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MLS head gasket choice, measuring poke

sim

sim

Mere Drab Mob
Joined
Jun 12, 2010
Location
Port Moody, BC
I was lucky enough to find a low mile 1995 B230FD. I have it on a
stand with the head off, and I'm trying to determine the piston
deck height so that I can order a Cometic MLS head gasket of an
appropriate thickness for tight squish.

This motor will get a 15g turbo from an 850 and will have a 6500
RPM rev limit. I plan to keep the boost reasonable.

The motor is in great shape, all I'm planning to do is clean it
up, drill and tap an oil return, swap out the cam and the
intermediate shaft, and stick it into my 245.

I believe the correct tool to measure poke is a bridge that can
span a cylinder and hold a dial indicator. I'm not sure where I
can get one, it is the sort of thing usually only used by a
machine shop, and something a machine shop could fabricate with
no trouble.

Lacking the tool, but having a dial indicator and a magnetic
base, I set the thing up with a flat ruler across the cylinder so
that the piston would push the ruler up once it was above the
deck.
Piston_Height_Measure.jpg


I'm not really convinced of the accuracy of this method, but it
was the best I could come up with to get an idea of the deck
height.

Here are the numbers I came up with:

Cylinder #1: 0.004"
Cylinder #2: 0.004-0.006"
Cylinder #3: 0.002"
Cylinder #4: 0.002-0.003"

This is the ruler set-up on #2 at full poke:
Piston_Height_Measure_poked.jpg


Any suggestions for a better approach to measuring the poke?

So, based on my measuring, a 0.030" gasket would be safe, and get
me into tight squish. I think the 0.027" would probably be okay
as well, but I'm not confident enough to order one.

Does this sound reasonable?

Also, any suggestions for prepping the block deck for the MLS
gasket? 600 then 1200 grit sandpaper wet?

Thanks for any pointers, I'm keen to get this thing into my
wagon.
 
not so fast there...

I would suggest that you rethink the 30 thou thick MLS HG choice a bit. 36 thou thick would be a wiser choice.

why so?

1) how many miles constitutes "low mile '95 B230FD"?

2) did you disassemble the bottom end and measure rod bearing clearances on all four rods? And what are the rod bearing clearances?

3) did you r&r the four pistons and measure the skirt diameters and the cylinder diameters to determine what the piston to wall clearances are? And what are those piston to wall clearances?

4) your methodology for 'measuring the poke'.....aka 'measuring the piston deck height to see if it is positive [above the block deck] or negative [below the block deck]'....is well intentioned, BUT is such that it is too prone to inaccuracy. [laying the ruler 'across' the pistons side to side brings 'piston rock' into the measuring process.]

....the variations in positive piston deck height that you report indicate either inaccuracy due to how you measured it, or rod bearing wear resulting in that variation in reading.

IF you haven't checked and measured the rod bearing clearances and the piston to wall clearances because it is such a 'low mile B230FD' and you don't have to do that because of the 'low mile' condition, I would suggest that that is not a wise course of action.

What makes it unwise is your choice of going 30 thou on the HG thickness and the choice of a 6500 rpm redline. Those two choices, combined with the unknowns [rod bearing clearances/wear & piston to wall clearances] results in a risk laden decision.

Effective squish action/wave/effect can be achieved with squish clearances tighter than 35 thou, down to 30 thou. To go tighter than 30 thou squish, you have to make sure that things are as right as you can make them in order to ensure that your motor will live.

I have several motors on the road running under 30 thou squish. And one running 24 thou squish. THAT one I made sure that the rod bearing clearance were where I wanted them....1.5 thou....and piston to wall clearances right at 1.7 thou.

.....and that motor does not see over 5500 rpm....

my suggestion is to go with a 36 thou MLS HG.

good luck

TF
 
If I'm reading it right, cyl 2 may be .006 positive, so a 30 hg would net 24. A 27 would net 21. It then becomes an interference motor with No valves. ;-)
 
stealthfti said:
I would suggest that you rethink the 30 thou thick MLS HG choice a bit. 36 thou thick would be a wiser choice.

why so?
Click to expand...

Thanks for weighing in Thomas, I follow your posts with
considerable interest.

1) how many miles constitutes "low mile '95 B230FD"?
Click to expand...

