Jump to content

Should my piston look like this???

Mr Raiders

Recommended Posts

Hey everyone, been a while sense I have posted anything so here we go.....So I had a problem with overheating a while back and found I had no compression in one of my cylinders. I drove the truck around a little bit and let it sit for the most part till I finally decided to put it in the Brother in-laws garage and we tore it down to see what the problem was. Course I was hoping for the best an got the worse, but that was kinda expected. Of course I'M not a noob on this forum so here are the pictures to tell the story. Cause who wants to read.



Opened her up.






Everything looked good with the top end, nothing broken, bent,

or scored.



And this is what I found after removing the head...ouch




I'm not completely sure but this is not a good thing right???...lol


So my main question is this, has anyone seen this happen before? If so is there something that caused this or will cause it to happen again?

I'm going to replace the piston and rings and have the head looked at to make sure none of the valves are stuck, tho they all look fine.

It's kinda hard to see but at the edge of the picture is a bunch of what seems to be carbon build up. seems like there has been a problem

here for a while. Anyone have any suggestions on how to clean the carbon without scoring the piston walls???



on a side note anyone have any idea what connects here???

Link to comment
  • Replies 42
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic



Probably from detonation. (pinging) Running while overheated can do this, the motor will ping form the extra heat and erode the piston away. Also could be lean mixture causing a hot spot that melts the piston away over time.


The crap on the cylinder wall is where the exhaust squeezes past the piston into the crankcase. It may have melted aluminum on it too and it may also have eroded metal off the wall..


Is the cylinder scraped or scratched up and down?



That piston is junk and the cylinder may need to be overbored and a larger oversize piston fitted. It will depend on if the cylinder is worn where the exhaust gasses have been getting past the piston. It's critical that this area be sealed tightly by the piston and ring or it will have too much blow by when rebuilt.


OR it may need honing and a new piston. Hell anybody can do that.

Link to comment

Ya it does suck, but I have the meens to fix it, I hope.


Thanks Datzenmike for the info, I was hoping you would post. The wall seems fine. Nice an smooth no scaring. I was figuring that the carbon stuff was also aluminum, So how can I get it off- With out takin it to a shop???

Link to comment

Should be able to hone that bore out real nice. Get rid of the ridge at the top of wall too I would guess.


Actually, if there is a ridge, there is wear so measure top and bottom of piston travel and see if within limits including maximum taper before bothering to hone. It may need to go oversize. A small ridge (under 0.005") can be left to aid in sealing compression. Also the ring should easily compress into the piston as it passes over the ridge when being removed.

Link to comment

Thats exactly how my #1 piston looked. The machinest said everything was within specs, I found a new set of pistons for $40 so I went ahead and bored it. To bore, hot tank, install pistons on rods, and install rings cost me around $200.



Link to comment

Ok so I did some more work today



Here's the head sitting in the back of my truck, looks clean.



The underside, carbon build up, the valve is out were its supposed to be



Other side, same thing



Got the piston out with no problems



Hard to see the damage, it's a little burnt, probably normal...I've

never pulled a piston before.



Here's a better picture of what happened.



And from the top.


The cylinder wall looks good, no scar's or scrape's. The carbon build up

on the edge of the wall has some aluminum from the piston, or rings.

And there were a bunch of little pieces of the rings in the bottom of the oil pan.

I'm lucky nothing got anywhere and made things worse.



I cleaned up some parts too, had to take a picture of the manifold it came out so damn good.

I'm gonna get a carb kit an rebuild that, buy a piston an new rings, hone out the cylinder, and

get new seals, than put her all back together if everything specs out.

Link to comment

That burning look is not normal. Used pistons do not look like that.



Hard to see the damage, it's a little burnt, probably normal...I've

never pulled a piston before.


As Mike said, if the ridge wear is very light, you can just lightly hone the cylinder and fit a replacement piston. If you use the same size piston there is no need for boring, which requires machining.


To check the ridge, run your fingernail along the cylinder. It it doesn't catch on the ridge (if you cannot feel it), it's probably just fine. Measure it with a micrometer to be certain.

Link to comment

All those are caused by prolonged detonation (pinging). That's why it is so important to set the timing correctly and also listen for pinging even after it it timed correctly.


As much as 75% of detonation can be inaudible~ FYI....

Link to comment

Yes, but that is less likely to kill your pistons. Also loud exhaust is a problem because it makes it hard to hear the pinging. With a quiet exhaust you can hear dangerous detonation.


Either way, to be safe, set your timing with a timing light. If you set the base timing more advanced than stock, it may idle smoother but likely will exceed the safe full advance at high RPMs. You can check it at high RPM to be sure. Datsun distributors have a +- 2 degree tolerance (4 degrees at the crank) so what works for one engine may be 4 degrees different on another. The factory settings are the "safe" settings, which is around 30-34 degrees for an L20B. Obviously you want it closer to 34 than the 30 to obtain the most power.

Link to comment

All those are caused by prolonged detonation (pinging). That's why it is so important to set the timing correctly and also listen for pinging even after it it timed correctly.



As much as 75% of detonation can be inaudible~ FYI....


Not directed at the OP... That's why you should loose the header and fart cannon on street motors. Waste of time and money and they are loud only for the sake of being loud. A header can cause lean running... if pinging, you can't hear it.

Link to comment

Timing, yes, but more likely the combo of lean condition coupled with either correct, or too advanced, timing. The photo documentation there of the pistons say that a high mileage engine was run lean and poorly tuned. It was audible but no one was listening, or didn't know what to listen for.

Link to comment

Thanks guys for all the info, I'll post some pics of the plugs tomorrow. Gonna get some more work done on it. I did run the truck at a high rpm for 4 plus hours straight on the highway when I moved, damn 4 speed. Than all kinds of stuff started happening after the next 2 hours. I'm sure that's when the piston finally blew out.


I also noticed when I pulled my oil pan my oil was milky, seems to be coolant getting in some where, I'm gonna look closer at the block to make sure she's not cracked somewhere. The head gasket is in like new condition and is aftermarket so I'm thinking it didn't come from that, course there was no sealant on the gasket at all, not sure if that's the recommended installation for that type of gasket, but seems odd.


Anyone have a clue were a good place to get a new piston would be, I searched online a bit, but I am not sure the piston I found is the right one. I'm hesitant buying something like this online without being positive i"m getting the right one, and I have never had to purchase a piston ever before.


Thanks again everyone, you guys rock.

Link to comment

High RPM won't cause that problem, no worries there. The L-series safe RPM is 7000 RPM, you can do that all day.


Gasket: Normal head gaskets do not use extra sealer, but the pure copper $100 gaskets do.


Piston: To find the right piston you have to look at yours for a oversize mark (+1 or .050 or something). Then get one to match with the same dish. It would be difficult to find a single piston aftermarket, as the dish varies a lot. Best to look in the wreckers, or post a Want To Buy here with photos of all sides of the piston.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.