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Stonejag

VR6 valve guides - Bentley misprint?

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Hi all,

 

What's the deal with replacing valve guides in a VR6 head? The Bentley says they should be installed from the camshaft side which makes sense, but it also says they should be removed from the camshaft side! Surely pushing them out in this direction will cause the shoulder on the guides to smash through the channel they're supposed to sit in and damage the head?

 

It also says "Cylinder heads on which the valve seat surface can no longer be refaced, valve guides have been previously installed, or cylinder heads that have been refaced down to the minimum dimension cannot be refaced further." Do you really have to buy a new head if valve guides have been installed more than once?!

 

Getting my head skimmed this week so I'd like to get them done soon! There's enough slop in the exhaust guides that I'd like them done for peace of mind, but not if it's going to ruin everything...

 

Cheers

 

Stone

Edited by Stonejag

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The original valve guides do not have the shoulder and will press straight out.

All replacement guides have a shoulder to allow ease install to the correct depth. Only problem is that depth is not correct if you install aftermarket cams and the valve seals can be crushed. The shoulder can be machined off if you can set the depth accurately.

I suspect that Bentley are referring to heads which have had the shouldered guides installed, and cannot be reused because they would be pressed out from the wrong side, although I really don't see a problem if your careful with it.

Make sure you use decent guides some are too soft (BGA), I've found Febi to be ok.

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Hello Stone,

 

Yes that is correct the new valve guides should be installed from the cam shaft side of the cylinder head.

I think what the manual means is the old warn guides should be pressed / knocked out through from the valve seat side and out through the cam shaft side so to speak.

I think it might just be the way they've worded it in the manual ?? Like what SCIENCE has said above.

 

I believe you can only replace the valve guides two or three times on the VR6 or on most aluminum cylinder heads,

as there is a greater risk of the head cracking under the 1 tone of pressure used to install the new valve guides,

plus normally the valve seats have to be replaced or machined so as to keep the valves sitting central,

again the valve seats can only be machined so many times,

what I mean is the seat can only be set back in lower to that of the height of the compression chamber.

Basically any machine work done to a cylinder head will weaken it as you are reducing it original dimensions making it thinner.

 

Same issue really when it comes to having the cylinder head skimmed it can only be done a few times,

as it increases the compression this is due to the depth of the compression chamber being reduced each time the head is skimmed,

it's the head of the valves that can also protrude so most people have a slightly thicker head gasket fitted to compensate for this,

but mainly it's down to the fact that the overall thickness of the head face can be come too thin and the issue with the compression chamber.

 

 

But yeah the answer to your question is yes the cylinder head will have to be replaced if it's already been machined to it's max limit.

 

 

Bottom line is really when you've had new valve guides fitted,

should then have the head skimmed and valve seats replaced or machined and have valves lapped in to new valve seat surface.

 

I've heard some people have said you can't or shouldn't machine VR6 valve seats ??

 

£300.00 to £450.00 is what it may cost to have the guides replaced and have the head machined

 

 

Hope this helps.

 

Si :thumbleft:

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Thanks both - though Si is wrong about head skim raising compression ;) (the head is flat and the combustion chamber is in the piston, so to increase compression you'd have to skim the block).

 

When buying OEM replacement valve guides do they have the shoulder too? I'll be using Schimmel 263s. Seems strange that they wouldn't always use shouldered guides and just instruct you to push them out from the valve side, but hey...

 

My replacement head has never been skimmed before - so when it's done this time it will be factory dimensions again as the corroded areas were built up with weld so only the welded areas need skimming. My skim will be dirt-cheap so I'll see if I can find a cheap place to do the guide reaming and seat machining and leave them if the guides are within tolerance.

 

Cheers,

 

Stone

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Hello Stone,

 

Yes that is correct the new valve guides should be installed from the cam shaft side of the cylinder head.

I think what the manual means is the old warn guides should be pressed / knocked out through from the valve seat side and out through the cam shaft side so to speak.

I think it might just be the way they've worded it in the manual ?? Like what SCIENCE has said above.

 

I believe you can only replace the valve guides two or three times on the VR6 or on most aluminum cylinder heads,

as there is a greater risk of the head cracking under the 1 tone of pressure used to install the new valve guides,

plus normally the valve seats have to be replaced or machined so as to keep the valves sitting central,

again the valve seats can only be machined so many times,

what I mean is the seat can only be set back in lower to that of the height of the compression chamber.

Basically any machine work done to a cylinder head will weaken it as you are reducing it original dimensions making it thinner.

 

Same issue really when it comes to having the cylinder head skimmed it can only be done a few times,

as it increases the compression this is due to the depth of the compression chamber being reduced each time the head is skimmed,

it's the head of the valves that can also protrude so most people have a slightly thicker head gasket fitted to compensate for this,

but mainly it's down to the fact that the overall thickness of the head face can be come too thin and the issue with the compression chamber.

 

 

But yeah the answer to your question is yes the cylinder head will have to be replaced if it's already been machined to it's max limit.

 

 

Bottom line is really when you've had new valve guides fitted,

should then have the head skimmed and valve seats replaced or machined and have valves lapped in to new valve seat surface.

 

I've heard some people have said you can't or shouldn't machine VR6 valve seats ??

