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Semi-building a B20 to measure squish

R

red92s

New member
Joined
Jul 17, 2009
I'm in the process of rebuilding a B20, and pretty much everything I've read suggests that shaving the block a bit to get better squish is a pretty worthwhile endeavor if you are taking the block into the machine shop anyways.

So, here is where I'm at. I have new +.040" Mahle pistons mounted to stock rods with new wrist pin bushings. I have a block that has been checked for cracks and measured to insure the bores are within spec for +.040" pistons. The bores will need to be honed but this has NOT been done yet. The crank was measured up and will be fine with stock size bearings and a polish.

I'd like to avoid multiple trips to the machine shop, primarily because it's logistically complex for me. I'm aware I could probably take everything in to them, have them assemble the rotating assembly, measure, take it apart, and machine . . . but with shop labor in the $65/hr range that will become cost prohibitive. Also, this is very much a learning project for me and I'm having a lot of fun doing things on my own.

Is there any problem with using the old crank/rod bearings (both in good shape, but will be replaced before final build), my newly assembled rod/pistons, and the un-honed block to take measurements for squish calculations? Will I trash the new piston rings on the un-honed cylinders if I am only turning the motor over a couple times to take some measurements?
 
You don't have to deck the block to get good squish, unless you're stuck on using the stock HG and its thickness.

But chances are if you assemble it, you can just measure the piston height at TDC and just buy a HG with the appropriate thickness to suit. MLS has various thickness options. And you can get copper HG's in a variety of thicknesses.

But measuring your old rods and pistons probably isn't a good way to guess who the new parts will fit in there.

EDIT: I think I read that wrong, I thought the new pistons were .040 over and the block wasn't, and would need to be honed, so I wasn't thinking you'd be able to fit the new pistons in the block at all until it was honed. But if they fit as is, I'd pop the rings off (no point in dealing with them) and install it and get a measurement on deck height. Unless the old bearings are really thrashed, shouldn't be much of a factor. Just get the measurement, look at it vs. the available Cometic thicknesses, see if you really need any deck height shaving at all, you might not.
 
Last edited:
red92s said:
I'm in the process of rebuilding a B20, and pretty much everything I've read suggests that shaving the block a bit to get better squish is a pretty worthwhile endeavor if you are taking the block into the machine shop anyways.

So, here is where I'm at. I have new +.040" Mahle pistons mounted to stock rods with new wrist pin bushings. I have a block that has been checked for cracks and measured to insure the bores are within spec for +.040" pistons. The bores will need to be honed but this has NOT been done yet. The crank was measured up and will be fine with stock size bearings and a polish.

I'd like to avoid multiple trips to the machine shop, primarily because it's logistically complex for me. I'm aware I could probably take everything in to them, have them assemble the rotating assembly, measure, take it apart, and machine . . . but with shop labor in the $65/hr range that will become cost prohibitive. Also, this is very much a learning project for me and I'm having a lot of fun doing things on my own.

Is there any problem with using the old crank/rod bearings (both in good shape, but will be replaced before final build), my newly assembled rod/pistons, and the un-honed block to take measurements for squish calculations? Will I trash the new piston rings on the un-honed cylinders if I am only turning the motor over a couple times to take some measurements?
Click to expand...

test assemble--use a couple of main bearings, snug the caps.
Slide one piston and rod with rod bearing in place.
(Key part of process) LOOK.

Where's the piston crown relative to the block deck?
Is it below?
Or flush?

Only YOU can tell us that, we don't have the parts here.
The object---which by the way is QUENCH (which we get by shooting for zero deck)---is to get the piston FLUSH or maybe .004"/0,1mm out of the block

Your test assembly will tell you what YOUR crank, rods, and block and pistons add up to rather than me stating idealised:
Block height 231,7mm
Rod c-c 145mm
1/2 of stroke 40
Comp height of piston 46mm

If you do do the numbers (subtract 1/2 stroke, and rod c-c from Block deck hgt you'll see the piston is down the bore about 0,7mm---bad.

(Those a B21 and B23 numbers---B20 is supposedly same---I say supposedly cause i haven't measured---and nobody else here ever talks dimensions to the 0,1mm. dunno why?)

So cold assembly , measure , and report.

Machine shop will charge you the same if you say "kiss to clean" or "take her down 0,xx mm" May as well enhance QUENCH
 
Both the new pistons, and the old pistons, are +.040". I dropped a piston during disassembly and dented it pretty bad, hence the new pistons of the same bore. So the block is already sized for the pistons (just not honed yet).

