2G BC 272 cam problems

[schumy]

I had my head rebuilt at a local engine shop, and I have bc valves, springs, retainers, and 272 cams. They also clean, resurfaced, and reground the head. it was assembled there on the block. After so much rereading everything I knew, I got fed up with the car not responding to anything I tried to tune, I ripped off the head to find that at top dead center (for each cylinder) none of the valves where fully closed. The car had the right timing, and it never jumped. You might be asking why I couldn't tell by the sound of the car that something was wrong, but it really didn't sound that off just a little lopey. Now could this be a bad cam or a error by the builder?

 

[cait sith]

Brian Crower cams need to be degreed (carefully). They are notoriously off center.

 

[schumy]

That cant mean everyone can it? I mean as in every product

 

[vingasoline]

My bc272s were installed straight up and the car ran and tuned fine. To me this issue sounds like the tbelt was off a tooth. Which would cause the problem you are describing. What was your vac at
Idle? Mine was 14 @800. With adj cam gears its 19@1000

 

[schumy]

It would bounce 1-2 like I was told and it was around 23 24 in. at idle

Now that I think of it I was getting reqlly high vac. on decel

 

[WES_393]

Are the valves always open, or just open at TDC? Which valves are open?

 

[schumy]

Are the valves always open, or just open at TDC? Which valves are open?

No they shut eventually but alittle after maybe like a degree. I have adjustable cam gears and they r still on 0 and I have never touched them. Intake and exhaust r open

 

[BogusSVO]

Doubt the base circle of the cam is off that much to hold the valves open. I figure for the cam base circle to hold the valve open, the base circle would be +.030 or more larger than stock cam base circle.

2 things that could be the issue, The first, and fairly common, the HLAs were not bleed down for the install. The second is the valve tip height is out of spec and not allowing the valve to fully close.

Noe since you have had the engine running and trying to tune, Along with the head being built at a local shop, if they more accustom to building domestic V8 engines, they easily could have over looked the valve tip height. If it is off bu more than .005 or so over/under min/max spec.

Also keep in mind that everything manufactured has a spec range. The cams, and the adj cam gears. So if the cam alignment pin is supposed to be at "True" TDC, the range for it considered +/- 1 or 2*. the same can hold true for the Adj cam gears locating hole.

So if both products were at the far edge of acceptable manufacturing spec, then you can be off as much as + or - 2 to 4*

(The numbers used are to give an idea, and not the true specs used in the manufacturing of the products)

Now what can compound the issue is how much the head and block have been milled.

 

[schumy]

Doubt the base circle of the cam is off that much to hold the valves open. I figure for the cam base circle to hold the valve open, the base circle would be +.030 or more larger than stock cam base circle.

2 things that could be the issue, The first, and fairly common, the HLAs were not bleed down for the install. The second is the valve tip height is out of spec and not allowing the valve to fully close.

Noe since you have had the engine running and trying to tune, Along with the head being built at a local shop, if they more accustom to building domestic V8 engines, they easily could have over looked the valve tip height. If it is off bu more than .005 or so over/under min/max spec.

Also keep in mind that everything manufactured has a spec range. The cams, and the adj cam gears. So if the cam alignment pin is supposed to be at "True" TDC, the range for it considered +/- 1 or 2*. the same can hold true for the Adj cam gears locating hole.

So if both products were at the far edge of acceptable manufacturing spec, then you can be off as much as + or - 2 to 4*

(The numbers used are to give an idea, and not the true specs used in the manufacturing of the products)

Now what can compound the issue is how much the head and block have been milled.

The head has never been apart so I cant imagine them taking to much off he said the head wasent that tweaked. But yeah I see what your saying, I think that just a simple adjustment of the cam gears should do it and nothing was bent so it really cant be that much off. Thank you everyone for your help

 

[nitrouskris]

The head has never been apart so I cant imagine them taking to much off he said the head wasent that tweaked. But yeah I see what your saying, I think that just a simple adjustment of the cam gears should do it and nothing was bent so it really cant be that much off. Thank you everyone for your help

How can you rebuilt a head if the head was never apart ???

 

[Kapok6]

Doubt the base circle of the cam is off that much to hold the valves open. I figure for the cam base circle to hold the valve open, the base circle would be +.030 or more larger than stock cam base circle.

