Diameter Chevrolet 216 pistons?
 

I must be doing something wrong, but I can't seem to find the STD diameter of a CMP Chev 216 cast iron domed piston. The Maintenance manual does show detailed measurements of main bearings, camshaft and crankshaft journals, but no detailed info on piston diameter(?).

I am preparing to measure the parts in my engine to determine if parts are still STD, or if someone has rebuilt it in the past already. I have 5 pistons marked "BB" on top and one that is slightly different with "STD" on top, along with some markings that are hard to read. The 6 pistons seem to have the same diameter, meaning they good all be STD, but I need better measuring tools and specs to really be sure.

Does anyone know where to find detailed info on piston and bore sizes?

regards,
Alex

Alex,
Standard bore size is 3.50 inches

pistons were available in .003", .010", .020", .030" and .040" oversizes

Hi Richard,

Thanks for your responce. 3,5" bore of the block right? So, the piston would be slightly smaller???
Alex

Well yes, you would have the piston clearance to subtract, don't forget to measure thrust to non thrust side (ie opposite to pin) of skirt. A micrometer is best way to measure.

bore
 

Alex

When measuring the bores, you can use a telescoping gauge. Cheap gauge sets are available on ebay , you use a micrometer to place across the telescopic gauge. The best gauges are Moore & Wright made in England or similar, the modern Asian sets are OK but you get what u pay for .

Measure at the lower end of the bore, right down inside, this is where the bore wear is at a minimum . I've got new sets of Hastings rings +30 and NOS domed pistons +30 and 40 . Standard pistons , I reused the standard pistons in the C8 engine with a hone and chrome faced rings from another engine ! , only problem is a rattling gudgeon pin on one piston, hear it after warm up but its not the end of the world.

Hi Alex. I'm not sure how much you know, so don't be offended if you know all this.
If I recall correctly, ).010" wear, or 0.003" oval, and it's time for a rebore. Those old tollerances were the deal back in the day, for a patch up,but today they're hardly acceptable.

Piston to bore clearance is such that the bore is honed so that the piston slips on a 0.002" feeler, but locks up on a 0.003" feeler. (feeler sitting down the thrust face of the bore)
So a 0.002" piston to bore clearance.
The piston is measured on the thrust face (90 degrees to the gudgeon pin) just below the oil ring. This is the biggest measurement you get on a cam ground piston.
My info covers 1935 to 1951, but does not say anything about the difference between cast iron and alluminium pistons. Each should have a different spec.
Measuring the bottom of the bore will tell you what the bore size was, but what you need to know is what the maximum measurement is. That tells you what you have to do. For example if you have 0.012" wear, you need to find some 0.020"O.S. pistons, for your motor re-conditioner to bore and finish hone to.
For me, I would buy some good quality modern piston rings for it.

pistons
 

One good aspect of the old domed cast pistons is the upper ring grooves usually don' t wear ( widen ) , unlike the softer Al. alloy pistons which often have worn top ring grooves that require machining and spacers fitted ( expensive) . George Russell did a set of Ford pistons 85hp for me years ago, machined the grooves and supplied spacers for the top ring groove.

Down side is the domed chev pistons can be weak around the crown area and they have been known to fracture , the crown flies off . I think this tends to occur if you are a harsh driver eg constant high revs .


In the 50's there were a few after market manufacturers of the Chev 216 pistons ( and valves , springs, and other engine parts ) . I bought a set of Al. domed pistons ( when the US postal service had sea mail ) , weighed them and they are 25% lighter than the cast iron ones. I believe POLSON here in Aust. made 216 pistons ?

I sandblasted the original C8 cast standard pistons with a fine abrasive , I think its called 'peening' . Had no issues with it so far.

Mike, Lynn, Richard,

Thanks for your ideas and suggestions, very much appreciated!

No worries, Lynn. I think most of you have noted already ,by the questions I asked, that I am a newbie to rebuilding engines :D, which is also the reason why I will have a specialist look at my engine. I only have a caliper, but he has proper kit to measure accurately. First up will be to examine all the parts for damage and wear, which is why I was looking for measurements, so we have something to compare it with. Somehow it's difficult to find the nominal diameter of the pistons, to me that seems odd, but it might be due to my lack of knowledge on the subject.

I am not planning to go to the races with my C8, but I do want to have a reliable engine without blue smoke. My budget is tight, so if the pistons can be re-used I will, if not, ....well..I will have to wait and see.

Alex