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  #16 (permalink)  
Old 12-15-2011, 09:15 AM
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Quote:
Originally Posted by techinspector1
Best way is to use a bore gauge to find the cylinder diameter and a 4-5 inch micrometer to find the piston diameter. Old school guys like me will turn the piston upside down, insert it into the bore, holding it by the wrist pin and inserting a feeler gauge down beside the piston and cylinder wall. You need to know where the piston is the largest diameter because the skirts are different diameters along the length of the piston. They will usually be the largest on the skirt opposite the wrist pin or close thereby. Just keep swapping out thicker and thicker blades until you get a snug fit. That will be the piston to wall clearance.

That's not a very accurate way of checking the clearance, aside all the issues of using a feeler gauge for measuring.
Most piston manufacturers have a datum point for measurement, which is not actually the widest point of the piston. The last set of Mahle's I had in had a measurement point of .708" above the bottom of the piston skirt. Lots of them list out .500"-.750"
Looking at a set of speed pro's I have in stock and they say to measure 1.820" down from the top.
Neither of which can be accomplished using a feeler gauge. So, your first response is correct, bore gauge and mic is the only way to go.

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  #17 (permalink)  
Old 12-15-2011, 11:36 AM
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Quote:
Originally Posted by Plaintoast
Neither of which can be accomplished using a feeler gauge.
You need to know the fattest point on the piston to use a MICROMETER to find the diameter. You DO NOT need to know the fattest point on the piston to find piston to wall clearance with a FEELER GAUGE BLADE. You simply plunge the piston into the bore upside down, like I said, and use whatever thickness of blade will slide between the piston skirt and the bore.

I'm well aware that the current line of measuring equipment is superior to the feeler gauge method, but how many millions of cars and trucks are out there every day, providing service to their owners, that were clearanced with a feeler gauge? This stuff ain't rocket surgery.
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Old 12-15-2011, 02:43 PM
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Quote:
Originally Posted by techinspector1
You need to know the fattest point on the piston to use a MICROMETER to find the diameter. You DO NOT need to know the fattest point on the piston to find piston to wall clearance with a FEELER GAUGE BLADE. You simply plunge the piston into the bore upside down, like I said, and use whatever thickness of blade will slide between the piston skirt and the bore.

I'm well aware that the current line of measuring equipment is superior to the feeler gauge method, but how many millions of cars and trucks are out there every day, providing service to their owners, that were clearanced with a feeler gauge? This stuff ain't rocket surgery.

You need to know the datum point to find the clearance. If that datum point is .750" up from the skirt, how will you insert the feeler gauge at precisely that point to measure the clearance?
You simply can't.
I'm not saying it's not done the way you're saying, but I'm saying the only ACCURATE way to do it is a bore gauge and mic.
Much the same as thousands of motors are assembled using plastigage, but you won't find it in a professional shop, because the correct way is, again, with a bore gauge and mic.
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Old 12-15-2011, 05:38 PM
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Quote:
Originally Posted by Plaintoast
You need to know the datum point to find the clearance. If that datum point is .750" up from the skirt, how will you insert the feeler gauge at precisely that point to measure the clearance?
You simply can't.
I'm not saying it's not done the way you're saying, but I'm saying the only ACCURATE way to do it is a bore gauge and mic.
Much the same as thousands of motors are assembled using plastigage, but you won't find it in a professional shop, because the correct way is, again, with a bore gauge and mic.
I haven't built hundreds of motors, but I've built several dozen street and street/strip motors over the years and have never owned a bore gauge. You're hanging your argument on ACCURATE. Yep, no question about it, a feeler gauge blade isn't as accurate as a bore gauge. My argument is that it simply isn't that important on a motor that will be used in something other than a race car. I suspect that it wouldn't matter if the piston to wall was 0.0015" or 0.0020" and you could certainly get that close with a feeler gauge. Also, although I have a complete set of mics and calipers and know how to use them, I find no problem using plastigage for checking bearing clearance. Again, it won't matter whether the rod bearing is 0.0015" or 0.0020" on a street driver and you can get that close or better with plastigage.

New fellows and gals need to hear the other side of the story. This stuff just ain't rocket surgery.
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Old 12-15-2011, 09:36 PM
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Quote:
Originally Posted by inquiring_mind2
the pit we run is 200 ft. I wasn't sure if I cud run new pistons in the bore I have. I'm nesting up the bottom end of my motor. right now I have I beam rods that are press fit. I wanting to yo to H beam rods. but from what I have seen is the are floating.
Aside from what the piston to wall clearance is now, the cylinder has to be honed for new rings. So the honing will need to be accounted for when measuring the piston to wall clearance and fitting the rings.

The file-fit rings will work fine as long as the cylinders are honed for them and there's not much taper. But before committing to anything you need to measure the taper, as well as any ridge that may have formed at the top of the cylinder. Otherwise if the new pistons were to have a top ring groove any higher than the pistons you're replacing, the top ring of the new pistons will hit the ridge at the top of the cylinder.

Ideally you'd want a set of 0.060" over pistons (regardless of whether they're for a stock length rod or a longer rod) that were on the high side, diameter-wise. Unless you were going to overbore to the next oversize, that is. You'd also want the top ring to be positioned no higher than in the previous pistons, for the reasons mentioned above.
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Old 12-16-2011, 06:59 AM
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I should prolly add that even if the top ring of the new piston is positioned the exact same as the last piston, if the ridge is pronounced enough the new rings can still butt into the ridge because they're not worn in to it the way the original rings were that formed the ridge in the first place. Guys will sometimes get away w/this in beater engine "rebuilds", but in an engine seeing 8K RPM blasts, it's another matter altogether.

AFA long rod/short rod for a BBC build, there are well sorted out combinations that you can choose from. Most all the stroker bottom end assemblies will give you a selection of rod lengths to choose from, and there's just no good reason (IMO) to not use at least a 6.385" rod.

There are many opinions on rod length vs. stroke, to put it mildly. Like those of Smoky Yunick, who has said to stick the longest rod in that you have room for. Some might argue that a lot of water has passed under the bridge since Smoky made those remarks. And while that is true, look at the current crop of GM Gen III/IV engines to see what the trend is w/con rod length. Or look at the highest-tech 4 stroke racing engines around: F1 motorcycles (~2:1 R/S ratio), NASCAR Cup (~1.9:1) and F1 car (~2.5:1).

Let me say again- these is the CURRENT trends- this could well be superseded yet again w/whatever the next generation of engine design brings. In some cases (production engines in particular), packaging the engine into a chassis has as much- or more- to do w/the architecture of an engine as maximizing performance does.
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Old 12-16-2011, 08:17 AM
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In reading this entire thing again, you have never said WHY you're replacing the rods or the pistons, for that matter. Your current rods seem to have served you OK and you can have them measured and resized if needed, replacing the bolts if you think it's needed for around $200 or so. Your 427 already has a 1.63 rod ratio which is respectable. You've also never said what rpm's you hit in your runs? Many factory 396/427's went 7 grand with factory rods. Once again, if you're determined to get new pistons and rods, spend another $500 and get the crank and go the 496 cid route. In the mud, I guarantee the extra 70 cid and torque produced will do far more than any jerking around with rod ratio will ever do. It will be the best $500 you ever spent.... then you can start thinking of cylinder heads, spend some real money and make some real power.
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