ericbauer Posted August 2, 2013 Share Posted August 2, 2013 Hi all, I've been looking all over the web this morning for stock unshaven block/head heights. I'm just looking for a quick confirmation that these numbers are in fact accurate? All L24 L26 L28 heads = 4.248" / 107.9mm without a head next to me (I'm at work) I assume this is measured valve cover to headgasket surface All non-diesel L28 blocks 207.9mm measured from from crank centerline (ie main cap flat surface) to headgasket surface Just got both my L28 blocks back and they needed 8 thousandths and 10 thousandths removed to be flat again. PO didn't have any cam tower shims and yet I'm pretty sure the head is shaved too, on account of how close the NISSAN JAPAN on the side of the head is to the headgasket surface... Thanks for your help, -e Quote Link to comment Share on other sites More sharing options...
Xnke Posted August 4, 2013 Share Posted August 4, 2013 I measured three heads that I suspected to be uncut when I got them, and they were 108.0mm, 107.9mm, and 107.9mm. There are both 0.015" and 0.020" thick shims out there, although Silver Seal still sells the 0.020" thick shims. Quote Link to comment Share on other sites More sharing options...
ericbauer Posted August 5, 2013 Author Share Posted August 5, 2013 Thanks Xnke! Quote Link to comment Share on other sites More sharing options...
ericbauer Posted August 6, 2013 Author Share Posted August 6, 2013 Measured both my blocks now that everything's back from the machine shop... N42 3.1L stroker from Japan 265.52mm OAH from HG surface to OilPan Surface - 57.9mm from OilPan Surface to MainBlock Surface (ie crank centerline) = 207.62 In 35+ years it's been shaved just 0.28mm (0.011") The machine shop just removed 0.010" to make it flat again - so this was probably the first time it got shaved... Matching N42 head from the same Japanese stroker 105.7mm from HG surface to ValveCover surface... It's been shaved 2.2mm (0.086") ! ... Combustion chambers have been re-worked heavily and measure only 36cc PO didn't have cam shims under cam towers... odd no? I drove that car for about 5 months before I thought it was time for a rebuild (low cyl comp due to worn rings) Meanwhile my F54 2949cc Made In California block: 207.7mm So it's only been shaved 0.2mm (0.008") in its 30+ years Machine shop said he just removed 0.008" so it too was the first time it got shaved That makes sense because I pulled the F54 out of the junkyard myself - guess PO of that ZX never rebuilt his block. Matching E88 head P&P'ed by Rebello 106.9mm - so it's only been shaved 1mm (0.04") Quote Link to comment Share on other sites More sharing options...
Tony D Posted August 6, 2013 Share Posted August 6, 2013 This is because, like most "rebuilds"... They likely aren't required! My stuff pulled in the 80's and stuck on the shelf flat/straight/unmolested is by alchemy turning from Al to Au! Muehehehehe! Quote Link to comment Share on other sites More sharing options...
Leon Posted August 6, 2013 Share Posted August 6, 2013 How did you diagnose the worn rings? Low cranking compression doesn't automatically dictate a rebuild. Your long-duration cam bleeds quite a bit of compression at cranking speeds, so a compression gauge will show a lot lower than stock numbers. Was there a leak test done? How much oil did it burn? This rebuild may not have been warranted... Quote Link to comment Share on other sites More sharing options...
Leon Posted August 6, 2013 Share Posted August 6, 2013 Great research on the measurements though, it's good stuff for the archives. Regarding the cam tower shims, shimming is typically done to correct valve timing. The shims may have been avoided by design, to get the cam dialed in. Quote Link to comment Share on other sites More sharing options...
ericbauer Posted August 6, 2013 Author Share Posted August 6, 2013 Hi TonyD Hi Leon When I first bought the car it was showing 150psi across all six, a leak down test done at the time showed the rings were on their way out. No prior history came with that car, and neither I nor the seller knew exactly what was inside... I just had a really good feeling about it... glad I snapped up that car when I did. Five months later, when I decided it was time for a rebuild, my comp had dropped to 95-110psi across all 6. On the head / block numbers, I wouldn't mind if someone confirmed that oil pan to crank centerline is indeed 57.9mm. I measured both my blocks and that's what I came up with, but I never found an official dimension for this somewhere on the net. Quote Link to comment Share on other sites More sharing options...