The car this engine came out of had 87,000km (~55,000 miles) on
the odometer. I bought the car from a young man with a family
after he rear-ended someone less than a year into his ownership
of the car. For a sixteen year old motor, it is in very good
condition.

2) did you disassemble the bottom end and measure rod bearing clearances on all four rods? And what are the rod bearing clearances?
Click to expand...

No, I have not touched the bottom end yet.

I wasn't really planning to, in fact. This thing is in fine
shape. On the other hand, now is the chance to tear into the
thing. It may be better to just tear it all the way down, I'll
get it cleaner that way.

3) did you r&r the four pistons and measure the skirt diameters and the cylinder diameters to determine what the piston to wall clearances are? And what are those piston to wall clearances?
Click to expand...

Nope.

4) your methodology for 'measuring the poke'.....aka 'measuring the piston deck height to see if it is positive [above the block deck] or negative [below the block deck]'....is well intentioned, BUT is such that it is too prone to inaccuracy. [laying the ruler 'across' the pistons side to side brings 'piston rock' into the measuring process.]
Click to expand...

Yes. I realize that the method I used is less than perfectly
accurate. I guess the best device would be a "bridge" that holds
a dial indicator and has a ground flat surface so it can be zeroed
on the deck and then placed across the bore. I described the tool
I needed to both a good local auto parts place and a good tool
store and the countermen both could picture what I was talking
about, but had no suggestions. Really, it is the kind of tool only a
machine shop would use, and it would be a trivial job to make up
such a tool in a machine shop.

I measured the poke the best way I could think of, given what I
had in my garage. The piston deck height is certainly positive,
and seems to me to be in the range of 0.002"-0.006".

Any idea where a correct tool could be had? Or a better method?

....the variations in positive piston deck height that you report indicate either inaccuracy due to how you measured it, or rod bearing wear resulting in that variation in reading.
Click to expand...

The variation does have me worried. I guess that is a big reason
to tear down the rotating assembly.

IF you haven't checked and measured the rod bearing clearances and the piston to wall clearances because it is such a 'low mile B230FD' and you don't have to do that because of the 'low mile' condition, I would suggest that that is not a wise course of action.
Click to expand...

I hear you.

What makes it unwise is your choice of going 30 thou on the HG thickness and the choice of a 6500 rpm redline. Those two choices, combined with the unknowns [rod bearing clearances/wear & piston to wall clearances] results in a risk laden decision.

Effective squish action/wave/effect can be achieved with squish clearances tighter than 35 thou, down to 30 thou. To go tighter than 30 thou squish, you have to make sure that things are as right as you can make them in order to ensure that your motor will live.

I have several motors on the road running under 30 thou squish. And one running 24 thou squish. THAT one I made sure that the rod bearing clearance were where I wanted them....1.5 thou....and piston to wall clearances right at 1.7 thou.

.....and that motor does not see over 5500 rpm....
Click to expand...

I did read about the 24 thou engine you built. I must have missed
the 5500rpm redline, that does change things considerably.

Also, I'd like this engine to last a long time. I need to design in
some margin for future wear.

my suggestion is to go with a 36 thou MLS HG.

good luck

TF
Click to expand...

Thanks for the advice. I appreciate the time you take to share
your experience.

I'm not trying to build a crazy monster engine, I do try and do
as much of the work on my car myself as possible. I'm learning
a whole lot on the way. It's pretty cool to live here in the
future where experts are accessible.

I will either order a 0.036" gasket, or tear into the short block
further. 0.036" will get me under 0.035" squish on all four,
provided my measurements are any good.

This motor is going into my '79 wagon, where it will truly be a
"Stealth FTI", third-row seat and all. Thanks again.
 
RTV between the block and head, tighten head bolts to TAF ft lbs and run it.
 
745 TurboGreasel said:
Straight edge and feeler gauges in at least 4 locations per piston.
It's pretty tedious, but you can do it.
Click to expand...

What I was going to say.

Measuring along the centerline is good because pistons can rock some and throw the measurements off at the sides. But then again, the sides are where the sqush pads are on the head.

But the minimum clearance to aim for that I've heard casually tossed around (and which I've used to build a couple of redblock motors, pushrod and OHC) is .032".
 
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