Fitting Billet Main Caps and Line Boring on a CNC Machine Center

MCH86GN

Well-Known Member
Aug 20, 2008
1,238
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Put a billet cap on #1 as well. Its only $80 more. The rear cap is outrageously priced from Pro-Gram......and not really needed.


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I grabbed the #1 billet main cap. In the grand scheme of things, what an extra $80. I almost bought the rear cap, I'm glad I didn't. As far as the rear cap, is it just that the stock one is pretty beefy already or because it isn't subject to as much force as the center or a combination of the two?
 

EVIL

BANNED FOREVER
Jun 30, 2008
2,198
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MA
Question.

When the two billet main center caps go bad and the end caps are fine like in the above pictures, does that mean that there was a problem with the align bore and or align hone?

Could metal derby in the oil cause the same effect without damaging the end caps?

This situation appears to have happened more than once.

thanks
D
 

hensleyt

Active Member
Feb 20, 2008
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Hawaii Big Island
Dank GN #17 post I see a crack in the block?
I have not maged my old block ,I don't see a crack but I sesupect one .
I will grind this crank before I install in my spear
 

TURBOELKY

Well-Known Member
Jun 27, 2005
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This is an interesting post. My factory block is at 2.672 on main journals right now, never align honed or bored since it left the factory. Looks about .015" undersized in comparison to your measurements in first few posts. Currently squaring up the block.
 

turbodave231

Moderator
May 24, 2001
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TurboElky,

That sounds way too tight to be correct. The factory spec for the main bearing bore is 2.6870 to 2.6880.

The appropriate tools needed are a dial bore gauge with .0001 resolution, and a 2-3 micrometer with .0001 resolution. Measuring properly is not an easy task either.

Are you sure you didn't measure 2.6872? That would be "in spec" towards the tight side......just the way I like to see the mains.
 

TURBOELKY

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Jun 27, 2005
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It is possible I was off .010, I just did a quick measurement with a pair of calipers. I was looking around the shop for some inserts to set the block up to index the thing before we deck. Regardless, looks like I'll be making some inserts on the lathe so I'll double check with a bore gauge first.

I'm just learning the machine side of things so my machinist I'm sure will double check what I'm doing before I cut anything. Bottom of the block was off .015 and the top was off .008. from square.
 

turbodave231

Moderator
May 24, 2001
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When you say the top and bottom of the block, are you talking about the deck? If you measure that much out of square from the intake side of the deck to the exhaust side, the decks are rolled. You can verify a rolled deck if the opposite deck has the same error only in reverse.

When you have rolled decks, The problem is that if you correct that much by squaring the decks, the bores will be off (or not 90 degrees to the new deck surface. This can be corrected but it takes a larger overbore to clean up the cylinders. I would verify the bore position relative to the decks by sweeping the bore up and down with a dial indicator.

I use a CNC machine center to measure deck position relative to the cam bore. If the cam bore is zero degrees the decks should be 45 degrees and -45 degrees from the cam bore. I see many OEM blocks with LOTS of error. Some can effectively be corrected, and some cannnot. For a street engine, a little bit of error won't hurt a thing......the engine doesn't know the difference.
 

TURBOELKY

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Jun 27, 2005
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I'm sure my terminology was not correct. I was talking about squaring up pan rail for girdle and top of block where manifolds seals. Haven't got to the deck yet, but thank you for the info.
 

turbodave231

Moderator
May 24, 2001
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Understood!

Squaring the pan rail is smart move if you are using a girdle. When installing a girdle, I machine the caps to be .003" proud (above) the pan rail. No stupid shims that way. Check the girdle for flatness too. I have sent a couple out to be surface ground because they were nowhere near flat. The manufacturer will tell you its not a problem.......that may be OK for some guys, but not for me.
 

TURBOELKY

Well-Known Member
Jun 27, 2005
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Understood!

Squaring the pan rail is smart move if you are using a girdle. When installing a girdle, I machine the caps to be .003" proud (above) the pan rail. No stupid shims that way. Check the girdle for flatness too. I have sent a couple out to be surface ground because they were nowhere near flat. The manufacturer will tell you its not a problem.......that may be OK for some guys, but not for me.
This is exactly how me and Cook talked about installing it. Giving the caps a little pre load on the caps, no shims then align hone the mains while girdle is torqued on the block. There is a metal supply shop in the LB that can flat ground metal so for sure I'll be checking the girdle for flatness. Great info.
 

turbodave23101

New Member
Jun 12, 2015
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You will need to remachine the oil pump pickup flange area. Cutting the pan rail will extend into that area.

I made a special fixture to measure from the pan rail to the top of the cap. It sits on both rails and goes over the cap.

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