Tony D Posted August 7, 2013 Share Posted August 7, 2013 (edited) I would never have rebuilt an engine with 150psi compression. Run it for 10K miles and see where it goes. Leak down on rings likely means they weren't seated yet. It's not uncommon for BMW engines to show decreasing ring leak don every check to 30,000 miles and slightly beyond! Once you reach that point, you then start charting. A single compression and leak down check IS NOT criteria for a rebuild...ESPECIALLY with 150# across the boards. The subsequent check (5K miles later?) showed something amiss. Equally dropping like that (same gauge used for testing is my assumption) seems to indicate ring/bore finish mismatch. I refer to my prior statement regarding unnecessary overhauls, as most would have just gone on the leak down alone and disregarded the compression. Always observe the trend in 1/3/5/10K miles and see what happens. People get so itchy to tear into an engine it overpowers the sound diagnostic process. Good to see you followed up...and didn't just jump to it. What did you find on the rings? Another engine rebuilt that dumped in <10K miles due to something or other done in the overhaul process. "Keep it together and run it!" LOL Edited August 7, 2013 by Tony D Quote Link to comment Share on other sites More sharing options...
ericbauer Posted August 7, 2013 Author Share Posted August 7, 2013 Hi TonyD Maybe I wasn't clear, so I'll restate:When I bought the car it showed 150 across all six It seemed a low number at the time, and a leak down test was performed which showed rings were leaking Car came with no history so it was impossible to know how long or how recently the 12:1 high comp stroker had been built I enjoyed the car thoroughly until end of July last year, that's when it started to feel like it was losing power - redid comp test and found all six in the 95 - 110 psi that's when i decided to tear it apart... anyways yeah... moot point now! lol... Quote Link to comment Share on other sites More sharing options...
Tony D Posted August 7, 2013 Share Posted August 7, 2013 No no no, I understand what you're saying. I'm just reinforcing your logic...as you stated it went from 150 to 110 in 5K miles no reason to go further. Likely it was a bore ring finish mismatch. This is very common in rebuilds... One of the reasons I tell most to leave it alone. They have 135 across all ix, no excessive oil consumption, no big leak/blowby issue.... If they rebuild on the cheap, they end up with a scenario like you describe: 150, dropping quickly to 100 in less than 10K miles. You don't KNOW how many "rebuilt" L-Engines I've seen that have low compression at <10K miles (lower than the rings they pulled out with 250K+!!!). It's the basis for my "chances are you will be worse off in a year than if you just left it alone" comment of times! If you rebuild, and rebuild RIGHT, it's not a "cheap" process. I suspect your engine got an "economy rebuild" and unfortunately you paid the price having to do it over. My bet is, with the rings going away that fast, it was a ring-bore mismatch during a comparatively recent "overhaul"--they just don't go away like that, not even on 13.75:1 engines! I agree with the process you used...I should have been clearer on that point. Quote Link to comment Share on other sites More sharing options...
Tony D Posted August 7, 2013 Share Posted August 7, 2013 An example: Bought two 1980 280ZX's at lein for $100 each. Both runners. After a month of hard driving to free up everything one was 145 across the board, one was 150 across the board. 225K and 195K respectively. I took the 195K engine to a kid who saw the mileage, and despite my warnings to the contrary re-ringed it with a flex hone because it used about 750ml in a 1,000 miles. The 225K engine was similar, I put it in my Fairlady. Now, years later, I am STILL using about 750ml of oil in between 1,000 to 3,000 miles (autoX vs driving interstate 80 at 85-110mph...) that was close to 80,000 miles ago. Compression has yet to change. His engine, well... Oil consumption was "up due to break in"... Compression was initially 175 (wet) and 165 (dry)... That went away within a year.he was to 120psi and a solid quart every 500 miles by the time he pulled the engine back out. Sadly he sold the car and bought a 91 IROC. Teenagers...what can you do, eh? That engine sits back in my shed. I'll hone it right one day and put it in something. But all the time he spent on parts...wasted! As with you...I got a Vair that supposedly was "hot motor"... When I pulled the cover to find lightened rods and forged TRW Slugs I actually shouted into the house to my then-wife "hey! Bring me the camera, that guy told me THE TRUTH about this engine!" LOL Sometimes you get a pig in a poke. My good fortune offset your engine separated by 20 years! Quote Link to comment Share on other sites More sharing options...
ericbauer Posted August 7, 2013 Author Share Posted August 7, 2013 haha ok cool - got it! Thanks TonyD ! -e Quote Link to comment Share on other sites More sharing options...
Dkyle Posted August 8, 2013 Share Posted August 8, 2013 Can I ask about the term " ..bore ring finish mismatch" ? Are you referring to the ring gap? If so, then what is proper gap for flat top pistons under a P90 head with just less than 9:1 CR ? Quote Link to comment Share on other sites More sharing options...
ericbauer Posted August 8, 2013 Author Share Posted August 8, 2013 I believe TonyD was referring to the actual honing finish pattern on the cyliner walls, as in how they are sanded / in what overlapping directions - the FSM is pretty specific, so are ring manufacturers. Quote Link to comment Share on other sites More sharing options...
Tony D Posted August 9, 2013 Share Posted August 9, 2013 Exactly, you finis the bore for chrome rings and install cast iron, you're in for problems. End gap is specified as thousandths of an inch per inch of bore. Frankly, going wide is better there than going tight. Quote Link to comment Share on other sites More sharing options...
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