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Discussion Starter · #1 ·
The Flattops are in and the block is bored 50 over. Haven't put the head on yet, waiting for the weekend. Interesting enough, the pistons stick out above the block just a wee bit. Add the gasket to the deck and I think it's not a problem. But I just wanted to know if it's normal for them to stick out like that?

Now that it's bored 50over and with flattops on an N47 head, what's the best way to find out displacement and compression? Hopefully somebody has already done this or has a formula I can use to calculate it myself.
 

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I'm not an expert, but here's some info.

> The Flattops are in and the block is bored
> 50 over. Haven't put the head on yet,
> waiting for the weekend. Interesting enough,
> the pistons stick out above the block just a
> wee bit. Add the gasket to the deck and I
> think it's not a problem. But I just wanted
> to know if it's normal for them to stick out
> like that?

Should be 0. All 6 cyl L series blocks have the same block height (oil pan gasket surface-head gasket surface should = 10.467) This can be difficult for you to measure without proper tools. The best thing to do would be to measure how much this 'wee bit' is. If it's something like .005 or so, then I wouldn't worry. An uncrushed stock head gasket is 1.25mm, and when it is crushed, usually ends up being 1.15mm or -.005. If they are sticking up just a 'wee bit', did the machine shop deck the block just a 'wee bit'?? Some shops will machine up to .005 off to do a clean up cut.

> Now that it's bored 50over and with flattops
> on an N47 head, what's the best way to find
> out displacement and compression? Hopefully
> somebody has already done this or has a
> formula I can use to calculate it myself.

Down to the formulas;
To find the total displacement of your engine:
Formula: pi/4 x piston (cm) x crank (cm) x no. of cyl.
79mm Crank
86mm +.050 = 87mm Pistons (.27mm clearance)
Cylinder volume = pi/4 x (8.7)(8.7) x 7.9 = 469.63cc
Total engine volume: 469.63 x 6 (cylinders) = 2817cc

Compression ratio: (assuming no head milling)
Formula: cyl. volume + chamber vol. /chamber vol.
N47 stock: 44.6cc
Stock Gasket: 1.25mm = 7.6cc
Cyl. volume: 469.6cc
Chamber vol.: 44.6cc + 7.6cc +0cc (flattops) = 52.2cc
C.R.: 469.6cc + 52.2cc / 52.2cc = 9.996 = @10:1

If you use a 1mm HKS gasket, substitute 6.5cc for the 7.6cc stock gasket.

If you milled the head, you need to know how much was milled, and what the chamber diameter is, so you can determine how many cc's are in it. Or, even better, cc the head, then you'll be sure.

240Dave
 

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Discussion Starter · #3 ·
Got it...

Steve,
We speak again! I just got my 78 running (.040 over with flattops) and mine stuck WAY out. I was really worried as to whether they would cause a problem or not. Let me run this scenario by you - the head has been shaved twice due to head gaskets and the block was decked as well. Judging by the rust pits, they probably took off a bit because the car sat for 4 years. I am proud to say that it all went back together like clockwork and the clearance turned out to not be a problem with the head gasket and head on. Should you run into clearance problems, Nissan motorsport makes a real thick head gasket for performance motors (it ain't cheap!) but it gives you additional clearance if needed. Hope this helps

Tim McGovern
White 78 280

> The Flattops are in and the block is bored
> 50 over. Haven't put the head on yet,
> waiting for the weekend. Interesting enough,
> the pistons stick out above the block just a
> wee bit. Add the gasket to the deck and I
> think it's not a problem. But I just wanted
> to know if it's normal for them to stick out
> like that?

> Now that it's bored 50over and with flattops
> on an N47 head, what's the best way to find
> out displacement and compression? Hopefully
> somebody has already done this or has a
> formula I can use to calculate it myself.
 

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Discussion Starter · #4 ·
Re: Got it...

> Steve,
> We speak again! I just got my 78 running
> (.040 over with flattops) and mine stuck WAY
> out. I was really worried as to whether they
> would cause a problem or not. Let me run
> this scenario by you - the head has been
> shaved twice due to head gaskets and the
> block was decked as well. Judging by the
> rust pits, they probably took off a bit
> because the car sat for 4 years. I am proud
> to say that it all went back together like
> clockwork and the clearance turned out to
> not be a problem with the head gasket and
> head on. Should you run into clearance
> problems, Nissan motorsport makes a real
> thick head gasket for performance motors (it
> ain't cheap!) but it gives you additional
> clearance if needed. Hope this helps

> Tim McGovern
> White 78 280

Hey Tim!

Thanks for the good info. How's your engine running? Got pinging? What gas are you using? Is power up significantly?

Thanks again

Steve
 

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Discussion Starter · #5 ·
None so far

> Hey Tim!

> Thanks for the good info. How's your engine
> running? Got pinging? What gas are you
> using? Is power up significantly?

> Thanks again
> Steve

Hey Steve!
The engine is running great - I just got around 300 miles on it so I have yet to re-torque the head and readjust the valves. No pinging at all - I am running the Amoco 93 gas and I have the timing set WAY up and it does not ping. I have not gotten on it full throttle yet, nor had I driven the car before I got it, so I can't vouch for the power statistics, but I used to work for the Z Shop here in Atlanta for 3 years and I drove a boatload of Z's during that time. This one definitely keeps up with the Joneses - even for an automatic (that will be changed soon!) it has as much power as just about any Z I have driven - well - save for my 240 and 260,(highly modified) but for a stock fuel injected car - it is very strong. I hope yours turns out the same way - you should be able to tell a bit of a difference in seat of the pants acceleration. Good Luck and keep us posted - the car looks like it ought to be very sweet when its done!

Tim McGovern
White 78 280
 

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Re: None so far

Steve,
I have an 83 block with the standard flattops, and have just a week on it, about 200 miles. Anyways, when I put my pistons in, I thought the same thing, they stuck out of the block by about 20-30 thousands. This worried me, so i checked it when the head went on, make sure nothing was hitting. It is running good and no problems yet. I could see on the pistons when I cleaned them that the part of the p79 head, the little point area, was shaped on the pistons, outlined in carbon basically. I do not think those things stay far aay from the head. I am about to pull the engine apart that came out of the car, another 83. This time though I have a cam, will bore it over, use either a N42 or N47 head, I am not sure which it is that I have, but it is off a 280z injected car, it that the N47? Anyways, I have other mods as well, can't wait. Let me know how yours runs. By the way, does the n47 have hardened valve seats? The bigger valves? I have not pulled it from my other engine, I am getting a collection here, two extra engines, 4 cars, extra 5-speed, etc. Good luck with yours this time.
Joshua L.
 
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