just checking my piston to deck height. now should i be measuring A and B or C and D? then averaging the two. ive read that doing both sides are best to take into account the rock of the piston which would be C and D i assume?

whats the consensus on this?

thanks

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Forget all four, and find the highest point of the piston relatively to a straightedge placed on the deck. The average does not matter, the maximum does. The goal is to ensure the piston and the head do not touch during operation, by ensuring a minimum piston-to-head clearance

That being said I found no significant variation across the piston crown in my engine

If you are worried about deck height variation, that is covered. The FSM instructs to check that with a straightedge, and the allowed distortion limit is 0.10 mm

A,B,C and D are for measuring the bore rather than the deck height. Here is what the Datsun FSM says, other repair manual (Haynes, etc) also cover this

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Well I've measured one of the Pistons at position A and B and there is slight variation. A is .23mm and B is .30mm and that's using a bridge and dial gauge. Is that OK as I'm working out my CR.

That's only 0.07 mm, which is not very significant. For estimating Compression Ratio, you can just use the average. Piston rock doesn't apply as that only occurs in operation (and will average out in any case)


Better yet you can directly measure the piston height re Compression Ratio when you measure the piston dish. Smear some grease around the edge of the piston (down in the rings lands) then put plexiglass over the bore and see how many CCs it takes

You gotta measure dish, right? So do it all in one measurement. This measures the CCs of the Piston Dish + CCs due to Piston Height


After measurement, rotate the piston down and wipe off the excess grease

Hi when you do deck height you put the oil rings on number 1 and nunber 4 piston
put the crank in with the bearings front and rear mains put pistons in then use a dial clock measure the center of piston (deck has to be clean) then rotate clock to deck measure ,then do rear piton @TDC,NOW you know if block is on angle, facing is done of sump face so when mount it in machine you clock it in and shim it if need be , rods will play a part if lengths are different ,Having deck @0 is ok you may have to check piston to valve
depending lift and dur & valve size and piston type chapo

What is the goal of measuring the piston this way, just to see if all four match? Center of piston is the dish, so measure the center of piston won't give you piston-to-head clearance?

Ok measure at edge , blocks some times run up hill or down IF you need ZERO deck max comp(square deck) front to rear
you may have to shim block in mill before facing deck, You still clock block in on mill (after you know, up or down at 1 end on bench) it maybe flat on sump rail and deck runs up or down hill or Rods maybe out or tunnel or a combo of all
Mark chapman (chapman brothers racing engines perth au)

When ever I have machined an Engine what was done was to mount the block on a fixture where the main bearing tunnel sat on a round machined straight bar, The block was then surfaced in situ on the mill that way the deck is parallel to the crank.

Next operation is to use an old school strap on boring bar which due to no vibration issues work far better than the fancy vertical mill type boring machines because the length of the head on those machines means they are not as accurate and you end up having to leave too much material so when it is honed there are no boring bar marks left.

We soon worked out those style boring machines are rubbish compared to a million year old strap on machine because the extra cost of stones worn out in the sunnen honing machine soon outweighed the ease of setting the block up and just running the block along with the table to bore the cylinders compared to unbolting the strap on and refitting it to do each cylinder one at a time.

Because the strap on fits onto the just machined block top face that is now square to the crank your cylinders are perfectly square to the crank when done that way, Followed up with a nice plateau hone job done with a torque plate and all is good.

It does make a noticable difference to making sure the piston heights are even (Well at least with quality aftermarket pistons)

So if machining a block those are the requirements for a really good base to build an engine from, If your local machinist does not have the gear to do it that way find somebody who does as the improvement in frictional losses will be well worth it.