Skimming brake discs on milling machine

[Munkul]

Along with cylinder heads.
Obviously when you loosen all the bolts they will no doubt change shape slightly - but bolt them down again and surely they will become flat again?

Cylinder heads definitely warp, its very easy for them to do so They'll twist and stretch by a few thou, but the contact between cylinder seals has to be perfect. When a HG goes, an alloy head often warps - put it together with a new HG and it will go again in a few thousand miles.

I never did a HG on an alloy head without skimming it at the same time... for £40 it wasn't worth it. Worst head I've seen personally - 12 thou was machined out of it. Good heads - 3-4 thou.

Haven't played with engine internals for a few years now though, I certainly don't do it for pleasure any more!

 

[fizzy]

Wrong... cylinder heads definitely warp, its very easy for them to do so. They'll twist and stretch by a few thou, but the contact between cylinder seals has to be perfect. When a HG goes, an alloy head often warps - put it together with a new HG and it will go again in a few thousand miles.

I never did a HG on an alloy head without skimming it at the same time... for £40 it wasn't worth it. Worst head I've seen personally - 12 thou was machined out of it. Good heads - 3-4 thou.

How can they warp if they are held down by perhaps 12 bolts? And if they do doesn't the right retorquing pattern fix that?
Perhaps they were like that in the first place?
Surely the fault lies more with the gasket? I can understand head skimming to remove corrosion etc.
A lathe bed will twist or warp depending on what it is bolted to. If you adjust the lathe bed bolts you can take the warp or twist out.

 

[Munkul]

It's the effect of localised heat when a HG gives way between cylinders, or if you run it dry.

It's not like a lathe bed, because you don't take a blowtorch to your lathe and heat random spots up.

If no HG problems were detected and the engine is rebuilt for other problems e.g. bottom end, then yes, your head probably doesn't need skimmed, but I always got it done anyway.

 

[PhillipM]

I know a few old race engine builders that would never skim a head back flat - they bolted it down and put it in an old oven at 250c+ (basically as close as they could get it to 300c) for a day and let it straighten itself back out, then they'd run stone over it as light as possible just to clean it. Stopped the cams, etc, ending up on a slight twist or slant,

 

[Mr Fro]

Thanks for all the input guys, much appreciated!

@Brad93 - that's one of the things that worry me about the mill, I don't want to end up with a scallop on the disc.

It's a 2015 F11. I told the wife that if I had to have a family car that I at least wanted a quick one.

I'll hopefully get the wheels off and have a squint at the weekend then figure the way from there.

It would be great if it was just a build up of gak or something simple but I won't hold my breath!

 

[Melvyn Best]

Check your callipers are free, one of my brothers had an intermittent brake judder which I traced to a sticking front calliper

 

[Morrisman]

This is why your pads last a long time, if they only worked on abrasive friction you'd put a new set of pads in every few thousand miles!

I had 45,000 miles out of the front brake pads on my BMW bike. They just don’t wear out, as I rarely use them hard, or even ride hard, and much prefer to use engine braking.

 

[minimutly]

I know a few old race engine builders that would never skim a head back flat - they bolted it down and put it in an old oven at 250c+ (basically as close as they could get it to 300c) for a day and let it straighten itself back out, then they'd run stone over it as light as possible just to clean it. Stopped the cams, etc, ending up on a slight twist or slant,

Yep, the cam tunnel / caps warp with the head. Spent many an hour rubbing down bearing caps, lapping an old cam in the head to get the cam bearings somewhere near right. It usually worked, provided the warp wasn't too bad.

 

[Munkul]

One cannot take much off an overhead cam cylinder head if it is to correct warpage - the cam bearings will no longer be in line.

The cost of replacement heads may well be saved by relieving the warpage, by heat treatment, in these cases.

CI heads are retorqued while hot but alloy heads must be cold before retorquing. A good reason, I suppose, for the industry move to yield-point head bolts - even though they charge an arm and a leg for those manufacturer-supplied parts.

