How much should I shave my 280z Head

[gengis]

Hi All,

 

I have never posted any thing to this forum before however have had many hours of great reading. Thanks to all for this.

 

I have owned my 260 for 26 years now and in the early days it was my daily drive. During its life it has been stored for extended periods while I had children etc. 5 years ago it was turned into a dedicated track car which generally gets a day or so each year at Philip island or Sandown to live the dream. I was getting it ready for a track day or two this year and whilst changing the oil discovered the dreaded milky colour and a river of silver. I took off the rocker cover and discovered the cam had been damaged and I have noticed that what I believe is water had entered the cylinder closest the fire wall of the engine bay. I have taken photos of the spark plugs and one of the cam for those to comment.

 

I know on my last track day 2 years ago the car over heated, I have since changed the suspect stock radiator for a race one and am in the middle of installing an oil cooler.

 

Previously I had installed a Wade cam the number stamped is 487 stamped on the end I would like to know if any body knows the specs of this cam as I just found out Wade cams has closed it's doors for good in February this year. I need to purchase another cam and would like to go bigger and not less than the damaged cam I have now.

 

While I take the head off to change the head gasket I thought I would have it shaved a little to gain some compression.

 

My questions are as the bottom end is stock what is the highest compression I should look to obtain and where should I go take my N42 head to be cleaned and shaved. I live in Mentone, Melbourne. The wade cam in it now pulled really well up to 6500 however I did not push it any further as I know it is an old engine and I did not want to break anything. I have Triple dellortos extractors and 2.5 inch exhaust straight through.

 

I am in now way a mechanic, however probably just like most guys on the forum love car stuff and engines and want to tackle this myself and perhaps learn a couple of things along the way. Any help would be appreciated.

 

Keep well,

 

Gengis

 

[d3c0y]

How much has it had off it already?

[dat2kman]

Clive Cams bought all the stock, patterns, machinery and blanks from Wades, that grind is quite common, well known.

You can get away with a trueing cut, on a virgin head of around 10-15 thou.

When you get into the 20 thou upwards, ie when half of the stamped "Nissan N42" is missing along the bottom, between no1-2 spark plugs,

Then you need to some stuff to your guides, and chain tensioner plunger mount block.

I posted the details in this, this morning, i think topic was about Chain Tensiioner or something.read that.

 

You head gasket is a bit ordinary now, with that milky oil.

Can get away with just replacing it, but head will need a trueing cut, 4 to 6 thou, surface finish of Ra40 ( the shop will know what that means)

[gengis]

The head has never been off. So it has never been shaved.

 

Great news about Clive cams, I will look them up and see if they can provide another cam perhaps one jump up. I did read the post about chain tensioner earlier thanks for that.

I will change the head gasket as suggested thanks Dat2kman....

 

Any body know a good machine shop as I have never done this job before...

[C.A.R.]

You don't shave the L-series head for more static compression - PERIOD.

It'll just turn into a giant mess...

 

Bring/send it down to Les: www.lescollinsracing.com - Les is THE L-series guru.

[Zedman240®]

Don't go to a too high comp ratio unless you are planning to run some exotic race fuel; keep it at a realistic level and run PULP 98 or more if you have access to it. Also, with a bigger cam, think of better valve springs, lash pads etc etc..

[jamo240]

If it has a Wade 487 grind in it, it was probably a reground stock cam, so with a reduced base circle (look it up) would have had thicker lash pads installed to retain the correct rocker ratio/wipe pattern. I ran a 487X for a while (inlet/exhaust profiles reversed), which Wades claimed was a good turbo cam, but it was not...I digress.

 

Anyway, it was ground on an L24 A grind, and needed thicker lash pads to keep the rocker geometry right...

 

If you go a bigger cam, you will have to make sure this is right, or have someone else set it up for you.

 

Also...I use Orgers for machining in Bayswater. They used to machine the RB26's in the GTR's, and there's not much they can't do out there. Besides...pressure testing/facing a head requires no special Datsun knowledge to do.

 

Jamo

[gengis]

Thanks for the advice, At what compression ration is to high to run on 98 at the pump fuel...

 

I was thinking to increase the compression to 10 to 1. And how much would be needed to shave of the head to achieve this. Or is this something they would know at the machine shop?

