why are billet caps needed w/forged crank?

[Jerryl]

Actually, billet caps are a PAIN to install, and REQUIRE machine work. They are NOT a simple bolt-on. Line boring and honing (and usually elongating the stud holes, too) are required to get them to line up. This also requires the use of a shorter timing chain, too. The cap installation needs to be done by a competent machinist, or they can screw up your block. I personally prefere a girdle over steel caps. I think it does a better job, and it only requires a line hone, not a bore. The reason to use either caps or girdle, is that the main caps are pretty weak. I've seen lots of broken ones. If you have the money, now......do it and be done with it.

+1
Besides the line boring, timing chain etc issues, you can also get misalignment from the trans input/converter.
Maybe not much to worry about, but, still misalignment.
LOTS of "junk blocks" because they are machined incorrectly for caps by incompetent machine shops.

Skip the caps, get the crank and install a girdle!

 

[turbofabricator]

What Jenius tried that?

I was responding to a question above: Legit question. All questions are. All ideas are worth considering. ('cept Marx's)


Also, when line boring a block, the machinist "tries" to only remove material from the cap, and not the block. Easier said than done.

Another issue stated previously:

<<"The chain will be under sized.I never figured on getting a smaller one due to the tensioner in our engines.>>

Using a tensioner with a double roller chain is not a good idea. The double roller chain will chew the tensioner up in short order. No one offers a link-belt timing chain (factory style) in undersize. Not at least as far as I have been able to find.

Now back to the original question. As stated before, If you are spending the money for a Forged crank/rods/pistons, I would say, have the block either steel capped, or girdled.

 

[TURBOELKY]

Typically caps fail because of crank flex. A crankshaft can flex ALOT and not crack or break. 4340 forged cranks are typically MUCH stronger and more rigid than a stock cast iron crank. So, in theroy, the stock main caps should live under much higher power levels. With that being said, a girdle (or steel caps) would be a smart addition to ANY engine (Buick V6) build that is expected to make over 575 hp.
The stock crank can live at 700 hp, but at 701 hp, pistons start swapping cylinders.:biggrin:
When line boring/honing a block, a small amount of material is removed from the block side of the bore to make it round. The main caps are surface ground to make the bore smaller. (ground on the parting surface) It is then machined back to specifications. By doing this, the crank centerline is moved UP in the block, closer to the cam, requiring an UNDERsized timing set. (.005"/.010")
Buy offset grinding the main journals on a crank, you will effectively throw the whole crank out of whack by the amount of offset. Some pistons will be near normal, some more stroke, some less stroke, depending on where you "time" the offset. Kind of like just flipping your flat tire over.......After-all, it's only flat on the bottom side!

THANKS guys. I am aware of knock, and wont deny i've had plenty at times and blew my head gasket clear out of my block this thread was not started so I can get out of putting caps on and justifying it, but to see how different peoples opinion's are. I think turbofabricator knows what i'm talking about with the flex of the stock crank, and cracked caps. More than likely Bison, that cap broke due to your crank flexing, does'nt mean your crank had to break too. Why do some people not run billet caps on stage engines? I know the webbing where the main caps bolt down are thicker, but the caps looked the same to me:wink: My machinist charges about $500 for the 4 caps installed, plus the price of the caps makes that pretty steep. I might see how much of a difference in price an engine girdle would cost me.