pistons and it is up to you to find out if they fit or not . and if they don't then it is up to you to fix it up. the way i do it i take the pistons in with the block and say i want these pistons in this block. then if it is not bored right it is on the guy i paid to bore the block. seams like my way cuts out a whole lot of coulda woulda and shoulda.
Just buy .030 over pistons and you will be just fine . That is a common first re bore size . They can go up to .060 with no problem . Now is the time , depending on what you will use the engine for and if you will EVER decide to use a forced induction of some sort or nitrous to consider buying forged pistons or not . They are very costly BUT if you use any of the above now is the time to choose your pistons . Also the rings will be another thing to considerer too . I would recommend Molly rings since the block was bored . RetroJim
Now after you start to get confused and things start sounding screwy , now you know why guys buy the crate engines ! If you buy a short block you won't have to worry about the pistons . RetroJim
Maybe you should save up some money and do it right all at once? If you're going to step up to forged pistons, then I'd suggest holding off on the final hone until you have the pistons in hand. Cast pistons usually fit right at the nominal bore size.
yeah, I went to school for performance engine building and have practiced it a bunch during and after college-theres A LOT to know about engines, and no one knows it all. This is a pretty simple question that can get you a lot of very complicated answers. another general fact that hasn't been mentioned yet that I'm not sure you're aware of-modern pistons (I'm not sure if all of them, but most of them) are "cam ground" meaning they aren't perfecty circular. ALWAYS MEASURE ON THE THRUST SIDE of the piston. This is the long side. They do get circular when up at operating temp, but when on the bench in your hand they are oblong.
here is my advice... all the info here is very accurate however i wouldn't piece together a motor as it usually more expensive. the cheapest way I've found is find a running core (smoking whatever) dissasemble it and send it to the machinist. let him or her do their job. one can usually get a good core motor for very cheap and have all the parts IE crank and rods. depending on what motor you want to do and the power you want to make you can use the heads too. t
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