Kinematics

I purchased this project car a few weeks ago, with rod knock. I pulled the head today, and everything seems to be in good shape. Though, there is an odd mark on the #1 piston that I can't decipher. The arrow is pointing at it in the picture below.

To clarify, this 'mark' is actually complete absense of any carbon on that section of the piston. It doesn't appear to be scratched or bent or dented in any way. Each cylinder (upon quick visual inspection) is free of any scoring or groves on the cylinder walls.

As you can see all of the pistons are blackened as normal. However, there is that one little section on the #1 piston that is clean. I have suspicion that this was coming in contact with the head, however there are no pertaining marks on the head to confirm this or not. For good measure, here is a picture of the head.



#1 piston is on the far right on the picture. Intake side is on the upper half of the head. Inspecting the head, I don't see any physical damage done to it, though I haven't cleaned it up to take a closer look, or do any measurements to see if the head is warped or otherwise damaged.

The point of this post is, am I wrong in my suspicion that the piston was slapping the head, or is there another possible answer? I won't find out about the condition of the bottom end for at least a few more weeks, but I'm guessing the source of the knocking is from a stretched rod, rod or main bolts.

Thanks for any input.

Edit: Additional note, all the valves seem to be in good alignment, and none are bent.

185db

i am no mechanic or expert, however i once seen a piston rigged into an engine that did not belong. Because of that the engine was knocking and eventually stopped working at all. what we found when we took it apart was much the same thing you are seeing here. In order to make the piston fit (it was to tall) they used a rotery tool and actually carved out metal from the top of the piston in the shape of the head and made the shape of the valves too. The shape where the valves was was lower and rougher than the rest of the piston and was also shinney...??? To this day it still boggles my mind. Dont know if this will help but just thought it was a good place for it...



In theory it seems that the actual combustion in the cylinder may not combust in an area that does not fallow a particular shape. This may be why these areas are not being “scorched”. I cannot say what might cause this, but you may be able to turn your crank and see what the peak of the piston is compared to the other pistons.

BigVinnie

iTrader: (2)

I believe if you look at cylinder ones dome in the head you can see some welting caused by warping. The head is screwed...
It is probably fixable with some welding, machining and polishing.......

wikd240

i have seen this on a few other motors i have worked on and rebuilt... all have also had a rod knock... but i dont think this is the cause of the problem... i suggest that you clean the head extra good and have it checked... on the motors i worked on and rebuilt never had another problem again... and all of the pistons blocks and heads checked out to be good... i dont know what causes the clean part on the pistons... but it casued me no problems...

Kinematics

Thanks for the worthwhile advice. This motor certainly does have rod knock however. I'm suspicious of the rod bearings. We'll see when I tear it down, I suppose. On 240sxforums.com someone wrote me up a good bit of information, so hopefully that can help me determine the condition of the head and the rest of the motor.

Thanks. Good to see others are at least familiar with this situtation

wikd240

if there is a rod knocking... count on the crank being trashed as well as the rod... i have never reused a rod that has been knocking...

just in case you dont know what happens when a rod knocks... it usually means the bearings are so worn out that they have desintegrated... or in the case of a spun bearing the bearings have actually overlapped each other and one side of the rod has no bearing surface between it and the crank... in either case each revolution of the crankshaft causes the journal to smack into the unprotected rod and causes the knock... doing damage with every knock to the rod and the crank... it would have eventually broke the rod and caused even more damage to the motor... busted rods are usually what causes a hole in the block...

i would suggest at least a new rod... and you will need a new crank unless you have a good machine shop that can resurface your crank... but even with the crank resurfaced you will still need a new rod to be safe...

if you go with a crank from the parts store it will come with new bearings that match the machine work that was done on the crank... usually oversized bearings... unless you pay 700 bucks for one from nissan... you will end up with a remaned crank...

when getting a remaned crank... befor you install it check all the holes that are threaded into the crank... make sure none of the threads are messed up in anyway... it would also be a good idea to have a shop check it over to make sure it is correct... take them your block main bearing web and rods so the can make sure all the tolerances are correct... it pays to have it checked so you dont install a crank and bearings that will just knock again...

i would pay them a little cash to have them put it all back together again just so you know...

but good luck on the motor rebuild... and if i can be of any help just let me know...

Kinematics

Thanks for the help wikd240. I've got the motor pulled out and halfway torn apart, I have to wait on a crank pulley puller to get it all the way apart, but at least I know now what the issue is with the motor.


Lots of metal shavings. Rod #1 has about 3-4mm of play to it on the crank.


Here's the main piece I've found so far. Looks like rod bearing to me.

I'll probably add a few more pictures once I get the oil pump off and the rest of the assembly removed. Thanks for reading.

wikd240

yup you are going to need a new crank and at least that one rod... unless you are going boosted... i would suggest that you add the sohc rods and pistons to gain you a little power and compression...

Kinematics

I am going boosted, but not immediately on this motor. I have a KA I'm sticking in out of a 97 with 70k that I'll be boosting. That's going in hopefully in a week or two. Going to be collecting turbo parts over the winter and spring, and hopefully boost around summertime if all goes awesome.

This block I'm tearing down is going to just be for spare parts. I might just do a rebuild on it or sell it, or go nuts and spend the money it will take to make the block bulletproof and just run ridiculous amounts of boost . 9s?



That would be pimp. But for now, let me get this **** running first.

BigVinnie

iTrader: (2)

If you want to hit 9 seconds, you will want to look into a fully counter weighed stroker crank. Almost all the KA's I have seen with the half counter weighted only hit mid 10 seconds.