Cracked piston?

southpw · Post by **southpw** » Wed May 25, 2016 5:08 pm

Am i looking at a cracked #4 piston? If so, do all 4 need to be bored out and a larger piston installed or just the one? All four pistons have .030 stamped on them. Does this mean the bores have already been opened up .030"?

4x4M38 · Post by **4x4M38** » Wed May 25, 2016 6:15 pm

Brad,
It doesn't look good, but magnafluxing or pulling it is the only
real check. The other concern should be that chunk out of the edge.

Yes, .030 stamped means it has been already been overbored
to 30 over. Only looking at the bore will determine whether
you need to bore more. Possibly all you need is a new piston
and a home job.

The real question here is what caused the piston to crack if
it is. You have to solve that first, otherwise there may be another
cracked piston and maybe more damage. It may be worth
the trouble to check the rods, as well as the valve train.

southpw · Post by **southpw** » Wed May 25, 2016 7:41 pm

Thanks Brian. I will being my bore gauge home from work on the weekend. My dial caliper reads bores 2 and 3 at 3.155" so hopefully its just a piston swap.
I dont know much about engines so i will google the reasons and see what comes up. Maybe i will find a paragraph about it in one of my manuals as Well.
Brad

jason · Post by **jason** » Wed May 25, 2016 8:07 pm

what octane gasoline were you using? A cracked piston could be the result of detonation or it could be a cheap piston.

Post by **wesk** » Wed May 25, 2016 11:05 pm

Save yourself a lot of time and worry. Pull the piston, bead blast the piston clean. Then inspect it. A crack you should be concerned with will be visible to the eye and confirmable with a simple dye penetrant check. Often very thin layers of carbon and lead form on the top of a piston and flake off a bit. What you see right now could be that simple.

I wouldn't get to excited about that small notch. Many builders will do that to a piston to mark the spot that goes towards the front of the engine. I stamp a small arrow in the head of each piston and a number so I can keep them properly organized and re-install them in their original/correct positions.

4x4M38 · Post by **4x4M38** » Thu May 26, 2016 6:05 am

Hi Wes,
I remember marking the pistons that way from old Hot Rod
days now that you mention it.

Makes sense.

jam51m38cdn · Post by **jam51m38cdn** » Thu May 26, 2016 7:02 am

your pics indicate to me that your engine is a fairly new rebuild.030 oversized is a pretty small amount,due to my machine shop told me he can go .080 over before he has to sleeve the block.The notch in the piston is from the manufacturer of the piston.Many of your questions can be answered by a good machine shop.Mine has been doing willys engines for 3 generations. I would put an indicator on the crankshaft and check end play and make sure your within tolerance.A lot of machine shops like to reface the face of the front main journal which actually rides against the rear of the front main journal and determines the crankshaft end play. If the machine shop takes to much off excessive end will be the result,and then your pistons WILL NOT be in the center of the bore. There is is no way short of spray welding that face and turning the crankshaft to the correct dimensions to achieve the correct endplay. Usually you will see wear and scaring inside the bores front to back in the block if this has occured. Jim

southpw · Post by **southpw** » Thu May 26, 2016 6:12 pm

Thanks so much for all the replies. I bought this engine used but heard it run and sounded like it was running good. He had pics of the recent rebuild but not sure if that is when block was honed. I do not know what fuel he was using and if riming was optimal.
So i opened up my manual and figured out how to pull the piston. Stopped at a hot rod shop here in town and asked if they would take a look. I am practically a virgin when it comes to engine blocks. Anyways, he tells me those are surface scratches, likely from the time they were installed. He grabbed scotch brite pad and rubbed them out. He had to put elbow grease into it but i dont see the marks anymore. I left as soon as i could, feeling a bit awkward.
I now come home and decide, since the piston is out, i would measure the dia. of piston.
I get a .016" difference from size parallel to camshaft and perpendicular to camshaft. 3.134 to 3.150 time to go pull out the book.
Thanks again for all the help.
Brad

charlesfitton · Post by **charlesfitton** » Thu May 26, 2016 6:16 pm

Piston are oval when they are cold....

southpw · Post by **southpw** » Thu May 26, 2016 6:16 pm

I can see now in pic #2 of post 1 that i see the "crack" lone extend onto the block. I was using a razor blade in a handle to scrape carbon earlier.

southpw · Post by **southpw** » Thu May 26, 2016 6:17 pm

Thanks Charles.

southpw · Post by **southpw** » Tue May 31, 2016 3:51 am

Just to follow up, i pulled all 4 pistons and broight them into work and a quality control inspector on the night shift did a LPI (liquid penetrant inspection) on all 4 pistons and found no signs of cracks. I feel much better now that he was able to confirm that it was in fact just a scratch on the top of piston.
I guess i can put them back in and continue the build.
Thanks for all the help.

southpw · Post by **southpw** » Sun Jun 05, 2016 8:43 am

It looks like when the block was bored 0.030" oversize and new pistons were installed, the original rings were reused. I get a gap of 0.075" which i believe should only be 0.008-0.013" Please correct me if im wrong before i order 0.030" oversize rings

Post by **wesk** » Sun Jun 05, 2016 9:21 am

Bores are finished carefully by final honing to match each cylinder specific piston which is then stamped with the cylinder number it belongs to. This often results in small bore variations to ensure proper piston fit. Ring gaps are filed to fit one piston in one bore. A wee bit of mix and match on re-assembly can easily lead to the error in gap you see. As can excessive wear to a 0.030 OS bore. Are you inserting the ring using a piston the full length of the piston into the bore before you measure gap?

Before you order anything you should confirm ACCURATELY each piston diameter, each bore diameter and then match largest diameter piston to larger bore and so on and then match rings to bores based on gaps. When you have ascertained positively that you cannot custom match all four bores with correct fitting pistons and rings then a decision must be made. All this said be sure you are using TM 9-1804A and check all ring clearances, not just gap.

Is a new overbore needed or not? Are new pistons needed or not. Are new rings needed or not.

southpw · Post by **southpw** » Sun Jun 05, 2016 9:52 am

Thanks Wes. I took ring off of piston #3 and inserted ring 2.5" down from top of block into cylinder #3 and made sure ring was at same depth all the way around.
This is how i got this gap size.
Are you saying i should check gap when i push ring in like photo below? If so, I still get that oversize gap. Surprizing the gap is so large compared to the spec of max 0.013"

I have also measured bore diam with an inside micrometer and is right on the .03 oversize size. Measured in multiple directions. I need to bring home a bore gauge to check full depth of bore as my hands cant fit very deep to check diam all the way down the cylinder