Block recess depth

[Nick Jones]

Got my block back from the machinist.  Has had "a very light tickle across the top to true it up" - which could be true as the engine number is still clearly visible.

However, block recesses now measure at 0.35mm (0.014") which doesn't seem like much......  I've seen figures of anything between 0.025" and 0.038" being quoted as the correct, or at least acceptable depth. Anyone know what the book figure is and what the minimum you can get away with is?

On a related subject, deck height is less than expected.  This is slightly complicated by the pistons not being level in the bores (another story!) but two are more or less flush and the lowest is 0.0065" down.   I always understood that they are about 0.025" down from the factory.

This all seems to point to it having had at least 0.020" off but I'd not expect to see the engine no anymore (or only the ghost of it)?

Cheers

Nick

[andy thompson]

Recesses are 20 thou as standard - give or take a thou

[andy thompson]

Hang on - scrub that number - I may have been still waking up - let me measure a virgin block I have in the shed

[piman]

Hello Nick,

ideally you want to match the piston heights, to even out the compression.

Alec

[andy thompson]

lagerzok wrote:

Recesses are 20 thou as standard - give or take a thou

Make that 0.025" give or take a thou at least on the unmolested block I have here


Scroll down here - 0.008" taken off block and engine number starting to "ghost"

http://www.74tr6.com/machiningblock.htm

[Nick Jones]

piman wrote:

Hello Nick,

ideally you want to match the piston heights, to even out the compression.

Alec

Yes, this is the plan - to try mix and match and find the most even combination.  Pistons all measure the same so it's all in the rods and crank throws.

[Nick Jones]

Thanks Andy - nice link that.  Will have a better look later.

Nick

[piman]

Hello Nick,

"Pistons all measure the same so it's all in the rods and crank throws."

I'm a little puzzled, how do you know the pistons are the same, it's not usual to measure pin centre to piston top? I can't believe the crank throw will be wrong by 0.065", different length rods, where from?

Alec

[Nick Jones]

Alec,

Where did the 0.065" come from?  I gave 0.0065" (false precision!) as the lowest deck height with highest being near-flush.

Rods are the originals. I'll have a go at evening things up a little more by swapping parts around.  Piston measurements were a quick and dirty vernier between the top of the pin bore and the crown, just for comparison purposes.  There are variations, but they are very small.

Measured the recesses in my other block today and they are even less at 0.30 - 0.32mm.  That's not been messed with since it last ran though it is a factory rebuild so may have been decked before.  Having looked at and measured the new gasket I think it'll be fine.

Once I've figured the best piston/rod/bore combinations for most even deck height I'll build it up.

Nick

[piman]

Hello Nick,

failing eyesight it seems, the correct figure is much more normal. Forget what I said.

Alec

[LadyScrumpy]

you measure each piston height to the deck with a dial guage.  Ideally you level them all out so that the compression is the same in each cylinder.  This means that if you balance the crank then it stays balanced.  Higher compression in some cylinders can wobble your crank a bit! haha

Piston tops can be levelled off in a lathe if you know, or you are, a good machinist.

I thought that zero deck clearance was the target but that's just something from memory.  Ive got my landrover running on zero deck clearance.  She's a performace racing beast.  She's not.  At all.

[99176]

Much better to measure piston heights using micrometer depth guage at centre of piston.
Deck heights are a subject in themselves. I'll come clean what I run when everyone else 'fesses up'.

[andy thompson]

99176 wrote:

Much better to measure piston heights using micrometer depth guage at centre of piston.
Deck heights are a subject in themselves. I'll come clean what I run when everyone else 'fesses up'.

I'm going to run my 2100 with pistons 0.005" proud - I have run a 2500 with flush pistons and it lived - my current white PI has pistons 0.022" down the bore and I wish they were now a bit higher

[99176]

With +40 bores and 35cc chambers I run a deck height of +0.008''
This gives a low 'notional' CR, but with a 35-75 cam is no slouch on 95 unleaded.

[Nick Jones]

99176 wrote:

Much better to measure piston heights using micrometer depth gauge at centre of piston.
Deck heights are a subject in themselves. I'll come clean what I run when everyone else 'fesses up'.

I'm sure this is true but I don't have one.

I'm getting more consistent results now I'm turning the engine over a couple of times before measuring and measuring in the centre of the piston.

Measuring all pots with the same rod/piston combo (with dti in centre of piston)shows that the crank throws are equal within 0.05mm  (0.002").  Hopefully will get the best combination sorted over the weekend.  Looks like eventual deck height will be around 0.005".  I like squish - not that there's squish area worth a damn on the 2.5 head - 2.0 L much better!

Nick