Sealik Posted October 3, 2011 Share Posted October 3, 2011 (edited) Hello... Have some queries on my recent purchase. It's a 69 510 with a L28 in it. Price was good...and...can't quite say no to any older 4-sale Datsuns in this locale Looks like someone started this project years ago and never quite finished. Bolts missing here and there, old gas, no carbon on head...etc., still has the cross-hatching marks in the cylinders from honing/boring. The block is a N42 with P79 50 pistons...assuming the cylinders were bored 50 over Pistons are above deck by about .5mm The head is a E31....looks like it had recent work on it Also looks like it's been shaved...From front of engine..4.228, then 4.231 at center and 4.240 near back of head. Wedged it seems... Stock head is....4.248??? Additional marks on head Head ,intake (E46?) and SUs are from the 71 240Z I believe....SUs need to be cleaned etc. Anywho......trying to figure out my options to get the pistons to clear the valves, slight contact as seen per pic Looks like the block hasn't been decked....but I read somewhere that with different crank/ connecting rod/ pistons from the 240 and or the 280 will give you the .5-.6mm positive piston deck height??? Cam is stock(?)...around .413 Don't really want to pull the pistons and get reliefs cut into them.....but.....will a 2 mill HG work? Thanks Oh.....here's the ride... And 1 of my 720 4x4s with a recent LZ23 that I built.... Edited October 3, 2011 by Sealik Quote Link to comment Share on other sites More sharing options...
NewZed Posted October 3, 2011 Share Posted October 3, 2011 This calculator, which seems to be pretty good (I haven't seen a bad word about it)- http://www.ozdat.com/ozdatonline/enginedesign/ - might help you out. You can even put negative numbers in the head gasket thickness box to simulate a significant head shaving. It will tell you if you have piston-valve contact but doesn't say if you'll have piston "pop-up." And, no expert, but that piston-valve contact divot in your picture doesn't look slight, does it? Looks like it hit pretty hard and moved some aluminum. You can run carbs on an N42 or N47 head from an L28 (the common head for that block), they still have the threaded carb manifold holes. That would get your CR down from the >10 the E31 gives, if you want to avoid those potential problems. Not sure about the P79 or P90 heads and carbs, the threaded holes might not be there any more, like they are in the N42 and N47. Looks like a fun puzzle to piece together. Quote Link to comment Share on other sites More sharing options...
Sealik Posted October 3, 2011 Author Share Posted October 3, 2011 (edited) Looks like a fun puzzle to piece together. Thanks...I am somewhat baffled on what the PO did Wasn't sure on the negative deck height for the stock 280 flat top piston.....3mm????? So....if that is correct....add .5mm for mine that are above deck...multiply that by 2, add 79...equals...86 inch stroke? Think I'm gonna need 2 HGs to drop the CR below 10 and clear the valves........ Edited October 3, 2011 by Sealik Quote Link to comment Share on other sites More sharing options...
Tony D Posted October 3, 2011 Share Posted October 3, 2011 The best bet is that someone screwed up the timing chain install and on first crank-over the pistons hit the valves. It wasn't due to shaving or excessive lift...it was because the timing was off. That would be my bet. Quote Link to comment Share on other sites More sharing options...
Sealik Posted October 3, 2011 Author Share Posted October 3, 2011 (edited) The best bet is that someone screwed up the timing chain install and on first crank-over the pistons hit the valves. It wasn't due to shaving or excessive lift...it was because the timing was off. That would be my bet. Thanks Tony...you could be right because those marks on the pistons don't look fresh. I started the L last week...ran like ****...but it did flash up, chugged actually with bad gas and all Assuming it sat longer than the previous owner stated. What I'm trying to find out is where the stock flat tops sit relative to the block at TDC Then I could add my .5mm (above deck) and determine stroke...then compression ratio. Factoring in the head that has been shaved at least .010 of course I calculated the stoke at 86...assuming a stock negative deck height of 3mm Bore 86.2 Hg....1.2 E31 volume...42.4cc Deck height... .012 inches Would be 10.62 without the shaved head? Or... 9.84/2mm HG without the shaved head? First time calculating Cr sooooo... Maybe Edit....measured the stroke...it is 79mm Disregard my CRs Edited October 3, 2011 by Sealik Quote Link to comment Share on other sites More sharing options...
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