Lazeum Posted October 13, 2011 Share Posted October 13, 2011 (edited) I'm in the process of rebuilding my L28 engine since some lub issue occured and destroyed the head. Engine got contaminated with metal chips so I took it all apart to inspect & clean everything. Bottom end is F54, Flat top (+0.50), stock crankshaft. Mileage since last rebuild is small; less than 10k miles; pistons look new, no sludge anywhere. I've dropped the engine to a machine shop this week. Crankshaft needs some grinding & block needs to be honed (probably due to contaminated oil). It happens diameter difference between piston skirt & cylinder bore is measured at 0.08mm (0.0031â€) Checking service manual, I found that it should be: Bore diameter = Piston skirt diameter + gap – 0.02mm Where: gap + 0.02mm = 0.005 to 0.025mm => gap = 0.025 to 0.045mm Ring gaps with top ring was at 0.50mm (0.020â€) – Seem too big to me according to Service manual & “how to rebuild L engine†book. So I’m basically out of spec. My conclusion is I need to rebore the block and change my pistons with 0.040†overbore (+1.0mm) to do it right I see 3 options, budget is tight but I want to do it right: - I keep what I have – I just ask for honing & new rings. It is most likely the cheapest option but it doesn’t look like a wise decision based on comment above. - I source a set of 87mm pistons set – besides black dragon, I haven’t found what I’m looking for. I’m not even sure they are flat tops & I question the quality. - I find an old block & I rebore it to 86.5mm to match my current setup – it is hard to find though for a fair price here in EU. What’s your opinion? I am that far out from spec with my current setup? Edited October 13, 2011 by Lazeum Quote Link to comment Share on other sites More sharing options...
ap72 Posted October 13, 2011 Share Posted October 13, 2011 your piston to wall clearance and ring gap are right where they should be. I would hone it, new rings, grind/polish the crank, do the main bores if necessary, new bearings, and call it good. Be sure to clean everything VERY thoroughly before reassembly. Quote Link to comment Share on other sites More sharing options...
Lazeum Posted October 13, 2011 Author Share Posted October 13, 2011 (edited) Thanks for your answer I'm back home, my first move was to check "how to rebuild your Datsun OHC Engine". Page 66, I can read: Standard Piston-to-Bore clearance is 0.002 in. (0.050 mm). Absolute maximum clearance, or the difference between your piston & bore measurements, should be no more than 0.009 in. (0.229 mm) So with my 0.080 mm, I'm right where I should be However, I've surfed over the Internet to see what should be piston-to-bore clearance on other engines. Most of them are more around 0.0015 max. Ok, it is not a Datsun engine but the recommended clearance is most of the time below my measurements. So back to Datsun Service Manual, here's a copy. It is not in accordance to what the book mentioned above is stating. Is there something I don't understand I'm still confused - but I start to feel better! Edited October 13, 2011 by Lazeum Quote Link to comment Share on other sites More sharing options...
beermanpete Posted October 13, 2011 Share Posted October 13, 2011 The piston to cylinder clearance is typically larger than stock for modified engines, especially when using forged pistons. If you are building your engine using all OEM parts for normal use follow the factory specifications. If you are building a higher performance engine follow the guidelines from the piston supplier, the how-to book, or a trusted source such as the machine shop doing the work on your block and head. Quote Link to comment Share on other sites More sharing options...
duragg Posted October 16, 2011 Share Posted October 16, 2011 (edited) If I recall my machinist had me use 1.8 to 2.2 thou (.0018"-.0022") on my motor 3 months ago. The engine was a fairly recent rebuild and when disassembled I found +.50mm pistons with .004"+ piston to bore (sorry for mixing units) We blew it out the bores to use +1mm pistons and the tighter clearance. Edited October 16, 2011 by duragg Quote Link to comment Share on other sites More sharing options...
Lazeum Posted October 16, 2011 Author Share Posted October 16, 2011 I think it is a safe move. I'm leaning torward the same option... I need to find pistons then. What was the source you've been using for the pistons? So far, the only source I've found with 87mm pistons for L28 is MSA with forged Ross unit, quite overkill for my application (and $$$!) Is there eventually a Mahle pn# that could match? It might be the easiest route for me since a friend of mine is sellling the brand. Quote Link to comment Share on other sites More sharing options...
Lazeum Posted October 18, 2011 Author Share Posted October 18, 2011 I've finally ordered some new pistons at +0.030 in. (+0.75mm). Those are ITM units with new rings. Block will then be rebored, surfaced & honed. Price was very fair (200$ for the set) so even if situation was not that bad, it seems safe to follow this direction. Since my current pistons are in very good shape I might even get some money back with them at some point anyway. Thanks all for your comments & help. Quote Link to comment Share on other sites More sharing options...
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