I understand the intent of the measurements, and have the tools (deck bridge, dial indicator) to make them. I just wanted to make sure using the parts I have on hand (used bearings, unhoned block) wouldn't pose any problems.
 
which headgasket are you using. There are 3 factory headgaskets
.030"
.051"
.060"

If I were you I wouldn't set it up for 0 deck if you are using the a .051 or .060, I might go + deck height of up to +.030(dependingly.) And if using the .030 I think it would be good to know what you plan to rev it to before you set that one up for 0 deck.

But that's my opinion.
 
red92s said:
Both the new pistons, and the old pistons, are +.040". I dropped a piston during disassembly and dented it pretty bad, hence the new pistons of the same bore. So the block is already sized for the pistons (just not honed yet).

I understand the intent of the measurements, and have the tools (deck bridge, dial indicator) to make them. I just wanted to make sure using the parts I have on hand (used bearings, unhoned block) wouldn't pose any problems.
Click to expand...


Really only measuring accurately can tell you that.
But you can fake it with feeler gauges and your new pistons.

But next time never buy pistons untill you ask the guys at the machine shop to "Dial Bore Gauge the holes" and you watch.
That'll show in seconds if its round, not tapered and how worn it is..

A cleaned---dish wash soap and hot water---bearing and a 100w bulb is step1, but the best thing is to mike them with a ball tipped mike.
Shell specs are available online at King Bearings, and ACL
 
John V said:
But next time never buy pistons untill you ask the guys at the machine shop to "Dial Bore Gauge the holes" and you watch.
That'll show in seconds if its round, not tapered and how worn it is..
Click to expand...

John,
I only wish that sentence was more common knowledge! So important.
Jonathan
 
Stereophile33 said:
which headgasket are you using. There are 3 factory headgaskets
.030"
.051"
.060"

If I were you I wouldn't set it up for 0 deck if you are using the a .051 or .060, I might go + deck height of up to +.030(dependingly.) And if using the .030 I think it would be good to know what you plan to rev it to before you set that one up for 0 deck.
Click to expand...

This is a pretty basic B20 rebuild with mostly stock parts. Nothing terribly exotic. Lightly worked head with new seats, D or K cam, stock valve train.

I have not selected a head gasket yet, primarily because it's been hard to find information about their compressed thicknesses, and there seems to be a great deal of conflicting information on sizes that are available.

I know Cometic also offers a few MLS head gaskets, which I've been told do not compress. Any truth to that?

John V said:
Really only measuring accurately can tell you that.
But you can fake it with feeler gauges and your new pistons.

But next time never buy pistons untill you ask the guys at the machine shop to "Dial Bore Gauge the holes" and you watch.
That'll show in seconds if its round, not tapered and how worn it is..

A cleaned---dish wash soap and hot water---bearing and a 100w bulb is step1, but the best thing is to mike them with a ball tipped mike.
Shell specs are available online at King Bearings, and ACL
Click to expand...

Once disassembled, the block, crank, and pistons were all measured by the machine shop against the green book specs. They measured the bores and recommended a cylinder hone, crank journal polish, and piston replacement (my fault).

Whats the 100w light bulb for?
 
I really thought the only important thing was how close the piston gets to the bottom side of the head at TDC. Getting it *really* close pinches the air out of those flat spots on the sides of the bowl, and snaps/swirls/whatevers it into the central chamber, which makes a lot of very nice, useful swirl. I thought that was the main point of quench, but what's the advantage of having the pistons stick out past the block, other than to account for a stock HG that's thicker than you'd want the clearance to be?

I've heard that .032 - .035 is a nice tight squish clearance to aim for. Any tighter and you might touch (rods expand when hot, stretch under RPM loads?), and the slap/snap/pop/crackle effect of pinching the air in between to flat surfaces that nearly touch decreases rapidly with added clearance.
 
red92s said:
I know Cometic also offers a few MLS head gaskets, which I've been told do not compress. Any truth to that?
Click to expand...
They have some crimped rings around the bores to help with sealling, but the advertised thickness is the thickness of the steel sheets themselves, and you aren't going to compress flat steel sheets.
 
Stereophile33 said:
which headgasket are you using. There are 3 factory headgaskets
.030"
.051"
.060"

If I were you I wouldn't set it up for 0 deck if you are using the a .051 or .060, I might go + deck height of up to +.030(dependingly.) And if using the .030 I think it would be good to know what you plan to rev it to before you set that one up for 0 deck.

