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piston and block stamping


debbheff@gmail.com

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I'm working on my 521 L16 and came across numbers stamped into the block and matching numbers stamped into the pistons.  number one cylinder is has a piston stamped with number 1 stamped into it and  block has corresponding 1 stamped into it on intake side of block.  The remaining pistons (2 thru 4)are stamped with number 3 with 3 stamped on block on the intake side.   no other numbers on pistons to indicate size, over size etc.  Does anyone have info  into what these numbers mean.

 

Thanks

DH

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I do think it has to do with the manufacturing.  They cannot make everyone the exact, exact same size.  So, they measure and know which one was the best fit for that cylinder.  Its so they can slight variances and use more of the parts.

 

Someone with better words will make more sense soon. lol

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On the '78 620 truck L20B the cylinder variance is from 85.000mm to 85.050mm. If small you can always increase it but you can't decease the diameter. If only an 85mm bore were acceptable there would be a LOT of waste. You could probably break this down into 4?ns with a 1 stamp on a cylinder that is 85.000 to 85.010. A number 2 for cylinders 85.011 to 85.020 and so on.

 

The same applies to pistons.... 84.985mm to 85.035mm. These can be graded into 4 separate diameter ranges for a perfect fit clearance. This way a 0.01mm over large diameter piston can be the perfect fit for a 0.01mm over large bore. When assembling the engine the builder looks at the number assigned to the bore, (possibly stamped on the block or written on the build sheet) say, a #5 and select a #5 piston from the #5 bin and it will have the perfect clearance.

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All pistons could be quickly balanced to an agreed upon amount by grinding on the skirts. The difference between the smallest and largest piston diameters is just under 0.002" about half the thickness of paper.

 

I balanced some KA24E pistons to all withing 1/10 of a gram once. There are 29 grams in an ounce. The scientific scale I was using was so sensitive I had to turn the fan in the window off.

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Thanks, I may just pull the pistons out to check the rings, number 4 had dry compression of app 40 psi and wet compression of approx 150  so among the things that could cause that is collapsed rings from overheating.   PO had installed a new head gasket and did not have or reinstall the locating pins in the top of the block. the headgasket was not installed to align the coolant holes in the block with the holes in the block and there was sufficient evidence to indicate there was a lot of sealer poured into the coolant system  for what i'm assuming  was a leak at the back of the head.  I'm thinking there was not enough coolant circulated around number 4 to result in that cylinder really overheating and potentially causing the rings to collaplse.  Long shot i know but ive got it down this far so it would seem like a good time to check rings rather than put it all back together just to find out number 4 wont hold compression. The motor is torn down to the point where pulling it is just a matter of pulling the block and trans.   

 

Did I mention going down the rabbit hole only to find out it's really a black hole

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Dowels. You can cut 1/2" copper pipe and use them if slit lengthwise. The 10 head bolts would align the head/gasket/block enough to run and there's lots of wiggle room for the coolant holes. The gasket alignment is crucial for between cylinders and you don't want the edge sticking out into the combustion chambers. It'll run but better with the dowels.

 

The sealer may have been there from before the gasket was replaced and the damage to #4 already done. Make sure you check the head is flat and not warped from over heating.

 

#4 may have broken rings and ring lands.

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Thanks, i did the trick with the copper pipe.  as i recall, you mentioned that in another discussion, good info.  Checked the head straight all three ways,  block straight too.  Could be the sealant had been there for a while and  the more i think about it the more i'm inclined to bite the bullet and pull it apart, would really be the smart thing to do although smart aint my long suit.  

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Thanks, i did the trick with the copper pipe.  as i recall, you mentioned that in another discussion, good info.  Checked the head straight all three ways,  block straight too.  Could be the sealant had been there for a while and  the more i think about it the more i'm inclined to bite the bullet and pull it apart, would really be the smart thing to do although smart aint my long suit.  

 

Those compression numbers are a head scratcher for me too.  i double checked the low number and was really mystified the high number after oiling the cylinder.  just a fyi, the intake manifold was removed from the head at the time.  Also number four exhaust guide was broke off level with the head (that part under the spring)  i was thinking that might be a potential source for the low number but cant quite figure out how  a broken valve guide would do that.  its interesing to note the po or whoever put the head on installed the wrong valve seals.  chevrolet v8 3/8 seals are indicated to work on nissans but these heads apparently need 5/16 seals.  all the ports, intake and exhaust showed signs of having oil pushed or pulled past the seals and 4 exhaust was really coated with oil.

