why are billet caps needed w/forged crank?

[TURBOELKY]

what are the benefits of having billet caps when you install a forged crank? The crank and rods are the weakest links in the bottom, and please dont say no they are not. Yeah, they can make upwards towards 550 or so, but we all know thats on edge.

So why the need for the caps? I understand they keep the stock crank from flexing, but a forged crank should not flex near as much as a stocker.

I want to know if anybody is running a forged crank without caps or a girdle, and how their stuff is holding up. I've been asking alot of questions in the last few days, trying to figure out my best route. I was told to scrap the billet caps, and spend the money on a forged crank and rods.

 

[graza002]

what are the benefits of having billet caps when you install a forged crank? The crank and rods are the weakest links in the bottom, and please dont say no they are not. Yeah, they can make upwards towards 550 or so, but we all know thats on edge.

So why the need for the caps? I understand they keep the stock crank from flexing, but a forged crank should not flex near as much as a stocker.

I want to know if anybody is running a forged crank without caps or a girdle, and how their stuff is holding up. I've been asking alot of questions in the last few days, trying to figure out my best route. I was told to scrap the billet caps, and spend the money on a forged crank and rods.

From what I have been told, to use billet caps along with a forged bottom end is just an extra insurance policy against flex. Also, I guess it comes down to how much power you want out of your engine. If it were me, and money is an issue and I was only going to have about 500 hp, I;d hold off on the billet caps. With a forged bottom end, you know the engine can take future abuse, and with that being said, you could always add the billet caps later, after all it is a pretty easy install compared to a crank.

Good luck

 

[TurboCrazy]

I agree, a steel crank is much stronger and will flex less. Just remember that the main caps are what hold that nice strong steel crank in the block. After seeing broken stock main caps in 2 different motors, I will not put a motor together without at least the center steel caps. It is much cheaper than spitting a crank out on the ground!!!:wink: JMO

 

[turbofabricator]

From what I have been told, to use billet caps along with a forged bottom end is just an extra insurance policy against flex. Also, I guess it comes down to how much power you want out of your engine. If it were me, and money is an issue and I was only going to have about 500 hp, I;d hold off on the billet caps. With a forged bottom end, you know the engine can take future abuse, and with that being said, you could always add the billet caps later, after all it is a pretty easy install compared to a crank.

Good luck

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.

 

[Tony Shoaff]

Wtf

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.

Shorter timing chain?/ WTF are you talking about

 

[johnnyttype]

you could always add the billet caps later, after all it is a pretty easy install compared to a crank.

whats so easy about installing billet caps??? you need to tear the whole engine down and have the block machined!

I was told by Weber Racing to run only the two center billet caps with my forged crank... why spend money on a nice crank and have it held in the block by some seasoned, factory caps? im sure it could work but a lot of damage will be done if one of those caps fails!

i would say get the caps because you will already be getting the block machined for the new crank(line bore or hone i believe), so all that would be needed is less than $200 worth of caps, correct? hth

 

[QUIP'6']

Shorter timing chain?/ WTF are you talking about

alot of times when you line bore its brings the crank closer to the top of the block which causes you to need an oversized timing chain

 

[Tony Shoaff]

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.

alot of times when you line bore its brings the crank closer to the top of the block which causes you to need an oversized timing chain

If your closer why oversized You guys are killing me.

 

[chopped39]

The chain will be under sized.I never figured on getting a smaller one due to the tensioner in our engines. I just did a smallblock chevy and the chain was .003 under sized. I did the first three caps in my engine due to breaking all of them. left the rear main alone.

 

[Rodney87GN]

It's not so much the chain itself that's different in size (because they are the same size regardless if you need an oversize timing set or not)....but it's the 2 two sprockets that are oversized to compensate for the slack when you align hone/bore the mains. Rollmaster explains this pretty clearly...
http://www.romac.com.au/pdf/Centre Distances.pdf

 

[QUIP'6']

If your closer why oversized You guys are killing me.

you know what i mean larger fricken gears

 

[Just a Six??]