 

[jamo240]

To work out the material to remove, you need to know the head you have, and hence the combustion chamber volume and shape. If the area you are going to machine is 'round', then you can calculate (via pi x r^2 x h) the material to remove via a comparison of volume at BDC vs TDC (look it up). Once you know the TDC volume at present, you can work out how much to remove via machining to achieve the CR you want. You can either figure out the chamber volume using a burette, or look up the chamber volume of your head based on the type that it is (cast in the side of it...eg P90, N42, E31)

 

The CR the engine will tolerate without knocking is influenced by many factors other than the CR itself. Combustion chamber shape and the relationship with the piston are key, as is spark plug temperature. If you changed nothing else, I expect 98 RON would support 10:1, but guys with more experience in stock normally exasperated engines than me might have a more informed view (I'm a turbo guy!).

[d3c0y]

google a program called Lengine, it has it's flaws but will be a good starting point to play around with.

[dat2kman]

N42 heads come stock at 44.6 cc chamber volume, a dished piston adds 7 cc, a std head gadket is about 2 cc.

Stock comp is nothing huge.

Your engine is stock, virgin, and is prob in need of just more than the head off, leave block in situ.

Pull the lot out, and ask around either on here, or local engine shops, and do the job properly.

 

The block will need hot tanking, oil galleries cleaned out.

No need for bigger bore, run a FlexHone tool up the bores, give it a set of stock flatops, or retain your own pustons, and re-ring.

Naturally, new bearings, and have the block decked by 4 thou, for good head gadket seal.

The head, whip 4 thou off that, if it is warped, then may e a deeper cut.

Stick with stock size valves etc, a three cut valve job, or even a decent hand lapping, if seats are in good nick.

 

Some of the more cluey blokes on here, in Melbourne, could do this in their backyard sheds.

You spund like you just looking for a freshen up, not a full race screamer.

The svary thing, if you are on a budget, if it goes into a commercial workshop, it can get up there in the $$$.

 

[1600dave]

As Lurch says, excessively shaving an L-series head to increase compression isn't a good idea.

 

You will decrease the distance between crank and cam, resulting in a slack timing chain. Your cam timing will suffer due to this.

 

You can fit cam tower spacers to fix the timing chain issues. Then your rocker arm geometry will be out.

 

Next up will be an aftermarket adjustable cam sprocket as you may not be able to get the correct cam timing with the limited amount of factory adjusting holes.

 

And so on, you will be chasing your tail from one issue to the next.

 

Pushrod engines ? Go for it, shave away. L-series ? Keep machining to the minimum required for reconditioning work, ie a light skim to true up the gasket face.

[gengis]

All good advice, Thanks heaps,

 

So I will just shave the minimum off the head to ensure no warping, the compression will go up a little it will have to do as I see the knock on effect of adding  would need to be done to raise compression considerably.

 

I have just been to a cam regrind shop, they informed me that they could re grind my existing cam which they say is a 72 after I gave them the 487 number stamped on the end. They can grind it to a 74. I am not sure how they can do this?  Don't they need to add to the cam lobe in order for the valve to open longer thus more fuel go's in etc... to add Duration etc... I am no expert but I did ask them if they add to the cam however they said they did not but would regrind mine... Am I wrong here and should I go to another shop or are they telling me the truth?

[d3c0y]

they take the material off the back of the lobes to in effect move them outwards, get it?

[1600dave]

Yep, the cam "lift" is basically the difference between the "base circle" (the "non-lumpy" bit of the camshaft) and the "lumpy" bit of the camshaft.

 

You can increase the lift (ie the difference between the two figures) by either increasing the "lumpy bit" or by decreasing the "non-lumpy" bit. Reground cams typically use the latter method.

 

You will have to check the rocker arm geometry if you reduce the base circle diameter through regrinding, you may need different sized lash pads (the little bits that sit in the valve spring retainers, effectively a thick type of shim). Once you reduce the base circle dimension, you need to adjust the rocker arm upwards to maintain the correct valve clearance. If you move only the pivot end up (ie the normal way to adjust valve clearance), you upset the geometry. Basically, you need to move both the pivot end of the rocker and the other end (the tip that opens the valve) by the same amount. Pivot end is easy, only way to move the tip of the rocker up is the add thicker lash pads between the valve and the rocker tip.

 

L-series heads are a little tricky to get this stuff correct once you start messing around with them.

[gengis]

That makes sense, I did not think of the under side of the lobe...

 

Thanks to every body for their suggestion. I will let you know how I go...

 

Keep well,

 

Gengis

[dat2kman]

I'd suggest stick with the 487, good all round cam.