But that's my opinion.
Click to expand...

Thinking through this some more . . .

How is running a .060" head gasket with pistons coming +.030" out of the block going to be any safer than using an .030" head gasket and running a zero deck height? Seems like the net result (.030" clearance at TDC) is the same (?).
 
red92s said:
This is a pretty basic B20 rebuild with mostly stock parts. Nothing terribly exotic. Lightly worked head with new seats, D or K cam, stock valve train.

I have not selected a head gasket yet, primarily because it's been hard to find information about their compressed thicknesses, and there seems to be a great deal of conflicting information on sizes that are available.

I know Cometic also offers a few MLS head gaskets, which I've been told do not compress. Any truth to that?
Click to expand...

Yeah, pretty hard to compress a flat piece or 3 of stainless steel.
The stated dimension is what you go by.


Once disassembled, the block, crank, and pistons were all measured by the machine shop against the green book specs. They measured the bores and recommended a cylinder hone, crank journal polish, and piston replacement (my fault).
Click to expand...

OK, good.

Whats the 100w light bulb for?
Click to expand...

For seeing good.
I've done hundreds of motors with cast iron cranks which hold oil better than forged steel (but also can and do fracture from internal microscopic defects---literally stings of nitrogen bubbles which can be anywhere and which can and do eventually link up from inside out---unlike normal crack propagation in steel which begin on the surface...see further "nitrogen embrittlement") and often the motors are million milers (130,000 and up,) and often lackadaisical care (layers of disgusting goo and sploodge).
I've stripped motors and see bearings that look perfectly OK in the garage, but when i wash them up and get them spotless and clean and gotten them under a 100w bulb---then I can see that the lead/tin/indium overlay on the bearing shell is microns thick and I can see the copper faintly lurking. :omg:
That bearing is daider than a door nail. No point in even miking it..:raincloud:

But if after careful cleaning you see no hhint of copper, then mike it and mark it.
YBCossiebuild6.jpg


Then you check the bearing manufacturers data and decide.

Never put an unknown bearing into a motor regardless of how the box is marked.
Click to expand...
 
red92s said:
Thinking through this some more . . .

How is running a .060" head gasket with pistons coming +.030" out of the block going to be any safer than using an .030" head gasket and running a zero deck height? Seems like the net result (.030" clearance at TDC) is the same (?).
Click to expand...

Crazy talk!
Once you check it, whatever you do get the piston at least flush to .004 to .005 out.

This is yet another common "if this much is good then a lot more must be better"..

But I am always the calming voice of restraint.:)
Although there MAY, MIGHT, oOUGHTTA BE, possibly some additional benifits of reducing clearence more----the zero deck to 4/5 out has been proven for decades to enhance quench extremely well.
You B20 chamber is pretty similar to the little central bathtub shaped chamber on my beloved Saab's Ford V4....

When the EPA rule for NOX emissions came into force 1 Jan 71, the only way in pre-cat days to reduce NOX was to reduce combustion chamber temp, and the EASIEST was to do that was to simply lopp off the piston crown and set comp---everybody did--at 8:1
For '71 to compensate for this atrocity Saab began using a Ford 1700 V4 in place of the earlier 1500 V4 which was 9:1 and zero deck 9and in use usually had another gawd knows how much comp from the 1-2mm carbon baked on the crowns!!:oops:)

Those 1500s went like stink, accepted way more advance and didn't ping ever running used beer for fuel.
The 8:1 1700s ping like sonnybeaches even with half the 1500s ignition advance.

We jump ahead to 1986 and i was building myself o good rally motor using some Nissan L16 Carrillo rods i got (6 for $100) and my machine shop guy bitches me out for wanting to mill the heads 2mm to bump up comp more (MORE! MORE! Comp is GOOD:nod:)
So we jiggled things around and ended up surfacing about 1mm off so the pistons were0,1mm out.
Comp came in at about 10.85 with a 1mm compressed gasket and that bugger ran good on 89 octane---and real good on 92.
No Ping.

We've done the same on a friends Carrerra PanAmericana (and Chihuahua Express) Amazon with a B20 and my 160mm rods and flat top CPs.

No ping---and they run it hard, up to 137mph in an Amazon on an open road during the event.

Don't think too hard, just get those pistons flush to .004 out, run a .036 gasket and have fun on pump gas...
Don't be too greedy.
 
