Off with her head!

[alainmoran]

I'd rather not if I can avoid it.

Any reason you'd rather not?

having your head skimmed isnt all that pricey as it might sound, there is a place in preston that'll do it for about £30

[alex_holden]

As well as the extra time off the road and the additional cost and the hassle of finding somewhere suitable and getting it to and from them, I'm a bit reluctant because I had a lot of problems with pinking on a previous car that I think were at least partially caused by the increase in compression ratio after skimming the head.

[alex_holden]

The saga continues...

Can anyone confirm that the correct torque for the small rocker pedestal nuts is 25 lb/ft? After stripping two more of them I gave up trying to torque them down and tightened them with a spanner instead. If that figure (which I found in the "general data" section of the official workshop manual) is correct then I think my torque wrench must be faulty.

I eventually got everything back together (including checking that the head wasn't fouling on the water pump), torqued the head nuts down to 40 lb/ft, and started her up. Immediately I could see gas bubbles fizzing out of the head gasket right along the right-hand side of the engine (the old gasket did this slightly at the back corner). The idle sounded a bit rough and clattery too. After a few minutes, rust-coloured water started seeping out from under one of the head nuts. I looked in the radiator and saw emulsified oil.

So... Clearly the new gasket hasn't sealed. My plan of action is:

1. Have the head professionally tested and if necessary skimmed.
2. Obtain a higher-quality Payen head gasket.
3. Strongly consider buying a new torque wrench because I've lost confidence in it after the stud-stripping.

[alainmoran]

I've always found my (cheap) torque wrench to be distinctly unreliable at the lower settings (comparing the click against how hard I was pushing ... and dont even THINK about using it on left-hand threads)

FT has always worked great for me so far ;)


FT translation - *very tight* (apparently that is a technical term, one I learnt while working in a garage - as a pump attendant)

*edit a

[Orkney]

Bad luck Alex
Take it you roughed up the surfaces really well before fitting?
A race mechanic i Know showed me this one day when overhauling a v6 in a degenham roller skate for me.
Put cloths in the bores that were down, got a sanding block and a really coarse piece of emery and went to town !
I was amazed thought the guy was mad - turns out it gives a better seal than bolting down on slippery mirror like finishes. Have used the trick ever since and never had a problem.

[alex_holden]

Take it you roughed up the surfaces really well before fitting?

No, I've never heard that one before.

I'm going to get some new head studs too in case the old ones have stretched and the nuts are bottoming out on the end of the threaded portion.

[Orkney]

Well worth getting the opinions of all on this really - whos heard of it who uses it?
Its a bizzare thing in so much as the opposite to what youd expect. Apparently i was told that the scratches from the abbrasive cave a key for the Gasket. the part i found diffucult to comprehend was that the metal parts would actually compress into scratches only microns deep.

That particular v6 got a heck of hiding on several occasions and went on to be used as daily drive for years before the LSD gave up and i sold the car.

Other thought here Alex, seems very odd you had a problem along one side - take it the final torquing sequence after getting them spanner tight in a zig zag down the length isnt one sided?

[alex_holden]

I followed the correct torquing sequence as shown in the manual, tightening each nut a bit more on each run through to avoid any distortion, then after they'd all clicked I went round them a second time to make sure I hadn't missed any, tightened down the small pedestal nuts (stripping two as I said - luckily I had a full spare set from another head), set the tappets, then went round all the head nuts a third time out of paranoia.

[Orkney]

Borrowed from here

http://www.aa1car.com/library/ar996.htm

well worth a read of the whole page...


For years, most aftermarket gasket manufacturers have said surface finishes with a roughness average (RA) of anywhere from 55 to 110 microinches (60 to 125 RMS) are acceptable. The preferred range they have recommended is from 80 to 100 RA. Even so, as long as the surface finish on the head and block end up somewhere between the minimum smoothness and maximum roughness numbers, there should not be any cold sealing or durability problems with the head gasket (assuming everything is assembled correctly and the head bolts are torqued in the proper sequence and to the specified torque, too).
But like everything else, these numbers have been changing. These recommendations were primarily for older cast iron heads on cast iron blocks. As castings have become lighter and less rigid, the need for smoother, flatter surfaces has become more important. Consequently, some aftermarket gasket manufacturers now recommend a surface finish of 30 to 110 RA for cast iron head and block combinations, with a preferred range of 60 to 100 RA for best results.

Can a head or block surface be too smooth? After all, the smoother the surface the better the initial cold seal of the gasket and the less likely you are to have problems with coolant and combustion leaks.

Though most gasket manufacturers do not specify a minimum smoothness spec for aluminum heads that have MLS head gaskets, they do recommend a minimum of 30 RA for engines with aluminum or cast iron heads and a nonasbestos or graphite head gasket. The reason for doing so is because soft-faced head gaskets require a certain amount of lateral support from the head and block.

