My Pistons say "50" on top.. Normal?

I just finished taking my head off for lifter replacement. I thought that the motor was virgin, but I think the head has been off before, but with the motor in the jeep. The very back head bolt well not clear the firewall seam when you try to take it out,and I can see where someone bent the lip of the seam to get the bolt in.. Also.. my pistons say "50" on them. Which I'm afraid means .50 over. Most oversize pitons I've seen have a decimal.. Does anyone know if the stock Renix pistons say "50" on the top? This kind of scared me because I replaced the rod bearings about 6 months ago with stock size bearings.. the ones I took out did not have a oversize stamp on them so I assumed they were OEM.. 1987 model..

50 usually means a .050" over bore piston. You should have drained the coolant first and the way to remove the head with the rear bolt is tape it up so the thread doesn't engage then pull the head with the bolt... put it back in the same way.

Also, on the bearings if they are over sized it will say so on the back of the bearing.

I'd be strange for the bore to be .50 over.. and have the stock bearings.... but.. like I said they had no oversize stamp on them.. they were also as worn as they should have been if they had been in there since day 1.

as to the coolant.. I'm pulling the pan right after this to change the oil pump.. so I'll have all the coolant out of the oil.

It is odd to have the original bearings and an over bore...

Has anyone out there seen the top of a stock renix piston? Or know what the stock bore size is? I can get some calipers and measure it..

With a tape measure (I don't have any calipers) my bore is 4'' ... like SBC..

4'' would be .125 over; stock bore is 3.875. The RENIX can go over up to that .125 and use 4'' pistons, but not alot of people do it. Also when using a 258 crank and rods, that increases the displacement to 5.0.....sounds like you have .050 over pistons.

stephenspann27 said:

Has anyone out there seen the top of a stock renix piston? Or know what the stock bore size is? I can get some calipers and measure it..

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OEM bore for all 242ci engines is 3.875".

The RENIX short block I have on a stand out in the garage - which I know has the stock bore size (I measured it myself...) has markings similar to this on the piston tops:

FRT
-->
50

... with the arrow pointing toward the front of the block. However, the gudgeon pin is usually not directly on the diameter of the piston - it's usually offset slightly, for a reason I can't recall at the moment (but I think it has primarily to do with engine noise.) The offset amount is usually .050"-.060", so that may be what the "50" is referring to.

Most oversize pistons I've seen have a three-digit number for the oversize - you'd see it as, say, either "010" or ".010" - for a +.010" piston. Just like crank bearings are labelled with, say, "010" for a -.010" bearing.

Typical "inch" piston oversizes are .010, .030, .040/.045 (depends on manufacturer. +.040" is more common) and .060". A +.050" piston would be unusual enough to call "rare" - although I think it's a common metric oversize for a "clean-up" bore in a Japanese engine (I'd have to check that, I can't swear to it at the moment. I may be remembering that as a bearing undersize, and getting my wires crossed...)

I just looked, and that looks exactly like the top of my Renix pistons. The motor as far as I know has never been apart before last month. The motor is an 87 4.0, and has a 50 on the pistons, I believe it is stock.

Yea I just saw a pic of a piston in my haynes manual, and it says 50 on it. Like you said then, "50" is stock OEM.

I forgot that I had a stock piston from a 4.0L sitting on my kitchen table.... it says 50 as well.

And yes, the offset if to quiet the engine by reducing piston rock.

jinshiyang said:

thread insert,helicoil

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What do helicoils have to do with 50 on the piston? Spam?