21 stud deck height, gasket thickness, etc. questions.

Yoyodyne

Yoyodyne

Well-known member
I'm building a 21 stud, the block is ready for finish machining. Stock stroke, flat top pistons and heads. I'm trying to optimize the head to piston clearance at .040".


The block deck is 10.442. The piston compression height is 1.534". With the 3.75 stroke and 7' rod that gives me a calculated deck height of .033". The block isn't bored yet so I can't yet preassemble it to measure directly.

Reading here on the 'Barn tells me the Best Graphtite gaskets are a good choice for aluminum heads, and I think that's my preferred choice. Best tells me the compressed thickness on them (PN 507G) is .053 ±.003. .033 + .053 will give me a piston to head clearance of .086. This requires cutting the deck .046 to get the .040 piston to head clearance, and the pistons will protrude above the deck surface .013.


Engine guys - Is this a reasonable thing to do? Is a .045" cut on the deck safe? The deck seems thin even before it is surfaced at all. It's a 36LB block and I only have one shot at this.
Is there a thinner gasket to use? Cometic doesn't seem to make 21 stud gaskets, but I could modify their 24 stud gaskets if necessary.
A custom set of pistons isn't going to happen at this point for this motor.
 
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Deck should be left alone!! Just a light clean up if really required.
The heads are where the machining takes place to get that 0.040 to 0.050 piston to head clearance. Best to actually measure with pistons fitted and check using head without gasket and allow given compressed thickness of gasket or use gasket. This way you should be good. standard valve train?
Phil NZ
 
Phil, the head and pistons are flat top, not domed, I can't get the head any closer to the piston by machining the head.
 
I only machine the decks to true/square them up side to side - to get equal compression heights. I never machine the decks to tune the squish.

You really need to determine what your actual compression height is - given your crank, rods and block. You don't need a piston to do this - you do it with the crank in the block, a rod on each bank, with piston pins in (no piston) - and then mic the highest point of the pin travel - then subtract 1/2 the pin diameter. Now you know your compression heights side-to-side. You'll probably find they are not the same.

On some of my recent builds, I've ordered my pistons from Ross - and gave them the exact compression height I needed . . . to get the squish I wanted.

Question: Are your pistons actually below the deck surface??? If so, then I'd be looking for new/different pistons - should be no reason for that with a stock stroke crank and stock deck heights.

In a prefect world (using the gaskets you mentioned), you'd need to "pop-up" the piston a bit - OR, maybe use a different head gasket. You might look into the MLS ones from Cometic. They are not cheap, but you can get them in various thicknesses. What I don't know is if they make them for 21 stud engines?
 
If the crankshaft isn't ground yet get you could investigate getting the rod throws offset ground equal to the compression height the distance you need.
 
38 coupe said:
If the crankshaft isn't ground yet get you could investigate getting the rod throws offset ground equal to the compression height the distance you need.
Click to expand...

That would only work if you had really thick rod bearings - something that is probably not really available. Now, you could shoe-horn in a later crankshaft and offset grind the throws to exactly what you need . . . that could work, but there is a lot of work that goes into that whole deal.

I'll bet that once he does an exact compression height measurement, he may find his pistons are just fine (flush with the deck).
 
This is what I'm going to do with my late 21 stud engine, if I ever get going on it.

I'll use a .060 Victor gasket, with my flat 40-A1 aluminum heads.

If you surface your block to Ford's production spec of 10.437 and I would recommend that, you can get this.

With your CH of 1.534 + 7.0 + 1.875 (1/2 stroke) = 10.409. 10.437 - 10.409 = .028 deck clearance. Add in your head gasket (in my case .060), that's .088. Minus .040 = .048 x 2 and that's how much I would need to add to my stroke or, a stroke of 3.846 and the piston will be out .020, with .040 squish.

From there you just get a '48 or earlier crank, with 2.139 journals (can even be ground .030), have it offset ground and the mains ground to 2.399. You of course have to mess with the rear seal, too.

Send that '<48 crank to:

Mario, at L&M Grinding, in Redding CA (530 221 7652) and for about $250, or so, he'll grind it to those specs.

The extra cubes, will also help with compression.
 
Bored&Stroked said:
Info: Cometic does make 21-Stud gaskets . . . is varying thicknesses. Here is a link to Jegs for a .040 gasket. One thing to note, MLS gaskets usually require a very fine RA surface finish - or they might leak. Check with the manufacturer before you have your decks trued up and decked.

https://www.jegs.com/i/Cometic-Gaskets/245/C5870-040/10002/-1
Click to expand...


Thanks for that. I'll call Cometic. The only gasket I can find on their site for a 221 is a drain plug gasket.
I'm looking through my junk for a used std bore piston, I'll assemble a used rod and piston into the block and see what that shows me for deck height. Like you say, it could well be different than theoretical.
 
Yoyodyne said:
Thanks for that. I'll call Cometic. The only gasket I can find on their site for a 221 is a drain plug gasket.
I'm looking through my junk for a used std bore piston, I'll assemble a used rod and piston into the block and see what that shows me for deck height. Like you say, it could well be different than theoretical.
Click to expand...

