Oddmanout84 Posted February 21, 2011 Share Posted February 21, 2011 So I'm rebuilding my shortblock and after all my clearances for the crankshaft have checked out well, I moved on to gapping my piston rings. I remeasured each bore (overbored the F54 block .50mm last summer) with a bore gauge and vernier micrometer with the garage temperature at about 50*F, and came out with a 3.409" bore. That's about 86.58mm, where the machinists bored to based on my 86.5mm ITM cast pistons and me telling them it was going to be turbocharged to 20psi max. There were no specs or instructions written on the box of pistons or the supplied rings. I have not been able to find much about the rings online, either. The boxes have no brand label on them whatsoever, but thankfully they do have a part number: SWN30066-2 CYL. Searching around discovered that this in an NPR part number, but still no specs. I looked up average gap specs both in my Datsun rebuild books and on the internet. The rebuild book says between .013" and .017", for stock engines. The "How to Modify your Datsun OHC" book says .004" gap per inch of bore, up to .002" more for high power engines. Online sources from Wiseco says .005" per inch (.0055" per inch second ring) for street modified turbo/nitrous. This site: http://www.aa1car.com/library/ring_end_gap.htm says .0045" to .005" per inch for a street performance engine. When I squared my first top ring into the bore, the smallest feeler gauge I could fit into the gap with a light drag was .020". Using the equation above, that's about .006" per inch of bore, which puts the gap into the recommended for "race only" category in just about all the sources I found. ****. I'm wondering, is this way too excessive? It sure seems like it, seeing as I intend to use the engine for a hot street car, not full blown race. All of the pistons are really just similar to OEM cast specs, except they've got Swaintech coatings. The most boost I plan on running is about 20psi (when I replace the stock turbo with a T3/TO4E). I'd have a hell of a time returning the rings, since the box set they came in with the pistons was purchased on ebay over a year ago. The only other option I guess would be to find another set of L28ET rings meant for a .50mm overbore, and even then are they going to fit my current pistons (if the ring manufacturer was say, Nismo or Perfect Circle)? Or am I really just flying off the handle and .020" is a good gap to go with? Quote Link to comment Share on other sites More sharing options...
Oddmanout84 Posted February 22, 2011 Author Share Posted February 22, 2011 Any anecdotes on ring gaps used? Oil consumption issues? Quote Link to comment Share on other sites More sharing options...
Noddle Posted February 22, 2011 Share Posted February 22, 2011 You can file the ends of the rings, to make the gap larger. Quote Link to comment Share on other sites More sharing options...
mark Posted February 22, 2011 Share Posted February 22, 2011 (edited) Based on the number you posted at .005 per inch comes out to an end gap of .017. I can't feel .003 difference with a feeler gauge. I'd put it together and not think twice about it. That's about all the help I can offer. Edited February 22, 2011 by mark Quote Link to comment Share on other sites More sharing options...
Oddmanout84 Posted February 22, 2011 Author Share Posted February 22, 2011 You can file the ends of the rings, to make the gap larger. Well, I have a ring filer wheel, but it has the unfortunate inconvenience of only taking material away from the rings and doesn't add it! If my gaps were smaller than spec, I'd have no problem at all grinding them down. Unfortunately, they're a bit larger. Based on the number you posted at .005 per inch comes out to an end gap of .017. I can't feel .003 difference with a feeler gauge. I'd put it together and not think twice about it. That's about all the help I can offer. I hope so. I'd like to imagine that it wouldn't make much of a difference either, but I'm trying to keep tolerances as tight as possible. Guess that's why I'm freaking out about it all. At worse I'll just end up with a lot of blowby and oil burning? Quote Link to comment Share on other sites More sharing options...
mark Posted February 23, 2011 Share Posted February 23, 2011 Have you checked all the rings or just the top ring? Quote Link to comment Share on other sites More sharing options...
Oddmanout84 Posted February 23, 2011 Author Share Posted February 23, 2011 Have you checked all the rings or just the top ring? Just a few of the top rings. All results were consistent. I was seeing if i could garnish any input based on the top rings first, because if I needed a new set because of excessive clearance checking the second rings and oil scrapers seemed like a moot point. Quote Link to comment Share on other sites More sharing options...
mark Posted February 23, 2011 Share Posted February 23, 2011 Send jt1 a pm. He will know the answer. Quote Link to comment Share on other sites More sharing options...
Noddle Posted February 24, 2011 Share Posted February 24, 2011 I hope so. I'd like to imagine that it wouldn't make much of a difference either, but I'm trying to keep tolerances as tight as possible. Guess that's why I'm freaking out about it all. At worse I'll just end up with a lot of blowby and oil burning? oil burning ?, I suppose it would depend on the type of oil rings you have, the full type , 3 piece type I would think would not leak, the single type may leak a little, but how much more oil could escape from a 0.508 mm gap Nigel Quote Link to comment Share on other sites More sharing options...
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