It's not so much the chain itself that's different in size (because they are the same size regardless if you need an oversize timing set or not)....but it's the 2 two sprockets that are oversized to compensate for the slack when you align hone/bore the mains. Rollmaster explains this pretty clearly...
http://www.romac.com.au/pdf/Centre Distances.pdf

Not to get off topic but I have a 4 thous over due to caps ,line bore etc. The question I have always thought when measuring the chain for slack a person could turn the crank some so that there is More Slack on the 1 side or turn the crank so slack is closer to even on both sides. They never really say the proper way as when I did it I put the slack to 1 side & its good but its nice & tight for sure! I had .002 & a .004 chain & could justify either one really but stayed with the .004 as they do strech in time. So to the point. Turn the crank so the Tension has the slack going to 1 side when measuring is Correct?
Sorry for being So Long Winded!!

 

[daviss]

If the crank is moved up due to line boring, what does that do to the compression height of the pistons? A few thousanths could have serious effects on piston/valve clearance....Right??? Correct me if I'm wrong.

Steve

 

[RmvBfrFlght]

The change in piston to valve clearance is small. The change will be the COS of the angle of the bore times the change in crank location.

So moving the crank up 4 thou only moves the piston up 2.8 thou.

This is all part of fitting up the parts during the build process. Measuring items such as piston to deck height, piston to valve clearance and such. Also recall that mild cams and heavily dished pistons are common in turbo engines.

RemoveBeforeFlight

 

[daviss]

I understand. That was a great explanation.

Why is it necessary to change the crank alignment with the addition of steel mains. I understand the steel mains need machining for proper install/fit but shouldn't that be based on the original bore location? I hope I'm not hijacking, but I believe this is still on topic.

Steve

 

[RmvBfrFlght]

The steel mains 'pull' on the block differently then the stock main caps. Which puts the block crank bores a little out of round. Then the caps themselves need to be bored to match where the block crank bores are. To do this a slight cut then hone needs to be made on the block and caps. Can't really do the cut/hone make it all round without also taking a smidge off the block.

RemoveBeforeFlight

 

[bison]

what are the benefits of having billet caps when you install a forged crank? The crank and rods are the weakest links in the bottom, and please dont say no they are not. Yeah, they can make upwards towards 550 or so, but we all know thats on edge.

So why the need for the caps? I understand they keep the stock crank from flexing, but a forged crank should not flex near as much as a stocker.

I want to know if anybody is running a forged crank without caps or a girdle, and how their stuff is holding up. I've been asking alot of questions in the last few days, trying to figure out my best route. I was told to scrap the billet caps, and spend the money on a forged crank and rods.

I disagree. The caps are the weak link. Ive cracked caps. Never had a rod or crank failure. Only block and caps. 600 hp is no problem without any detonation. On the calculator the one in my sig is 670-680hp. Id feel much safer with a good stock crank and steel caps vs forged crank and stock caps. Though the stock crank adds a lot of stress from the flexing. When i broke the cap in one of mine years ago it didnt break in the middle. Broke on the left side near the bolt hole parallel to the bolt. If you are going to spend $2000 on crank,rods, flywheel and balancing, $600 more on the two caps and machining shouldnt be passed on. Its still amazing how long things last without detonation. One dose of detonation at 500+hp will destroy it. It also amazes me how many guys think they didnt have detonation when they really did. Spend your money on good tuning tools and use common sense when tuning.

 

[bison]

If the crank is moved up due to line boring, what does that do to the compression height of the pistons? A few thousanths could have serious effects on piston/valve clearance....Right??? Correct me if I'm wrong.

Steve

If the line bore/hone is done properly the crank wont move up much at all. A few thousandths wont mean $hit on 99.5% of the engines you see guys on this board running. On the stock block im doing now i only have .028 piston to head clearance. Id be a little concerned if i had full floated the rods. Pistons are out of the hole.

 

[turbofabricator]

I disagree. The caps are the weak link. Ive cracked caps. Never had a rod or crank failure. Only block and caps. 600 hp is no problem without any detonation. On the calculator the one in my sig is 670-680hp. Id feel much safer with a good stock crank and steel caps vs forged crank and stock caps. Though the stock crank adds a lot of stress from the flexing. When i broke the cap in one of mine years ago it didnt break in the middle. Broke on the left side near the bolt hole parallel to the bolt. If you are going to spend $2000 on crank,rods, flywheel and balancing, $600 more on the two caps and machining shouldnt be passed on. Its still amazing how long things last without detonation. One dose of detonation at 500+hp will destroy it. It also amazes me how many guys think they didnt have detonation when they really did. Spend your money on good tuning tools and use common sense when tuning.

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!

 

[Nasty Wendy]

Buy offset grinding the main journals on a crank, you will effectively throw the whole crank out of whack by the amount of offset.

What Jenius tried that?