 

£300.00 to £450.00 is what it may cost to have the guides replaced and have the head machined

 

 

Hope this helps.

 

Si :thumbleft:

 

Yeah, skimming the head won't raise the compression, and although the valves sit proud of the head face, they should protrude by the same amount as you can recut the seats to compensate. You still have about 5.5mm space in the piston chamber before you add the head gasket thickness. This does mean you you have more clearance on the valve stem seals by the amount you skim the head, which helps.

I think you would be lucky to install the guides with 1 ton pressure, and it helps to put them in the freezer for 24hrs before.

There is also a minimum deck height for the head, I think it's about 139mm, it's in the Bentley book although I could never find the height of the factory head.

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As a guide I got my head skimmed, dipped, new guides (which i supplied) and seats reground for about £200 by a reputable engine builder over here.

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[quote=Stonejag;974934

 

When buying OEM replacement valve guides do they have the shoulder too? I'll be using Schimmel 263s. Seems strange that they wouldn't always use shouldered guides and just instruct you to push them out from the valve side, but hey...

 

My replacement head has never been skimmed before - so when it's done this time it will be factory dimensions again as the corroded areas were built up with weld so only the welded areas need skimming. My skim will be dirt-cheap so I'll see if I can find a cheap place to do the guide reaming and seat machining and leave them if the guides are within tolerance.

 

Cheers,

 

Stone

 

I think they all have the shoulder. My valve stem seals were crushed by schimmel 263's when I had the guides installed at the height the shouldered guides sit. I bought Victor Reinz OEM seals, I think you can get shallower seals but I've never seen them for sale.

raty7aga.jpg

The top guide has been crushed by 1.2mm, same as the extra lift on the schimmel's.

So I've now installed the guides as low as possible, I've only tested the engine and haven't put any miles in it, but it seems good so far.

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There is also a minimum deck height for the head, I think it's about 139mm, it's in the Bentley book although I could never find the height of the factory head.

That bit I can help with - it's on Bentley page 15.19-15.20. 140±0.1mm from the factory, minimum refacing dimension 139.5mm.

 

I think they all have the shoulder. My valve stem seals were crushed by schimmel 263's when I had the guides installed at the height the shouldered guides sit. I bought Victor Reinz OEM seals, I think you can get shallower seals but I've never seen them for sale.

How long are your stem seals? Those look longer than the ones I have from BG automotive, mine are exactly 10mm long.

 

Once the shoulder's machined off, do you just push them all in to the same depth? Was hoping I can get away without doing them, but if it's going to knacker the seals it looks like I'll have to... I'll get a dial indicator on them tomorrow and see how bad the guides are.

 

Stone

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That bit I can help with - it's on Bentley page 15.19-15.20. 140±0.1mm from the factory, minimum refacing dimension 139.5mm.

 

Cheers, thats a great help!

 

How long are your stem seals? Those look longer than the ones I have from BG automotive, mine are exactly 10mm long.

Try one on a guide (with a squirt of oil to protect the seal) they sit just above the shoulder. Im not sure how high they sit though. I could measure one in a week or two. Or if your urgent try GSF, they are 95p each.

Once the shoulder's machined off, do you just push them all in to the same depth? Was hoping I can get away without doing them, but if it's going to knacker the seals it looks like I'll have to... I'll get a dial indicator on them tomorrow and see how bad the guides are.

 

Stone

Im pretty sure the shouldered guides sit higher than the standard guides by the thickness of the shoulder. I couldn't find a lot of information about this problem except that some people have had problems with the shouldered guides and high lift cams. I don't think it's common with the 263's as they would be more documentation of this problem. Maybe my 263's aren't 263's and have higher lift, It sure does go well :-)

Anyway, I installed my guides at 2.422" or 61.52mm from the top of the head, just about as far as you can, the only downside I can see is that more of the valve guide is in the airflow.

 

There is a surprising amount of play allowed in the guides, yours may be ok.

 

If you have the original guides installed can you measure from the top of the head to the 'base' under the valve seal.

Just to see how far out I am.

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Yeah sorry Stone I was saying in general regarding the cylinder head regarding the compression chamber,

as most cylinder heads have a compression chamber in them,

but yeah on your VR6 the head face is flat so like you say you've got the recess in the pistons for the compression chamber.

 

 

Si :thumbleft:

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How much are valve guides and valve stem seals from the stealers for 94 VR6 ?

 

Thursdaves your man for OE prices.

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Just stripped my cylinder head and I have some cracked valve guides, I had new ones put in and they had the shoulder on them, now I have ABT 268 cams so I’m guessing this is the issue,

Has any one else run high lift cams with shouldered guides and if so what was done to compensate and stop them from being crushed.

Would. Have to source none shouldered guides or possibly stick with an original untouched cylinder head.

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I know this is oldish but since I'm looking into this right now for my project and tumble across this thread. 

It looks like for higher lift cams, the non shouldered guides are recommended. In the description, they recommend for 288 and up but me personally, I would install those for 268 as well. 

Here's the link to them: http://techtonicstuning.com/main/index.php?main_page=product_info&cPath=2_13_61_484_340&products_id=3362

I hope this help! Cheers

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