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Thats about it, no valve sealing as intended thru use of valve seals at least on the broken guide at number 4 exhaust,  number four exhibited heavy oiling  all the others exhibited typical oiling of intake and exhaust runners like you would expect if the valve seals were worn out, in this case the seals were the wrong seal, too big so pulled oil past the seals. 

 

on number four the  seal remained on the broken part of the guide.  when i was pulling the seals off the guides, number four practically fell out in my fingers and thought hmmmm, thats odd came off to easy, then looked at seal and saw the end of the broken guide and thought hmmm  not good, no sir not good at all.  

 

So if you had to hazard a guess, would you expect the broken guide and the low compression number to be related?

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If there was a lot of oil getting into #4 via the valve guide area, it’s possible that there would be a lot of gunk buildup on the valve seats, causing them to leak. It’s also possible that the guides are very worn, and there’s a lot of play in the stem, again causing the valves to leak.

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Intake guides are under vacuum so oil is sucked down the stem if the seal isn't there. Exhaust are under pressure so oil getting down the stem is less of a problem. I will say that the exhaust valves are a hell of a lot more hotter than the intake that has air and evaporating gas flowing past them. In both cases the stem to guide clearance is very small with the exhaust the largest at 0.0015 to 0.0029. That's 1/4 to 1/2 the thickness of newsprint. The stem likely expands some from the heat. Excessive wear is indicated if you install the valve on it's seat without the spring and measure the side to side rocking motion at the tip. 0.008" is the maximum allowable.

 

How do you know these were the wrong seals?

 

If they were wrong, and combined with the dowels missing shows this engine was assembled by a neophyte. Perhaps the guide was broken by them as well. Going further, if 'rebuilt,' maybe the rings are in upside down or 1 and 2 rings swapped on cylinder #4.

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good point carbon build up on ring lands and rings.  guess that settles it, out she comes.  btw, i had shop replace the broken guide, reseated the all valves just to make sure any carbon was addressed, new nissan seals installed,  machinist went over the valves and indicated all the stems were within spec and guides were in good serviceable condition.  Head is ready to go, now on to the block.

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If you're pulling the pistons out, pay close attention to the rings before you remove them from the pistons. Look for stuck rings and broken rings. Either can indicate overheating, and can cause scratches in the cylinder bores. Also, if you plan on re-using the pistons, look at the skirts for signs of metal transfer from the cylinder bore. Overheating causes the cylinder wall to squeeze the piston and since the skirt is the largest part of the piston, it tends to scrape along the cylinder and can leave some nasty residue. Also, broken rings can cause deep enough scratches in the cylinder walls to warrant an overbore.

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On 2/17/2021 at 3:33 PM, bilzbobaggins said:

I do think it has to do with the manufacturing.  They cannot make everyone the exact, exact same size.  So, they measure and know which one was the best fit for that cylinder.  Its so they can slight variances and use more of the parts.

 

Someone with better words will make more sense soon. lol

 

You got that right !  Matching the block bore to piston diameter is the key.  Iff the numbers match the  assembler did his job.

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Keep in mind that this was matched when built. Over the miles there will be wear and those clearances, between 0.001 and 0.0018" will probably be just slightly larger. Maximum wear limit is 0.008" piston to bore clearance. In any case always keep the piston / rod assemblies in their original bores. The bottom of the rod and the rod caps will all be marked as to which cylinder they come from.  

 

 

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  • 1 month later...

continuing, i've started it a couple of time and no smoking although run time was seconds.  still sorting out coolant and heater hoses and still havnt got fuel tank back from shop.  on to other things front brakes redone as well as new fulcrum pins, did i mention i discovered the big Healy fulcrum pins are the same as the Datsun pins.  good info for anyone searching for pins, and cheap also, relatively speaking that is.

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Austin Healey? Nissan built some Austin cars back in the '50s. They copied a lot of their engine designs and this is why all Datsun oil pressure senders use British Standard Pipe Thread. Nissan vehicles still did into the 2000s and maybe still are.

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