Going over the pistons on my 27 motor today and noticed I have three types of pistons. They are all iron but three look like the light weight pistons except one of them doesn't have the holes at the oil ring groove. The fourth one looks like the heavy pistons with no rim at the skirt or holes at the oil ring groove. What do I have?
Definitely not a winning hand!
If you're on a very straight budget and if your cylinders are close to std (unlikely with that piston family inside) then I'm sure there are many T:ers who could give you a matching set of cast iron pistons free - but a better scenario is that you measure cylinders to decide if it's the +0.020" or +0.030" set of alu pistons you're ordering next
Check the rods carefully so you have the same style - you'll want to weigh them both in the heavy and light ends + in total to check if they're close enough for running in your refreshed engine.
Roger, I do have a spare 26 motor with standard iron pistons that match. I would like to go to aluminum pistons. How much taper and out of round is OK? The cylinder walls look nice. No scoring or pits.
If the engine runs well as it is, and it's just something you noticed while doing other things, I would leave it alone.
As to how much taper and out-of-round is o.k., you'll probably get a lot of opinions. Here's mine: I would probably not worry about anything under .005 for O.O.R or taper in a used motor. T's are very forgiving.
Cast iron pistons uses smaller tolerances towards the cylinders since they grow equally as the block when heated. Alu pistons have to have a lot of clearance, especially above the top ring, where an old used engine has the most wear so it's unusual to be able to use std size alu pistons in an old block. Myself when I started with T's I had what I thought was a nice engine with usable cast iron pistons, but I wanted less strain on the crank at higher rpms with alu pistons and found stds at a swap cheap - but when I checked them in my cylinders, there were like 0.030" play up by the top rings.. No go, I had to rebore for .0030"+ alu pistons, and that was the smallest overbore that would have cleaned it up. Still it would have been possible to drive with the original cast iron pistons for some more time - they were far from as floppy in the bores as the std alus.
I think the Service Manual says to bore if the taper or oversize is more than 0.0025"? Anyway, with that kind of mix of parts inside, your engine seems to have had a long and interesting life that likely calls for a rebore.
My original plan was to bore the block. I found a long crack in the valve chamber that was leaking coolant which is why I pulled the motor. I don't want to put money into the block until I'm sure the crack repair is working. Just exploring possibilities.
Interesting weights. I weighed the pistons. The heavy type piston is 1255 grams. The light weight types are 1115g, 1090g, 1060g. Quite a difference. I took the pistons out of the spare motor and they are all the light weight type and match.
Maybe the "spare engine" is in better shape??
No the spare motor needs a rebabbett job and valve job. It still has the old two piece valves.
There was a set of .010 over cast iron pistons with parts that I bought once. At the time I was piecing a 26 engine together and after honing it a bit the pistons fit great! The engine, that was assembled from parts, has been running well in my 26 touring. I wouldn't be afraid of the stock pistons.