Sounds easy and logical and I agree 100% with the method of mics and bore gauges. In a perfect world where the bores are dead on the pistons are perfect.

What are acceptabe tolerances on machined bores and off the shelf pistons ? You add these tolerances up and you don't have much to play with for 0.0015" ptw clearance on a hyperutectic piston. You can spend all day measuring over and over with the gausges and still not come up with anything you are comfortable with, then you can stick a 1 thou feeler in the gap and if it goes and a 2 thou doesn't your done.

 

You are correct. That's the difference between "blueprinting" a motor and just assembling a motor. I just wonder how many people have paid for supposedly "blueprinted" motors that are just thrown together like this.



Paul

 

How about how many ebay shops even check this at all ?

BTW mine was not thrown together I measured everything more times than any shop has the time too. Even with a brand new crank and freshly align honed mains I decided to go with -1 main bearing set

 

That's even more scary.


Paul

 

Hi vetters! It's getting back to good weather and I'm starting to ache for getting my ride, thus also to just look at all the awesome rides that you people have. I'm posting in this thread (instead of just ghosting like I've been doing the last few weeks) cause I believe there's something I should point out.

Technically the piston to bore clearance is the UNIFORM gap between the piston and bore when the piston is exactly centered in the bore and not touching any of the sides...i.e. if the bore and piston are perfectly round (ideal static condition, but pistons skirts (below the wrist pin) are slightly oval to allow for deformation from side thrust). However, although I don't see the spec in the overhaul manual the top of the pistons should be perfectly round by .0005" and should be replaced if out of round by .001" since an oversize piston of .001" is readily available, and an out-of-round piston usually means worn bore and the oversize piston should snug it up (a little honing will normally take the .001" off). It's not that big of a deal though until you start to see .005" out of round. Keep in mind this is on the TOP of the piston...which is where compression seal is the top priority...and the gap is desired to be minimized so that it can not be any larger than a specified amount or else too much blow-by will occur. Also, the piston is actually tapered so that the bottom of the skirt is wider than at the top, and actually oval as I mentioned earlier. With this in mind the piston to bore clearance is calculated by subtracting the piston diameter as measured at the top of the piston from the bore diameter measured at the exactly same location then dividing by 2. If you don't divide by two then what you are getting is the largest gap produced when the piston is pressing against the wall of the cylinder. Then go by the spec as listed...Overhaul manual for my 75 indicates 350 (except 245&250HP) has a minimum gap of .00075" with preferred gap of .0015" and max of .00275" and 245&250HP 350 min gap of .0035" with preferred gap of .00425" and max of .0060". This is the procedure I use because I am much more concerned about the gap at the top of the piston. I also check the clearance at the piston skirt (by borrowing a bore gauge) as described in the GM overhaul manual, and I've nearly always found them to be the same on good pistons and different on worn pistons usually the skirts were worn, but on cylinders that had burnt valves the top of the pistons were worn (bits of valve rubbing against the piston rim I suppose).

The GM overhaul manual (where I got the specs as I mentioned above) says that the gap is calculated by measuring the diameter of the piston on the skirt at [90º from] the centerline of the wrist pin with a vernier caliper or micrometer and subtracting it from the diameter of the cylinder measured 2-1/2" below the top of the cylinder with a bore gauge. The bore gauge is required since the tines of vernier calipers are only 1" or so long at the most (I don't have a bore gauge which is why I rely on the top of the piston for piston-2-cylinder gap and just measure the skirt for taper). The manual is right in front of me...page 6-34 under the section on Piston Selection. However, I am very sure that this measurement must be divided by 2 in order for the specs to be properly compared. Take the 350 (not HP) min spec of .00075". If you use just the difference of the measurements without dividing by 2 then it would mean if the piston is centered in the cylinder (like at the top of the stroke) then there is only a gap of at most .000375". And...uhm...a gap this small will not allow sufficient oil to be between the piston and the cylinder wall for lubrication and the piston will probably melt or just wear really really fast, and of course probably just stick when it swells due to heat expansion (especially if they are aluminum pistons.)

