hypereutectic pistons - p-t-w clearance question - urgent

[Belgian1979]

Guys,

Putting together a pretty much stock SBC 350. Never mind the reason.
Block is bored to 4.030".
Pistons I have here are Sealed Power H345DCP .030. which are hypereutectic. Have always used forged pistons in my other build.

Pistons have a thight fit and when measuring I seem to have virtually no clearance. What I measure is around .01mm or which is ca. 0.0004". Cannot believe they have to be this thight. I don't find any instructions in the set and cannot seem to find anything on the internet. The skirts are coated and have a blank space which seems to be the area to measure and which is where I set my mic on to set the base size.

Is this correct at all?

[BillK]

It might be correct.
1. Where are you measuring the piston ?
2. Is the bore exactly 4.030"

I have a couple of sets of those pistons at the shop. I can look on the box and see what it says but it might be Monday AM. A lot of modern coated pistons are installed that tight but it also depends on the application. Most clearances are listed as "minimum clearance' and they typically specify more for performance use.

[BillK]

Here is the Sealed Power catalog page. It shows .001" minimum clearance. It does not say where to measure them.
https://www.drivcat.com/overlays/part-d ... brandId=SP

[Belgian1979]

I measured in 2 different ways to verify my measurements (one with the mic set exactly at 102.36 mm (4.030") and one time with the mic calibrated at 103 mm and measuring how much smaller it was).
All bores measure at or around 4.030" and some of them are a tiny bit smaller at around 4.0295 or so. I would say the bores were done correctly. Normally I think the pistons would account for the difference but .01mm or .0004"???

I'm not really comfortable with this.

BTW: the blank dot on the skirt is approximately at the lower part of the piston pin, a little higher than the bottom edge of the skirt when looking at the piston right at the pin. The spot is smaller than the mic measuring pins, so I'm probably measuring on the coating, but nonetheless...

[rustbucket79]

The picture shows the window of bare aluminum which is the measuring point. The older version you had to measure over the coating, which is around .001” thick by itself. The coating will wear pretty quickly, but if you wanted to size the cylinders .0005” over std. oversize you’ll be fine.

[Belgian1979]

The picture shows the window of bare aluminum which is the measuring point. The older version you had to measure over the coating, which is around .001” thick by itself. The coating will wear pretty quickly, but if you wanted to size the cylinders .0005” over std. oversize you’ll be fine.

That's where I measured it, but like I said above my mic has round tips, so are larger than that long strip of blank aluminum. So I'm probably measuring on the coating.

[rustbucket79]

Yeah it’s annoying, Silvolite’s have a nice round area of bare skirt in which to measure.

[Belgian1979]

Yeah it’s annoying, Silvolite’s have a nice round area of bare skirt in which to measure.

So assuming you need to measure the bare alu and the coating is .001" (.0254 mm) then I would be at a total clearance of .0354 mm or .0014 which seems to be about right.

[rustbucket79]

Yep. Since it’s your engine, zero harm in scraping the coating off for a proper measurement.

[Belgian1979]

Ok, but how much clearance do you guys prefer on these?

[Belgian1979]

Scraped off the coating to get a round dot and then measured it. (not easy). I seem to get indeed around .030-035 mm which is .0012-.0014".

[David Redszus]

My MB M119 engine uses hypereutectic pistons. The MB factory installation piston to wall
clearance is: min 0.000157", max 0.001023". I bored and honed to the first oversize.

The machine shop insisted the pistons would never fit in the bore and would scuff immediately.
But when the bore dimensions are correct with respect to diameter, ovality and taper, the pistons
fit nicely. Even fully assembled with camshafts, the engine turned over easily by hand.

However, the MB also uses a hypereutectic aluminum block, not cast iron. The thermal expansion
of cast iron is much less than the aluminum, resulting in a much tighter fit when at operating
temperature. If hypereutectic pistons are fitted to a cast iron block, increased piston to wall
clearance would be necessary.

[Belgian1979]

My MB M119 engine uses hypereutectic pistons. The MB factory installation piston to wall
clearance is: min 0.000157", max 0.001023". I bored and honed to the first oversize.

The machine shop insisted the pistons would never fit in the bore and would scuff immediately.
But when the bore dimensions are correct with respect to diameter, ovality and taper, the pistons
fit nicely. Even fully assembled with camshafts, the engine turned over easily by hand.

However, the MB also uses a hypereutectic aluminum block, not cast iron. The thermal expansion
of cast iron is much less than the aluminum, resulting in a much tighter fit when at operating
temperature. If hypereutectic pistons are fitted to a cast iron block, increased piston to wall
clearance would be necessary.

Thanks David, but how much in this case and for a SBC would be ok? Personally I would think the manufacturer of pistons would undersize pistons so it correctly fits a block bored to exactly 4.030.

[BillK]

B,
Now I seem to remember another thread about this same issue with the bare spot being too small to properly measure. If you have .0014" now that you scraped the bare spot then it should be fine. The page I posted said .001 minimum so you will be fine.

[Belgian1979]

Thanks Bill