But again, the net result is to get a .032 clearance, if that's done with a thick gasket and protruding pistons, or a .032 gasket and flush pistons, or even slightly recessed pistons and a thin HG. I'm just trying to understand if there's any added benefit of having the piston poke out of the block some tiny amount, or is that just done to match up to a stock OEM .036 gasket?
 
JohnMc said:
But again, the net result is to get a .032 clearance, if that's done with a thick gasket and protruding pistons, or a .032 gasket and flush pistons, or even slightly recessed pistons and a thin HG. I'm just trying to understand if there's any added benefit of having the piston poke out of the block some tiny amount, or is that just done to match up to a stock OEM .036 gasket?
Click to expand...

The .004 out comes from a time when we didn't have a choice of compressed gasket thickness and had only 1mm compressed.

No benefit for sure from paying big money to the machine shop to carve away all day to take way more off than needed.
 
My $.02 on copper head gaskets: I had my block machined to .002 below deck height to run my shiny new .030 copper head gasket. It leaked compression cross cylinder and into the coolant jackets.
 
John V said:
Why in the world would you use a copper headgasket,???:omg: good gaskets have been available for over 10 years...:oops:
Click to expand...

Lapse of judgement. I put in a stock gasket just to get it running but I don't know what thickness. In a moment of frustration I ordered a stock B20 head gasket, I was unaware then that there were different sizes available. I plan on doing a .030 mls gasket soon.

I only brought up copper gaskets because they were mentioned earlier and I didn't want someone else to make the same mistake.
 
red92s said:
I'm in the process of rebuilding a B20, and pretty much everything I've read suggests that shaving the block a bit to get better squish is a pretty worthwhile endeavor if you are taking the block into the machine shop anyways.
Click to expand...

yes and no. Milling the block deck surface to raise the piston deck height is just one of the ways to do it.

Having done it that way a number of times, I much prefer doing it the other way: have the block surfaced to restore flat, parallelism and perpendicularity to the crankshaft centerline axis, and then use the closest thickness Cometic MLS HG to achieve the squish clearance I want.....once I then have determined the actual piston deck height and run the numbers.

So, here is where I'm at. I have new +.040" Mahle pistons mounted to stock rods with new wrist pin bushings. I have a block that has been checked for cracks and measured to insure the bores are within spec for +.040" pistons. The bores will need to be honed but this has NOT been done yet. The crank was measured up and will be fine with stock size bearings and a polish.

I'd like to avoid multiple trips to the machine shop, primarily because it's logistically complex for me. I'm aware I could probably take everything in to them, have them assemble the rotating assembly, measure, take it apart, and machine . . . but with shop labor in the $65/hr range that will become cost prohibitive. Also, this is very much a learning project for me and I'm having a lot of fun doing things on my own.
Click to expand...


Then, do it yourself and learn lots in the process.

Is there any problem with using the old crank/rod bearings (both in good shape, but will be replaced before final build), my newly assembled rod/pistons, and the un-honed block to take measurements for squish calculations?
Click to expand...

No. Just make sure that things are good and clean, and lube the bearings up with grease. that will help compensate for the worn bearing surfaces, and help you get a more accurate piston deck height measurement.


Will I trash the new piston rings on the un-honed cylinders if I am only turning the motor over a couple times to take some measurements?
Click to expand...

Remove the new rings from the new pistons so that you avoid the risk of cracking a ring during the dummy assembly to measure the piston deck height. If you've never removed rings from a piston before, practice doing that on your old pistons. Not a hard job to do; just requires patience and a steady, strong, and careful hand.


Not having the rings on the pistons will not affect negatively on being able to get an accurate piston deck height measurement. So what that the pistons will rock a bit in the bores. You'll be measuring the PDH down the middle....right above the wristpins....so any rocking of the pistons side to side will be irrelevant.

Besides the three HG thickness available in stock composition type HGs, Cometic has been offering MLS HGs for the pushrod 4 cyl redblock for a lot more years than for the SOHC redblocks......those came online in 2006, after Jonathan got Cometic to start making them.

The MLS HGs are available in .027in, .030in, .036in, .040in, .045in, and .051in.

Shooting for a squish clearance of ~.032in is a good goal. I go at least that tight. And have gone as tight as .024in. Anything over ~.039in is borderline ineffective squish.....it's just quench. And any motor has quench: that's just boundary layer inertness....the 'law of the wall' stuff. You have to overlap the boundary layers in order to have both useful quench and tight squish.


good luck

TF
 
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