pcakes55 Posted January 28, 2010 Share Posted January 28, 2010 So, I disassembled my motor (l28, f54 block) in class today, and come to find out it's been rebuilt not too long ago. The pistons are stamped ".75", and the bores measured out to "86.76mm" consistantly. So the block has been bored out .75mm over, clearly. But I'm trying to figure out if my pistons are dished or flat-tops. They are slightly recessed, but I can't seem to find out if the flat-tops came with a recess also, but just a smaller recess than the dished; Or are the flat-tops just completely flat with no recess. If these are indeed the dished pistons, I think I'll re-use them for my turbo build and save on $x-amount for forged low compression pistons. Can anyone clear this up for me? I will post pictures tomorrow if need be. Quote Link to comment Share on other sites More sharing options...
FlatBlack Posted January 28, 2010 Share Posted January 28, 2010 Flat top L28 pistons have no recess at all on them http://i254.photobucket.com/albums/hh81/flatblack280/Head%20Work/IMG_0086.jpg?t=1255450556 Could you post some pictures? I'd be interested to see what you have. Quote Link to comment Share on other sites More sharing options...
letitsnow Posted January 28, 2010 Share Posted January 28, 2010 Dished pistons have a round recess in the middle, it starts about .25" from the bore. Quote Link to comment Share on other sites More sharing options...
19762802+2 Posted January 28, 2010 Share Posted January 28, 2010 So this may be a stupid question but I thought I would go for it. If you look in through the spark plug hole in the head is it possible to tell if the pistons are dished or flat top? the reason I am asking this is because a previous owner put a N47 head on my N42 block and it looks like flat tops if I look through the spark plug hole. I would take the head off to be sure but I am not comfortable with my experience enough to take it off without messing up the timing plus I don't really have any reason to take it off otherwise. Quote Link to comment Share on other sites More sharing options...
FlatBlack Posted January 28, 2010 Share Posted January 28, 2010 This is the only picture I have of dished pistons, it's a BHG'd N42. http://i254.photobucket.com/albums/hh81/flatblack280/Head%20Work/100_2769-1.jpg Yes, you can look through the spark plug hole to see what you have if you rotate it up to TDC. The PO probably didn't convert to flat tops on a N42/N47 setup, that would bump to SCR up to the mid 9s I believe. Quote Link to comment Share on other sites More sharing options...
19762802+2 Posted January 28, 2010 Share Posted January 28, 2010 Yes, you can look through the spark plug hole to see what you have if you rotate it up to TDC. The PO probably didn't convert to flat tops on a N42/N47 setup, that would bump to SCR up to the mid 9s I believe. I have it at TDC on the compression stroke right now and I am 80% sure from seeing those pictures that it has flat tops. I will see If I can go get some pictures. I might have to dig up the previous owners details and ask some questions too. Thanks for the quick reply though. EDIT: I went and checked it out, it was hard to tell on #1 cylinder but I looked in #3 and #4 and the pistons are dished it was a bit harder to tell from the first piston because it was TDC but since the others were further down I could see the side opposite from the spark plug hole. thanks for the help guys. Also I tried to take some pictures but none came out good. Quote Link to comment Share on other sites More sharing options...
Leon Posted January 28, 2010 Share Posted January 28, 2010 I have it at TDC on the compression stroke right now and I am 80% sure from seeing those pictures that it has flat tops. I will see If I can go get some pictures. I might have to dig up the previous owners details and ask some questions too. Thanks for the quick reply though. If the engine is in good condition just do a compression test. That should tell you whether the pistons in there are flat or dished tops. I don't want to give a wrong number from memory, but you can search and look up compression values for different compression ratios. Quote Link to comment Share on other sites More sharing options...
junglist Posted January 28, 2010 Share Posted January 28, 2010 These are not OEM Nissan obviously, but they ARE L28 pistons pictured. These are what dished pistons look like: If the image doesn't load, go here: http://www.store.partsdinosaur.com/product2039.html Quote Link to comment Share on other sites More sharing options...
pcakes55 Posted January 28, 2010 Author Share Posted January 28, 2010 Thanks for the quick responses guys- I beleive that the early 280zx came with dished in the N/A and yeilded ~8.3:1, and then starting in (82?) they switched to flat-tops which bumped the CR up to 8.8:1 for a little more power. I'm going from memory so maybe this is completely wrong. BUT, are the dished pistons found in the N/A the same as the dished found in the Turbo motors? I'd assume the aftermarket (.75mm over) pistons that are in my block yeild the same size "dish" as the stock, and if they are the same as the turbo pistons, then I can just re-use these for my turbo build. Bad/Good idea? (as long as clearance between pistons and walls are within spec, of course) I will also be using the P90 head, not the N/A. Quote Link to comment Share on other sites More sharing options...