When the head is bolted to the block, the metal on both sides bites into the gasket to help hold it in place. Too much bite when the head is aluminum and the block is cast iron creates sideways shearing forces that result from the expansion and contraction of the aluminum head. Yet a certain amount of support is necessary to keep the combustion gases in the cylinders from distorting the gasket and blowing past it. This is especially critical in the areas with narrow lands and between the head bolts where there is nothing to keep the gasket in place but the gasket itself. In high output or heavy-duty applications where combustion pressures exerts even greater force against the head gasket, a surface finish that is below the minimum smoothness spec might lead to premature gasket failure.

In many instances, a premature head gasket failure because of leakage or burn through can be traced back to the surface finish that was put on the head and block. Of course, there may be other factors involved, too, like engine overheating, detonation, etc., or mistakes that were made during engine assembly such as not torquing the head bolts properly. But if a head gasket fails, there is always a reason why. Paying closer attention to the details of resurfacing, therefore, can help eliminate this as a potential source of trouble.

[alex_holden]

I must admit the head I'm having trouble with has a much rougher surface finish than the top of the block and the face of the original (leaded) head. The surface has fairly deep grooves from a flycutter that you can feel when you drag your fingernail across it and can see imprinted on the top of the failed gasket. The page you've linked to says:

One thing you do not want on the surface of the head or block is scratches. Every scratch is a potential leak path along which fluids and pressure can migrate. If a scratch is deep enough, coolant may find its way into the crankcase or cylinders before the engine is fired up. Or, combustion gases may force their way past the gasket into the cooling jacket or an adjacent cylinder eventually causing the gasket to burn out and fail. Either way, it's bad news.

Sounds like it might need a skim. Annoying, because the head had supposedly been rebuilt when it was converted to unleaded, but it was actually in a fairly poor state when I received it.

[Orkney]

Hmm its quite interesting this is. Seems some scratches better than no scratches but too big is also a problem.
I mean what sort of surface finish is the expected norm when a heads been skimmed - I'd certainly expect a texture from the flycutter.
Maybe you just need the grooves from the flycutter taking down a bit if its possible without going so far as to affect the compression?
Would be handy to find out the definitive answer to this for future reference.

[bmcecosse]

Flycutter finish really is NOT good enough - should be a nice ground finish - but doesn't need to be 'mirror'! Try tightening to 44 ftlbs (washer under nuts and threads oiled) - or if you get new studs (Y on top) and new Mini type nuts (with built in 'washer') then 50 ftlbs is fine. You must NOT tighten the rocker pedestal nuts finally until after the head has been torqued down - they cannot safely pull the shaft down against the valve springs - the head nuts do that. I assume you only stripped the fine thread on the top of the studs ? In which case - it's just been worn nuts - fit different studs and nuts and should be fine. these studs/nuts cannot influence the head gasket - they only help to secure the rocker shaft. Best to always take out the head studs and slightly countersink the holes in the block - then refit studs. Also thin layer of grease on both block and head - doesn't matter which way gasket goes. Payen is 'supposed' to be best gasket - but I have had no trouble in many many years using standard 998 Mini gasket at ~ £4 (Mini has 10:1 CR as standard) from local Autoparts. Don't condemn it too quickly - there will always be a bit of 'oil' in the water after working on the engine.

[alex_holden]

I got the head back off tonight, and I ordered a new Payen head gasket, studs, and nuts earlier today. I didn't torque the rocker pedestal nuts until after the head nuts. The rocker pedestal studs that stripped were new when I fitted the head 5000 miles/6 months ago, and two of the nuts were new yesterday - I really think the torque wrench was giving too much torque when set to 25 lb/ft.

I need to try and find somewhere to skim the head tomorrow.

[bmcecosse]

I just pull the pedestal nuts with a ring spanner until I feel the first 'stretch' on the threads. Easy enough to detect. Do you not have another head you can try ?

[alex_holden]

After stripping three I gave up on the torque wrench and reverted to doing it by feel with a spanner. No more stripped threads. The only other head I have isn't unleaded and one of its thermostat mounting holes is knackered from an unsuccessful attempt at extracting a sheared-off stud.

[bmcecosse]

Easy enough - just drill and tap 3 new holes for the stat - move it round slightly, and plug the old holes - and the 'not unleaded' doesn't matter a hoot. Doesn't sound like yours is unleaded anyway if there are no obvious seats.

[alex_holden]

I've just got the head back from Moorespeed in Padiham. He charged me £25 for the job, did it the same day, and offered me a lift home when I told him I was planning to carry it back on the bus. He said it wasn't properly flat - he had to take about 8 thou off to get it true. It now has a nice ground surface finish with some very slight pitting around the water galleries rather than the deep flycutter marks it had before.

[Orkney]

nice one ! dont suppose you asked them about roughness/smoothness for gaskets ?

[alex_holden]

Fraid not. I reckon a smooth finish will be all right as long as it's not polished until you can see your face in it - that's how they came from the factory after all.

Incidentally the Payen gasket does look slightly different - where the standard ones had a bare metal finish on both sides the Payen one seems to be coated with a thin plasticky lacquer.

[bmcecosse]

Was that Morspeed - Simon Morson ? The ground finish is what should be - flycutter finish is not good enough. But still spread a thin film of grease on block and head when fitting the gasket.