ttodhunter Posted February 18, 2022 Share Posted February 18, 2022 Hey all, I'm putting back together my motor and have found that (especially) the #1 piston, at TDC, is above deck at the rear and at deck at the front. I am measuring approximately 15 thou above deck, but only at the rear of the piston. I don't have a mechanics straight edge, but used a framers square across the top and can't fit a 10 thou feeler anywhere along the deck. I plan to use a standard FelPro HG which should still leave 20-25 thou clearance, but not sure if that's enough. I also worry that there is a problem somewhere that once I fire it up will reveal itself. Has anyone else come across a similar situation? The block was not decked since rebuild. Feel free to make me look dumb. N42 Block P90 Head L28+1 dished pistons Thanks for your input! Quote Link to comment Share on other sites More sharing options...
madkaw Posted February 19, 2022 Share Posted February 19, 2022 So are you saying the piston height is different from front to back? Can you rock the piston in the hole ? Stock spec piston height should be above deck . Quote Link to comment Share on other sites More sharing options...
ttodhunter Posted February 19, 2022 Author Share Posted February 19, 2022 Yes, higher in the back than in the front. I'm going to spend a little more time with it today and will try swapping #1 and #2 to see if the problem is with the rotating portions or with the hole. Thanks for the reply. Quote Link to comment Share on other sites More sharing options...
AydinZ71 Posted February 19, 2022 Share Posted February 19, 2022 That’s weird… I’m not a machinist, but .015” is a LOT. Can you “feel” the difference with your finger tips? Trying to rule-out your measurement device. What would naturally cause your piston to “pivot” on its axial travel would be improper ring gap or worn rings. Beyond that, it could be many things. My head is off, and I’m at -0.35mm (negative for protruding piston) on all 6. i boosted an L24 twenty years ago and thoroughly wore-down the rings to the point where the piston side-skirts slapped the cylinder walls. Detonation was involved. Quote Link to comment Share on other sites More sharing options...
NewZed Posted February 19, 2022 Share Posted February 19, 2022 On 2/18/2022 at 8:13 AM, ttodhunter said: L28+1 dished pistons 3 hours ago, ttodhunter said: try swapping #1 and #2 to see if the problem is with the rotating portions or with the hole What brand of piston? Add connecting rod to the possible cause list. If the problem stays with #1. That's probably what you meant by hole. Interesting problem. Quote Link to comment Share on other sites More sharing options...
ttodhunter Posted February 19, 2022 Author Share Posted February 19, 2022 Just got back home and will now put some time in. Yes, probably not a machinist grade measuring device, but you can feel the piston over the deck on the rear. These are the ITM pistons. I'm really hoping it's not the rods... I'll post back with results. Quote Link to comment Share on other sites More sharing options...
ttodhunter Posted February 20, 2022 Author Share Posted February 20, 2022 I went and measured all pistons, Front and rear: #1 F -1.5, R +13 #2 F +3, R +8 #3 F +3, R +8 #4 F -1, R +10 #5 F +3, R +8 #6 F +10, R -3 I swapped #1 and #6, and the readings remained unchanged with piston assembly #1 in hole #6 reading F -1.5, R +13. Same thing for #6 in #1 hole. I am now suspecting that the rods are bent, so I will take them and get them checked out. I'll post results, but it may be a while. Any other comments or suggestions are welcomed in the meantime. Thanks for all the input so far! Quote Link to comment Share on other sites More sharing options...
AydinZ71 Posted February 20, 2022 Share Posted February 20, 2022 Have your crank checked too while you are at it. The Nissan L series cranks are bulletproof, but they can warp in some cases. Just like the block/deck can. My diesel crank was off by .005 and it had to be straightened. Quote Link to comment Share on other sites More sharing options...
ttodhunter Posted February 25, 2022 Author Share Posted February 25, 2022 So, I took #6 to the machine shop to check for straightness. He put it on a template and found it slightly off, put the crank end in a vise and a dowel on the pin and did a few rounds of hammer blows. He got it to 'as straight at he could get it'. I took it home and put it in the hole, and it now measures F +4, R +3. This appears to be the root of the problem. The question now becomes: do I have him repeat for all cylinders, or do I get new rods. Would you all have concerns with rods that were bent being straightened in this manner? I worry, and I'm no metallurgist, that when they heat up, they may bend back to their previous state. Or, perhaps, that their strength has been compromised. He did note, as he was hammering, that they were very tough rods. Thanks again. Quote Link to comment Share on other sites More sharing options...
AydinZ71 Posted February 25, 2022 Share Posted February 25, 2022 Can you afford a whole new bottom-end? I bought mine 6mo ago for $350. That would be the most convenient option (Quick swap). If you find a set of spare rods, crack open your “how to rebuild” book and read carefully if you are attempting it yourself. You need to check clearances etc. i also believe the wrist pin is pressed in on OEM rod so, there’s that. What are your future plans? If you plan to race the car, upgrade to lightweight rods. Rods are also the weak-point on a turbo engine. About $1500 for a quality set from Pauter. Quick facts on steel: if the rods are heated to the point where they change “color”, the metal becomes annealed. This is the most ductile state of the metal as the molecules have rearranged themselves. If the OEM rods were heat-treated (I don’t know anything about OEM forged parts), you are losing the “hardness” that came from OEM treatment process. I would toss them and not reuse them, unless you know what you are doing and can re-heat treat and “quench”. This process is done BEFORE machining, as heat-treating can warp the steel Quote Link to comment Share on other sites More sharing options...
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