2 things that could be the issue, The first, and fairly common, the HLAs were not bleed down for the install. The second is the valve tip height is out of spec and not allowing the valve to fully close.

Noe since you have had the engine running and trying to tune, Along with the head being built at a local shop, if they more accustom to building domestic V8 engines, they easily could have over looked the valve tip height. If it is off bu more than .005 or so over/under min/max spec.

Also keep in mind that everything manufactured has a spec range. The cams, and the adj cam gears. So if the cam alignment pin is supposed to be at "True" TDC, the range for it considered +/- 1 or 2*. the same can hold true for the Adj cam gears locating hole.

So if both products were at the far edge of acceptable manufacturing spec, then you can be off as much as + or - 2 to 4*

(The numbers used are to give an idea, and not the true specs used in the manufacturing of the products)

Now what can compound the issue is how much the head and block have been milled.

Bogus, how is proper valve-tip height determined/measured?

How can you rebuilt a head if the head was never apart ???

You can take head, minus cams, caps, hla's, and rockers and have it milled with the valves, springs, etc still in.

 

[nitrouskris]

Bogus, how is proper valve-tip height determined/measured?



You can take head, minus cams, caps, hla's, and rockers and have it milled with the valves, springs, etc still in.

I know that but then it wont really be a rebuilt

I had my head rebuilt at a local engine shop, and I have bc valves, springs, retainers, and 272 cams. They also clean, resurfaced, and reground the head. it was assembled there on the block. After so much rereading everything I knew, I got fed up with the car not responding to anything I tried to tune, I ripped off the head to find that at top dead center (for each cylinder) none of the valves where fully closed. The car had the right timing, and it never jumped. You might be asking why I couldn't tell by the sound of the car that something was wrong, but it really didn't sound that off just a little lopey. Now could this be a bad cam or a error by the builder?

anytime I have had a head resurfaced I take apart the head. that is what most shops around here do

 

[BogusSVO]

Bogus, how is proper valve-tip height determined/measured?



You can take head, minus cams, caps, hla's, and rockers and have it milled with the valves, springs, etc still in.

Last pic in this thread shows the tool I use for checking valve tip height

http://www.dsmtuners.com/forums/newbie-forum/341297-how-replace-seats-aluminum-head.html


I would be careful about having a head surfaced with the valves in in, mainly the intake valves, they can be clipped depending how how much needs to be surfaced or if the head has been surfaced before.

 

[Kapok6]

Last pic in this thread shows the tool I use for checking valve tip height

http://www.dsmtuners.com/forums/newbie-forum/341297-how-replace-seats-aluminum-head.html

Thanks for the reference. I was more inquiring about the mathematics behind it. For example, is there a spec range of how high the tip height should be from a specific point on the head? I searched and searched a couple years ago in a haynes manual and of all the specs listed in the rebuild section for the head, I couldn't find anything pertaining to valve tip height, what it should be, and how to measure it.

 

[BogusSVO]

haynes is not going to have anything that detailed.

IIRC it is listed in the FSM. But I use the Automotive Engine Rebuilders Association Spec (AREA)

It is 1.920 -1.950, But some heads from the factory will be as short as 1.900, normally found on the early 1g heads.

But that is a starting point, and will work with most builds.

 

[Purple_Demon]

So say that a cylinder head was resurfaced with everything still intact ( I believe I spoke to you about this a few months back Bogus). What steps should be taken to ensure proper operation?

Just for kicks, this is how I got my head back from the machine shop:

Notice the nicks on the valves in the top left corner?

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[BogusSVO]

Well, you may have done the oil port mod before you had it surfaced.

Blow it out well with compressed air.

Give it a good washing, and a few drops of oil on the cam towers before you install the cams.

The nicks in the valves was one the first things that caught my eye. But they were not clipped too much so you should be ok.

With just the first 3 being clipped, I would measure head thickness front and back.

 

[Purple_Demon]

Will do. And Oil port mod was done afterwards(I forgot).

 

[Kapok6]

haynes is not going to have anything that detailed.

IIRC it is listed in the FSM. But I use the Automotive Engine Rebuilders Association Spec (AREA)

It is 1.920 -1.950, But some heads from the factory will be as short as 1.900, normally found on the early 1g heads.

But that is a starting point, and will work with most builds.

Thanks for the info.