Head Rebuild Problem - HELP!!!!!

[Huw]

Hi all

I have been rebuilding my cylinder head and have run into some problems, which I could really do with some guidance on resolving.

So far I have had the head skimmed, inserts put in the valve guides and had the valves re-seated. I have also renewed the springs, rocker arms; lash pads and the cam. All these items are stock items.

On checking the lash pattern I found the wipe pattern to be way off the lash pad centre. With the wipe pattern biased toward the rocker pivots (See diagram). Did not expect that. It was like this for each of the valves, so I can determine the machinist did his job and equalised all the valve heights.

So playing about with cut up feeler gauges, I found I could improve the wipe pattern by packing up the stock lash pad height by 0.020”, increasing the lash pad size to 0.140”. So far so good, I can get hold of these.

BUT by raising the lash pad height on some of the rockers, I was unable to set the correct lash gap. There was no adjustment possible due to the rocker being flush with the cam and the pivot wound as far down as it would go. Its like the cam towers have shrunk.

Can anyone shed some light on what the possible contributors to this problem could be and how I could go about checking them and putting this right?

All suggestions gratefully received as I’m at my wits end with this damn thing.

Thanks

Huw

 

[morbias]

Maybe they didn't grind the valve stems down to the right length.

 

[MaximG]

The lash pad sits in the lash plate that in turn sits on top of the springs held in place by the collets. How is grinding metal of the valve stem length going to alter this?

It sounds to me like something to do with the height of the cam towers perhaps / possible requires shims to restore cam geometry due to the head being skimmed.

Mike B

 

[morbias]

The lash pad sits in the lash plate that in turn sits on top of the springs held in place by the collets. How is grinding metal of the valve stem length going to alter this?

Because if the valve stems are too long then there will be no clearance between the rocker arm and cam?

 

[racer]

Quote
"It sounds to me like something to do with the height of the cam towers perhaps / possible requires shims to restore cam geometry due to the head being skimmed."

How does skimming the underside of the head affect the height of the cam towers within the head?
Shims are used take up the slack in the cam chain after skimming.

 

[zbloke]

How far where the valve seats cut back? and when the head was skimmed did they skim the top, where the cam towers sit, and the face or just the face?

 

[morbias]

Quote
"It sounds to me like something to do with the height of the cam towers perhaps / possible requires shims to restore cam geometry due to the head being skimmed."

How does skimming the underside of the head affect the height of the cam towers within the head?
Shims are used take up the slack in the cam chain after skimming.

That's what I was wondering... unless they shaved the top by mistake

To my small brain it seems more likely the valve seats are much higher up in the head than they should be, so the valve stems are sticking up further. But that's just from looking at a picture, I have no experience of rebuilding an OHC engine whatsoever.

 

[MaximG]

Quote
"It sounds to me like something to do with the height of the cam towers perhaps / possible requires shims to restore cam geometry due to the head being skimmed."

How does skimming the underside of the head affect the height of the cam towers within the head?
Shims are used take up the slack in the cam chain after skimming.

Because Mat usually one skims both sides of the head if the head is warped which nine times out of ten is the usual reason for a head skim. So on that theory the spring seats are unaffected by any skim works but the tower seats are, thus bringing the cam closer to the rocker arm which in turn takes up any adjustment. It kind of depends how many times the head has been worked on over the years. Knowing what the thickens of the head is at the moment will give some indication of this.

As for the valve stems needing grinding that just rollocks.

 

[Huw]

Thanks for your thoughts so far, but I can rule out the cam tower heights as the machine shop only skimmed the bottom of the head only due to one of the water ways needing some welding. I think Morbias might be right however.

I spent the week end stripping it all back down and measuring everything, at least three times. The installed valve spring height (spring retainer to spring seat height) should be 40mm. Mine were off ranging from 40.36mm to 40.7mm at the extremes. The majority were at 40.4mm thus giving me the impression of uniformity across the valves. Since any deviation in valve spring height will equate to the final valve stem height, mine are all out by 0.4mm or more.

So far so good. The specs say that the valve stem can be ground down to a maximum of 0.5mm. Getting the machine shop to do this should rectify the valve geometry I thought. Just to make sure I checked the valve lengths to ensure they have not already had the stems ground back.

Stock valve length for 79 280ZX with a N42 head should be:

Inlet 115.2mm with 44mm head
Exhaust 116mm with 35mm head

Mine measure:

Inlet 116.38 with 44mm head
Exhaust 117.36 with 35mm head

So it would look like my head has had the wrong valves installed at some point, which may account for the extremely knackered cam, rockers and lash pads. I have measured all 12 valves three times with two different verniers and a good old steel rule. Or am I measuring them wrong, from the head face to the stem?!?... The stems also look like they have had work done as the chamfers around the stem head are of differing thicknesses from valve to valve. So if these are wrong valves where the hell do these valves come from????

The valves are dished and have a Nissan symbol on the heads, the exhaust valves also have an S on the face. They are the only components on the head that are not worn out. Making me think they may have been replaced at some point but the car has only supposed to have done 48k. Anyone ever come across something like this?

P.S. Found a great book for anyone needing to get to grips with engine theory. Automotive Technology: A Systems Approach By Jack Erjavec. Best of all you can read it on line for free at Google books ( just as we'll as its £168 in print)

Huw

 

[morbias]

The FSM has a diagram of how to measure the valves; ie. not from the face (see attachment)

depending on how much your head was skimmed it might be a good idea to shim the towers instead and regain some chain tension at the same time.

To be honest it sounds to me like the valve seat has been ground back too far, or the head has had too many valve jobs in the past. But it would better to ask someone who knows what they're talking about.

 

[Huw]

Great stuff, thanks for that Morbias. My mistake as I suspected.

I think you are probably right about the seats, ******.

 

[Huw]

Would anyone have the correct dimensions for an N42 head so I can check mine against please?

Thank you.

Huw

 

[SKiddell]

Huw
when you say "spring retainer to spring seat height" are you measuring to the set of spring shim washers or are you including them, IE the head material beneath the spring.

Also don't get too hung up about small variations on installed spring length at this stage, that could be down to whoever "played" with it before using any old washers to shim the springs up.

 

[Huw]

Hi

I did the measurements from the underside ofthe retainers to the steel disks that sit ontop of the head material as per Tom Monroe's book suggests.

Cheers

 

[Ian Patmore]

Huw,
Off the top of my head the correct thickness is 4.218"....I think...Mr. F and I confirmed this on an old post somewhere..best check.
On a side note, I had a head have new valve seats put in, and when the valves were installed, the valve stems were all at different heights, thus every valve set-up would have needed a different lash pad. Ahh, z experts....

 

[Huw]

Thanks Ian.

I looked through the old threads as suggested and found the one from Mr F. The head thickness for a stock head is 4.248" or 107.89mm. Mines had a 0.14mm skimmed off. So should be ok with out shims. Back to the valve seats I guess.

Huw

 

[candy red]

Huw give Mr F a ring he will help you with your problems and advise you what to do or check and supply you with anything you need a top guy