FlatBlack Posted January 28, 2010 Share Posted January 28, 2010 Thanks for the quick responses guys- I beleive that the early 280zx came with dished in the N/A and yeilded ~8.3:1, and then starting in (82?) they switched to flat-tops which bumped the CR up to 8.8:1 for a little more power. I'm going from memory so maybe this is completely wrong. 79-80 Model years are N42 blocks, 81-83 are F54 blocks, both N/A and turbo. The turbo F54 blocks had the same 10cc dished pistons as the N42 N/A pistons, but the ringlands are thicker. The N/A F54s have flat top pistons, but it only bumped the SCR to 8.5:1 stock vs. the 8.3:1 from the N42. I have a 1mm HG [stock is 1.5mm] on my motor, and according to LEngine.exe it is at 8.7:1 now. Check out this Nissan overview with actual numbers: http://forums.hybridz.org/showthread.php?t=153226 BUT, are the dished pistons found in the N/A the same as the dished found in the Turbo motors? I'd assume the aftermarket (.75mm over) pistons that are in my block yeild the same size "dish" as the stock, and if they are the same as the turbo pistons, then I can just re-use these for my turbo build. Bad/Good idea? (as long as clearance between pistons and walls are within spec, of course) I will also be using the P90 head, not the N/A. I'm using the flat top pistons, everything is going great so far. The N42 should be fine with boost and proper engine management. Here are the details: http://forums.hybridz.org/showthread.php?t=153195 Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted January 28, 2010 Administrators Share Posted January 28, 2010 Left= Dished Middle= US Flat-top Right= Euro Flat top, (slight raised circular dome on top). Quote Link to comment Share on other sites More sharing options...
FlatBlack Posted January 28, 2010 Share Posted January 28, 2010 That's great, thanks for the pics, BRAAP. Do you have any L28ET/L28E/Late L28E side shots to compare the turbo to the N/A pistons? Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted January 28, 2010 Administrators Share Posted January 28, 2010 That's great, thanks for the pics, BRAAP. Do you have any L28ET/L28E/Late L28E side shots to compare the turbo to the N/A pistons? Maybe... I have some new in the box L28ET pistons, might have an old L28 piston or 6 laying about.... I'll go check. Quote Link to comment Share on other sites More sharing options...
FlatBlack Posted January 28, 2010 Share Posted January 28, 2010 Maybe... I have some new in the box L28ET pistons, might have an old L28 piston or 6 laying about.... I'll go check. I couldn't tell if the Turbo Supplement that I scanned was comparing the N/A dished piston or the N/A flat top piston, there is a dish in the diagram it looks like. Are the ringlands the same size on the dished and flat top N/A pistons? Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted January 28, 2010 Administrators Share Posted January 28, 2010 I couldn't tell if the Turbo Supplement that I scanned was comparing the N/A dished piston or the N/A flat top piston, there is a dish in the diagram it looks like. Are the ringlands the same size on the dished and flat top N/A pistons? Sorry, I do not have any more N/A pistons. Differences in the L-28 pistons... The dish; Turbo dish and N/A dish share the same volume and shape. Ring lands; All early pistons, use the same thickness of rings since the early L-series was produced, (2mm thick for the 1st and 2nd rings). Ring lands changed when the flat tops came into existence, (same time as the Turbo came to be). Flat top and Turbo pistons have a thinner top ring, (1.5mm). I am pretty sure that the Turbo pistons also have the ring pack set a little further down the piston to get the rings further away from the combustion heat, but not 100% sure if that is the case with the OE. That is common practice in the performance aftermarket for boosted applications. Hope that helps. Quote Link to comment Share on other sites More sharing options...
pcakes55 Posted January 29, 2010 Author Share Posted January 29, 2010 Here are the pictures I snapped after I cleaned them up today: Quote Link to comment Share on other sites More sharing options...
pcakes55 Posted January 29, 2010 Author Share Posted January 29, 2010 sorry about picture size, photobucket = PIA. Quote Link to comment Share on other sites More sharing options...
letitsnow Posted January 29, 2010 Share Posted January 29, 2010 They're dished Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted January 29, 2010 Administrators Share Posted January 29, 2010 They're dished Yeup, and they are the Turbo pistons as well. Note the thinner top ring vs the thicker 2nd ring. Quote Link to comment Share on other sites More sharing options...
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