Whew...OK hopefully I've made up in this one post for the months that I have been quiet. Also, when using two tools to make measurements that need to be subtracted, then make sure you measure them together to get the best accuracy. For example, in this instance you need to use a bore gauge to measure the cylinder 2-1/2" below the top, but use the vernier caliper (or micrometer or whatever was used to measure the piston diameter) to measure the bore gauge and use that reading instead of off the bore gauge. I've also found that the best tool for cleaning pistons is a wire wheel on a drill...no scraping tool should be used on the piston surface. Also, a feeler gauge can be used for an approximation as long as the gauge is very flexible as I am sure they all are when you get down to .001"...they are usually copper when that small...I don't think they are steel (which is less flexible and will not conform to the roundness thus result in errors) until the gauges are .0035" or larger. But, anyone trying to cram a steel feeler gauge between a piston and cylinder wall is going to scratch the surfaces of both the piston and cylinder...this would be bad...like I said copper is OK, but not steel.

And lastly a few thousands of an inch really doesn't matter that much cause we will probably drive it like we stole it during the summer and then just take it apart again in the winter!

 

The pistons come with instructions where to measure the diameter and it is usuallly at 90 degrees to the wrist pin and just below it. Try that with a vernier. The vernier at least the normal 6 inch or even 8 inch don't have deep enough throats to get a piston in.
Also digital verniers are not that accurate, the electronics can't be 100% trusted. A good mic other then abuse or cold once checked with the proving bar can be relied on.
I rough things with the vernier and use a mic for the true measurement.
Mics also require feel and you get that from use and testing on the checker.

 

Guess I'm going to have have to save up for a set of mic's and throw away the ruler, tape measure, and string for making these types of measurments.

 

Wonder what they used when my eng was put together in Mexico?

 

What I meant with the vernier is, if you have one with long enough tangs is that if you keep it at an angle and then lock the measurement you can then use it to set your bore gauge, that way you don't have to accurately know where to measure w/ a micrometer or if you don't have a large enough micrometer. The verniers w/ the long beaks are probably hard to get, I got mine from a machine sjop (it's a mitotuyo) and it's accurate to 0.002mm (still not enough for the real accurate measurements)

 

The engine manufacturers actually measure the bores with an automated bore gauge, and every piston gets measured with an automated micrometer system. Have you seen the letter stamps on stock block bores and pistons...? There will be an "A", "B" or "C" stamped. Depending on the tolerance range of the components, the bores and the pistons get an identifying mark. During assembly, the matching parts are used, and this assures that a known and consistent piston-to-wall clearance is achieved in spite of the tolerance ranges of the parts. Even the Mexican builders do this.

 

IF YOU CAN GET BY STRESSING ABOUT READING THE MIC YOU CAN FIND THAT THE MEASUREMENTS WILL VARY DEPENDING ON HOW YOU HAVE YOUR BLOCK PRE STRESSED...A GOOD IDEA IS TO TORQUE PLATE THE DECK AS WELL AS TORQUE THE MAINS TO FACTORY SPECS.WITH THIS YOU WILL GET TRUE READING ON THE BORE.WITHOUT IT MAY FAIL BY CASUAL INSPECTION...ALSO MACHINE SHOPS SHOULD KNOW TO CHECK THE BORE ON THE COOL CYCLE AFTER HONING. MOST BLOCKS THE CRITICAL AREA OR FLAW WILL BE AT THE BOTTOM OF THE BORE SO REALLY MIC THIS AREA

 

If you're going to be building a lot of engines, by all means - buy the correct measuring devices. I'm fortunate in that I live a few miles from a shop that specializes in high quality, high hp race engines. I take my pistons, rings, rods and crank to them when I get ready to build one. They hone to piston manufacturer specs, file fit the rings and fit the main and rod bearings. They have plenty of bearings to fit them perfectly as well. I can't afford the correct equipment and don't care to keep that many bearings handy, either. The way I see it, they're going to be doing my boring, honing, decking, balance work, etc. - just let them go ahead set all my clearances and I'm good to go. There's no extra charge for honing, $20 for checking bearing clearances and $30 or so to file fit my rings. I end up with an engine with perfect clearances, checked by folks that do it for a living, and I get to work with the guys at the shop.
BTW, they always calculate clearances. Feeler gauges are for plugs, ring gaps and valve lash, not setting bearing or piston clearances. They calibrate the bore gauge against the mic that they measure the piston or journal with so that they're measuring exactly the same. HTH