74 deg will start to get lumpy, higher idle, and be doughy up to 3500rpm

Dont get concerned with a skim shave off block and head, it will not alter comp, and you can run normal fuel.

 

As Dave above, you, or your workshop, needs to be fairly particular with lbe wipe on the rocker pads.

Plenty of shops will say YeahYaeh, we know how to do it, but ask them how many L series have they done!

Send peter mc, via Lurch above, a pm, he may be able to knock it up for you, and it will be done right.

He's over Moe way, but worth the trip.

[d3c0y]

Bigger cam the better! 72 is a girl's cam

[gengis]

I don't mind going a bigger cam, I have had the 72 for over 5 years and the car never rev's under 3500 as it is a track car only... If I end up doing the bottom end eg just a clean up then I would feel better about going passed the 6500 which I short shift at now as I had the age of engine in mind...

[gav240z]

they take the material off the back of the lobes to in effect move them outwards, get it?

 

No - not really, can you show me visually?

 

The pointy end of the lobe would be the front right? That's the end that pushes down on the rocker arm. So how does changing the backside of it do anything?

[dat2kman]

By undercutting the base circle diameter, ( the rocker pad then has to be moved upwards), and then taking a bit ff the lobe tip, and then reshaping the backside of the lobe, this gives duration, ie valve open longer.

[1600dave]

Here you go ! Some dimensions are exaggerated

 

Might even do one up to show how this effects rocker arm geometry after I get back from Yr 12 formal shoe shopping (for the daughter, not me. Honest....)

[dat2kman]

Geez Dave, she's growing up! That vid of her in the passenger seat of your Dat, around Eastern Ck is a hoot!

 

To see what can go WRONG, do the chart thing, with the geometry extremely incorrect, and graph that up!

It will show just how little incorrect setup, can have big effect on bad wipe, as well as not allow full lift at valve.

 

We do the 50 thou before and after, looking at lift, wipe etc, on a dummy assemble engine using very light springs.

 

Setting up the FJ24 was certainly an education, as the lobes act direct on a bucket, that has a lash cap under it, then changing the cam timing via adjustable sprockets x2, relative to TDC,,,,,

And on and on!

[1600dave]

And here we go, before and after fitting a reground cam.

 

In the top sketch (original cam), the cam lobe wipes across the inserted pad on the rocker evenly.

 

In the second sketch (reground cam), the base circle has been reduced in diameter. Therefore there is a gap between the rocker arm and the cam (gap is the amount the cam has been "ground" down) - therefore massive valve clearance.

 

No worries, lets just wind the rocker arm pivot right up, same way you would adjust valve clearance normally, except much more.

 

You end up with something like the sketch, rocker pivot needs to be wound up roughly double the difference (because the valve spring end of the rocker can't change position).  The inserted pad is now pulled a little towards the pivot side due to the angle of the rocker. I should have actually shown that the rocker tip on the valve is also moved towards the rocker pivot. Imagine if you like a ladder - you change the angle of the ladder, and the feet of the ladder move towards or away from the wall.

 

So, the correct way is to increase both the height of the rocker pivot AND the other end of the rocker by using a thicker lash pad.

 

the problem of having the inserted rocker pad not central is that the cam can run off the end of the pad and onto the rocker arm itself. See pic for what this does to the rocker (this was the last engine I built when I pulled it down - previous owner hadn't got the geometry right and destroyed the rocker arm and the cam itself. You can see the black-ish section on the left hand quarter of the pad where the cam hadn't been wiping and the wear concentrated on the opposite end.

 

Sorry to ramble a little, but the first few engines I built years ago I didn't pay attention to this stuff, resulting in a few issues. Its not intuitive and can be g=hard to get your head around.

 

Oh, and the shoes. Little girl is all grown up  :'(

 

 

[gav240z]

Hey guys,

Thanks for the diagrams they are helpful, I understand the importance of setting up the lash pads and having the correct wear pattern on the rocker arms. However what confuses me is the base circle is where we machine or take out material to alter valve timing.

 

I was always under the impression that it was the other end of the camshaft profile that was changed (the part that changes lift etc..) I thought it was the thickness of the lobe that dictated how long the valve was in an 'open state' for.

 

I'm still trying to wrap my head around how removing material from the underside of the cam alters duration. Since in my mind that part of the cam is not exerting any effort on the valves themselves and should infact be doing nothing (leaving the valves closed).

 

Perhaps I'm missing something obvious? For the longest time camshafts have been a mystery to me, so just trying to visualise how this changes the characteristics of the motor the valve timing sequence.