"Nasty Engine" to "Nice Engine".

pmac

Well-Known Forum User
hi pmac, out of curiosity do the pistons sit high because you've had the block skimmed?
or are the pistons just too high anyway because they're aftermarket?

Both reasons you mentioned contribute.
Every engine we build is different. Most of them are non standard spec/component.
Putting an all stock Nissan engine back is easier but ultimately boring.
The components used ie piston, cam, headgasket are all factored in the calculation.
There is block to piston and valve to piston clearance to think about.
 

pmac

Well-Known Forum User
Back to work after Christmas..........
Steve has made a start on the head.
Its a very "nice" P90.
As we are fitting aftermarket valves and springs the first job was a dummy build to check how much packing is required to get the correct installed spring height.
Good news....................two Nissan packers will do it without the need to get any custom ones made.:thumbs:
So its on with port matching and porting in general on the head.
Like several engines we have done this one has a newer style Mangoletsi inlet manifold so the head needs to be port matched to that using the template and dowels supplied.:bow:
 

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tel240z

Club Member
Today I did a "two for one " trip on the M1 and brought another block home along with the one in this thread..
I just want to divert a moment from the build to show anyone not familiar with the wide choice of pistons available two examples that are at opposing ends of the spectrum..
I am using an aftermarket replacement cast piston in this build. The other block has a special order JE slipper piston to be fitted.
There is a marked weight difference between them. Cast replacement 594G. JE slipper 297G
So On six pistons in an engine that add up to 1782g of extra rotating mass..
I may be able to get rod weight comparison too if anyone is interested.:confused:

The next step in the build now is to do a dummy build with one rod and piston to determine if they need licking down to size.
More pics and description as it happens.

Nice slipper pistons Pete, the black coating is it the break in coating or the tuff skirt variant ? would be interested a rod weight comparison but with what :unsure:
 

pmac

Well-Known Forum User
Nice slipper pistons Pete, the black coating is it the break in coating or the tuff skirt variant ? would be interested a rod weight comparison but with what :unsure:

Tuff skirt variant I think. They are going on some custom Carillo rods so it will be stock rod vs Carillo.
 

vipergts

Well-Known Forum User
Back to work after Christmas..........
Steve has made a start on the head.
Its a very "nice" P90.
As we are fitting aftermarket valves and springs the first job was a dummy build to check how much packing is required to get the correct installed spring height.
Good news....................two Nissan packers will do it without the need to get any custom ones made.:thumbs:
So its on with port matching and porting in general on the head.
Like several engines we have done this one has a newer style Mangoletsi inlet manifold so the head needs to be port matched to that using the template and dowels supplied.:bow:

Yes Pete, the very same as I used. Johns Template is very useful. I ported and matched my inlet to head on my TR6 the other day, no template but I could work through the short
manifold
 

pmac

Well-Known Forum User
Many tedious hours later and the inlet ports are almost finished.
The next job is to cc the head so Steve can calculate how much I need to get skimmed off it to reach the desired compression ratio..
The head and valves are now ready to be taken for final machining and checking and valve seat work.
 

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Many tedious hours later and the inlet ports are almost finished.
The next job is to cc the head so Steve can calculate how much I need to get skimmed off it to reach the desired compression ratio..
The head and valves are now ready to be taken for final machining and checking and valve seat work.

How do you calculate how much to skim off the head with the varying combustion chamber shapes of heads and if its been skimmed before etc? given the more you take off the more it reduces CC volume with each pass.


Seems like that could be a head scratching thing. Or skim to about what you think then remeasure and go back if its not quite right?
 

pmac

Well-Known Forum User
How do you calculate how much to skim off the head with the varying combustion chamber shapes of heads and if its been skimmed before etc? given the more you take off the more it reduces CC volume with each pass.


Seems like that could be a head scratching thing. Or skim to about what you think then remeasure and go back if its not quite right?

We just do our measurements and put it all in the L.series online calculator.;)

Then all you need to do is sprinkle a bit of Shamurai magic fairy dust on it:D
 

Jimbo

1978 260z in yellow
Club Member
How do you calculate how much to skim off the head with the varying combustion chamber shapes of heads and if its been skimmed before etc? given the more you take off the more it reduces CC volume with each pass.


Seems like that could be a head scratching thing. Or skim to about what you think then remeasure and go back if its not quite right?

if the head is slightly mishapen then you may find that skimming it will end up with combustion face cc greater at one of than the other so in effect the head is skimmed at an angle and not flat.
to correct it you can machine the combustion chamber to increase cc however its far better to calculate first like pmac said and get the machinist to skim the head accordingly.
ive seen it done where i work where they test for combustion chamber accuracy, if its out the machine has to be calibrated to account for the discrepancy across cylinders.
obviously it wont work if the fire face is banana shaped as the material taken off either end will reduce cc in either ends combustion chamber hence the need to remove material in the combustion chamber in that situation.
to be honest its chasing BHP that neccesitates doing it as in my opinion its a time consuming art form.
you wouldnt bother on a fiat punto or the like.
 
if the head is slightly mishapen then you may find that skimming it will end up with combustion face cc greater at one of than the other so in effect the head is skimmed at an angle and not flat.
to correct it you can machine the combustion chamber to increase cc however its far better to calculate first like pmac said and get the machinist to skim the head accordingly.
ive seen it done where i work where they test for combustion chamber accuracy, if its out the machine has to be calibrated to account for the discrepancy across cylinders.
obviously it wont work if the fire face is banana shaped as the material taken off either end will reduce cc in either ends combustion chamber hence the need to remove material in the combustion chamber in that situation.
to be honest its chasing BHP that neccesitates doing it as in my opinion its a time consuming art form.
you wouldnt bother on a fiat punto or the like.

I think you've misunderstood my question.

The combustion chamber isn't a circular hole where you can decide to remove 10cc, so given its equal shape you can work out volume and just remove that amount. All the various heads have their own chamber shape, ones that have been re-worked etc. So removing 0.25mm for example doesnt' reduce the CC by the same amount each time, without a 3d computer scan of the chamber you can't know how much to remove to hit a target CC? So do you make the first cut, fairly easy on a stock new head, then measure and do multiple skims?
 

jonbills

Membership Secretary
Site Administrator
You've got to cc the chambers to start with - say they measure 40. Say you want a chamber size of 37 to get the CR you want. You can use graph paper to measure the area of the chamber - put the graph paper over the chamber and trace round the edge. count the complete squares & treat the partial squares included as 1/2s. each little square is 1 mm2. Maybe you end up with 4000 squares, so 40 cm2. a 1mm slice of that is 4 cm3. so you need to have a 0.75mm skim to get to 37 cm3.

And yes, the calculation assumes that the the walls of the chamber are vertical, which they aren't of course. because they slope in, the volume you'll remove is a little below the estimate. so you will have to measure again and skim again possibly.
 

Jimbo

1978 260z in yellow
Club Member
So do you make the first cut, fairly easy on a stock new head, then measure and do multiple skims?

pretty much, but to be honest the people who do skimming professionally know from experience what to take off from the cc readouts and a couple of cc tests after a test skim and like jonbills said you can use the graph paper to predict roughly what to take off.
for example the heads we skim the company must have skimmed 5 of the same heads at different heights and measured the ccs as we have graphs with plotted cc against material removed so we know what to expect at different heights, albeit what i do is standard production not modifying to get higher compression ratios but the theory is the same.
the other problem of course is the combustion faces i have to deal with are machined so most are pretty equal opposed to those that are left rough cast which plays havoc with readings
 
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