If you find gaskets from Cometic, make sure you follow their advice on surface finish and have the block lightly decked to match their recommendations. Also, "square deck" it and ensure the compression height is the same on both side.

I give the exact amount I want taken off to the machine shop - so there is no guessing.

Hopefully your machine shop can mount the mains in a "saddle" such that they can truly square deck it. You should check with them and see if they have a mounting system that uses the main bearing bores.
 
Bored&Stroked said:
Hopefully your machine shop can mount the mains in a "saddle" such that they can truly square deck it. You should check with them and see if they have a mounting system that uses the main bearing bores.
Click to expand...
He does, it's a Storm Vulcan Blockmaster in good condition and it will be. He's been doing my race stuff for 30+ years, he's fussier than I am. Takes f-ing forever though... :)
 
I spent some quality time yesterday carefully measuring my parts. The same piston and rod in each corner bore gave me deck heights of .045" to .048" to the gasket surface of the block. This combined with a .053 gasket would give me a piston to head clearance of about .100". I have 3 sets of pistons, used Std, new.030 and new .100. All three have compression heights within a range of .005" total. The one I used was the tallest of the three at 1.534". The crank is fresh ground at.020, I used a .020 rod bearing shell. I checked the rod on a CMM afterward, the length is 7.0010" and both ends are round within .0002 and on size - 2.250 and .752. The block measures within Ford's nominal height. I believe the crank stroke to be correct. At .020 under it can't be more than .010 radius from the OEM stroke.



So - Has anyone here checked this and found similar results, or different heights? If all early motors go together this way with .100 deck, they would all be lazier than they could be, right? I can see that it leaves a lot of room for carbon buildup, I guess that's good for long life without service.


Cometic has a .040 gasket, that would give me .085.
To Get .040 piston to head with these parts I need to remove another .045. Am I correct in thinking that's too much to take off the deck of the block?


This motor is already Plan B after a half dozen false starts with junk blocks. To get this together in this lifetime I need to use available parts, custom pistons are not going to happen this time. Before I do that I'll start again with a 37 block, Merc crank, etc. and I need this one to go together this winter.


So I can cut the deck .045, I can just put it together as is with .100" deck, or I can profile mill the heads around the gasket opening and leave .060 of material extend down into the cylinder to close up the chamber to piston clearance. That's doable, but would be a PITA, and would require even more clearancing for valve to head clearance.


Thoughts anyone? Who else has measured this on a stock flat top 21 stud motor? And/or done anything about it?

Is it worth this effort to get .040 piston to head clearance on a 221 with a cam and 2 carbs?
 
Concerning decking the block...even if you do consider a fairly severe decking (may be ill-advised due to strength lost) check closely at the effect this may have on valve seat areas. I decked an 8BA block and at .020" stock removal it was nearly at the actual seating area on one side. Not sure the 21 stud is the same but worth a look.



Terry
 
I'm a retired aerospace, machine shop owner and still have my machine shop so, I know dimensioning.

I have Ford's drawing for the 21 stud block and the deck height of the block, is given at 10.437.

Your rod, piston (I'm going to use 1.53, estimating one of your new pistons) and stroke combo equals 10.406, that will equal a deck clearance of .031, if your block is the nominal height. With your 1.534 piston the deck clearance, should be .027. That means that your block is .018 - .021, over 10.437.

In that case, I would see no problem in cutting it down to the Ford spec of 10.437, maybe even .010 more.

If you could find another std rod crank, you could have it offset ground, to .030 under with a stroke increase to 3.78, gaining another .015, bring your deck clearance to .016 but, that's still .056 squish, with your .040 gasket.
 
If it was me, the next thing I'd check is the actual stroke of the crank - not hard to do if you have the right depth mic. I've not built early 21 stud engines, so I have no experience on what is "typical" in the regards to true deck height, compression height, etc.. I do know that I'd surely not want my pistons "down in the hole" - having a .040 squish is key to these things really running.

I would not be decking it to achieve the clearance - unless all other possibilities have been ruled out. The decks are thin on these engines, I don't like to weaken them. But - it all depends on the intended usage. What I would NOT want to do is deck the block if the issue is potentially the crank.

Lastly, is it possible the mains/babbitt were not done incorrectly? I'm not a babbitt guy, so I have no experience to fall back on - but just a thought.

You might PM 'Brian' from over in NZ - he seems to have built a lot of 21 stud stuff (performance oriented) - would be nice to see what he knows. ;)
 
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Measuring the stroke is one thing I knew was missing, I just ran out of time. Will do that next.

This is a 36LB, insert bearings. The main saddles appear untouched.


Good 36-37 cranks that will clean up at stock stroke are hard enough to find, finding one to offset grind isn't likely.
 
I have a early double slinger crank with 2.5 mains, that has std rods but with surface rust.

If you like, you could have it for $25 (cost of crating) plus shipping. I could even send it down to my crank guy first, for main and offset grinding. He gets good shipping rates.
 
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