Most heads have a cast-in point where no more can/should be taken off past that depth, usually around 1mm ish.
All the heads I've worked on personally have had yield point head bolts, some can be reused, some can't. Fiat engines, Iveco, Nissan engines, and PSA engines.
Had about 8 alloy heads skimmed either for myself or for cars I've worked on, none have had problems with cam bearings, and I don't butch stuff together, I make sure it's right...

I suppose we must agree to differ

 

[8ob]

Most engines are a compromise, cylinder heads don’t get pulled down evenly. The end set of bolts only get to compress half the gasket of those inboard but are munched up to the same torque, block castings distort as the head bolts are tightened so they are not flat either. Only engine worth a rub was the Steyer diesel that had the head and block cast as one, pistons pushed in from the underside and the crank fed in on bearing blocks from the end, no stress/distortion anywhere.

Bob

 

[merryman]

Only engine worth a rub was the Steyer diesel that had the head and block cast as one,

Leyland 500 diesel had the head and cylinders cast as one, singularly unpopular, as you had to take the engine out of the chassis to work on it. Leyland had installed a dedicated machine line to make the 500, doubtful if they ever got the money back, though they did soldier on with it for about 10 years in the Buffalo truck. Meanwhile the old 400 and 600 series engines were being made on general use milling machines and lathes from the 1940s/50s and continued to be made that way long after the 500 machine line was ripped out for scrap.

 

[Mr Fro]

Well now, I know you've eagerly been waiting for an update on this...

The runout I have on the discs are from 0.06 to 0.11 mm so within reasonable tolerance. Visually they all look fine - no gouging or uneven wear pattern.

However, I do have a thickness discrepancy of 0.06 mm on one disc. I'm not entirely convinced this could cause the significant juddering I experience while braking so I guess I'm going to have to investigate further...

 

[Munkul]

Well now, I know you've eagerly been waiting for an update on this...

The runout I have on the discs are from 0.06 to 0.11 mm so within reasonable tolerance. Visually they all look fine - no gouging or uneven wear pattern.

However, I do have a thickness discrepancy of 0.06 mm on one disc. I'm not entirely convinced this could cause the significant juddering I experience while braking so I guess I'm going to have to investigate further...

I refer you back to post #34.

It's not a dimensional issue!

 

[Mr Fro]

I refer you back to post #34.

It's not a dimensional issue!

Yep, I read that first time round.

 

[Munkul]

Yep, I read that first time round.

My point is, buy some new discs and have done with it

 

[Mr Fro]

My point is, buy some new discs and have done with it

If this was any other car I would have done that straight away but they're just so bloody expensive!

 

[Edward Teach]

If this was any other car I would have done that straight away but they're just so bloody expensive!

What do you call expensive ?

 

[Munkul]

What do you call expensive ?

He'll call it extremely expensive when he gets them skimmed and it doesn't work and he has to buy new discs anyways...

 

[Myles1970]

Make sure and clean & regrease your calipers/pins thoroughly, I've had 3 BMs with sticking calipers that caused judder. Worst was the e39 530d; the steering wheel could be seen turning left-right about an inch... Easy to diagnose by jacking up each wheel and rotating manually

The last set of disks I installed were MTec, a LOT cheaper and never caused me any issues

....then you can use your old ones for your next welding projects

 

[Johnnybravo]

Well now, I know you've eagerly been waiting for an update on this...

The runout I have on the discs are from 0.06 to 0.11 mm so within reasonable tolerance. Visually they all look fine - no gouging or uneven wear pattern.

However, I do have a thickness discrepancy of 0.06 mm on one disc. I'm not entirely convinced this could cause the significant juddering I experience while braking so I guess I'm going to have to investigate further...

Strange as it may sound, I have had similar issues in the past. Discs ,wheel bearings all changed and it still persisted. Changed the tyres even though they were like new,recently balancedI had no wheel wobble on the motorway….problem solved. I do have a vague recollection of reading something on here about certain performance type cars being very fussy when it comes to tyres…