Is there a way of telling if the valves in a head are bent? There are marks on the top of the piston consistent with lightish valve contact but they don't look fresh at all and there doesn't seem to be any sign of contact on the valves. I'm thinking the marks on the piston could of happened in the past but I'd like to be sure before rebuilding the engine and then finding there's a problem and having to strip it down again.

Also how can I tell if my head needs skimming?

I'm trying o keep costs to a minimum so I'd rather do this myself if possible than have to pay some one to do it for me

Cheers for any advice

test results post 11

Edited 13 March, 201115 yr by McLOVIN

So youve got the head off then?

A good way to make sure theres a good seal between the valve and the seat is to do the "vacuum test"

Bascally get some rubber sheet and create a seal between the nozzle of the vacuum cleaner and the port, so you need to make a donut kind of shape,

Have the head upside down with the valves pointing upwards and put some water in the "well" around the valve, get someone to turn on the vacuum cleaner which will create a vacuum behind the valve seat, if the water quickly dissapears you know its leaking. This tests the conecentricity between the sealing faces on the valve and the seat, and also tests the finish and if they need re-lapping.

If the valves look fine then I'm sure they will be, even very small contact between pistons and valves results in them being de-formed.

To check if the head needs a skim in turns of flatness then get a steal rule and lay it laterally across the head, then with a set of feeler blades check if you can get it under the ruler. I think the "flatness" tolerance is about .15mm or something on the polo heads?

To check if the head needs a skim in regards to surface finish it can only be a visible check to be honest unless youve got comparison gauges or specialist equpitment. Its pretty obvious when one needs a skim for the finish of the surface.

I would happily skim the head for you on the house if it needs doing. Check my build thread out for examples.

Hope that helps guy

Edited 11 March, 201115 yr by MaxMax

Is the engine together or apart? If it's together I'd do a leak down test.

If not, put a bit of engineers blue on the back of the valve, push the valve all the way into the guide so that it is seated then turn it from side to side (as if you were if you were lapping it in). When you take it out again it should be apparent if it's not making a good seal as the seat won't be totally covered in the engineers blue.

An cylinder head... whack a straight edge across it and check it with feeler blades to see if it's distorted at all.

An cylinder head... whack a straight edge across it and check it with feeler blades to see if it's distorted at all.

i normally use a torch and straight edge to check for warping if you can see light it's warped. but yeah the feeler blades are the correct method.

good point, torch is also good if you don't have any feeler gauges to hand!

turn the head over by hand dude, and see if theres any extra resistance in certian areas when turning, tourch is also usefull like above.

Edited 10 March, 201115 yr by mattpolo5491

if it does need skimming then if u have the head off, just take it to an engineering place and see if they will do it for cash, place here does them for 20 quid cash if its just a case of skimming the head

if you are in any doubt, whip the valves out and whack them in the chuck on a pillar drill, if they are bent or there is any runout on the valve heads you will see them wobble when the drill is running, often a valve which has made contact will have burn marks or light nicks at the point it enters the valve guide.. also there is a chance that the witness marks on the piston crowns are from a previous timing belt failure repair..

also a good starting place is (as has already been said) turn the cam over using a spanner and feel for any stiffness and check for consistent valve opening, a leak test is a good idea, just spray a load of wd40 into the ports and look for leaks at the combustion chamber face but this not a subjective test as if the head has been cooked up and overheated and is warped the centre valves often weap because the distortion prevents the valve from seating fully...

you don't say which engine type it is, but on some vw heads (vr6 and v5, some of the early 1300's, some diesels) you need to remove the valves to skim the head anyway because there is very little or no bump clearance on the valve. or in laymans terms, the valve head is flush or just proud of the surface of the head

if its going to be skimmed make sure that you remove the cam(s) hydro tappets/followers, sometimes even the valves depending on what engine you have and anything that might fall off and get lost. because the engineers usually charge to strip your head.

Sorry forgot to say its a AFH head, cheers for all the advice every one. I'm gonna go with the touch idea first as it seems like the simplest to start with and i'll see what that throws up and take it from there.

Skodaboy, I think the witness marks might be from an earlier failure, like I say there isn't any sign of contact on the valves, just the piston and they don't look fresh

just did the torch test and got what seem like some strange results?

i've just got a torch around the valves while rotating the cams and got some strange results. all the witness marks on the pistons are on the exhaust side yet there's only one exhaust valve that doesn't seem to be seating properly, yet non of the inlet valves seem to be sealing even tho there is no sign of contact on any of them or the pistons confused.gif

i've got a feeling that the unsealing exhaust valve isn't due to contact or the other exhaust valve in that chamber should be the same surely? and how could the inlets be bent if they haven hit the pistons?

on the 16v heads when the valves come into contact with the piston it will be on the very outer edge of the valve.

if like you say the valves are not seating properly then you ought to strip all the valves out and clean up the combustion chambers and valves then re-seat them by lapping them in with some coarse grinding paste, when the valves aren't seating properly the engine will be down on power..

on the afh heads you can skim them with the valves still in the head but you need to remove the cams and followers

on the 16v heads when the valves come into contact with the piston it will be on the very outer edge of the valve.

if like you say the valves are not seating properly then you ought to strip all the valves out and clean up the combustion chambers and valves then re-seat them by lapping them in with some coarse grinding paste, when the valves aren't seating properly the engine will be down on power..

on the afh heads you can skim them with the valves still in the head but you need to remove the cams and followers

Still a good idea to do a complete strip though, the fine metalic swarf will eat any moving parts in a few 100 miles.

yes true, but most decent machine shops will run the head through a cleaning process or two after being skimmed to get rid of the dust

yes true, but most decent machine shops will run the head through a cleaning process or two after being skimmed to get rid of the dust

Will they?

still wont remove the swarf thats found its way between the guide and valve trust me.

Much better to just do it properly and strip it all, replacing the guide seals for good measure.

trust me on this, i've worked in a engine repair workshop for many years specializing in repairing cylinder heads, for a home DIYer it's far better to get all the valves reseated and ready before the head is skimmed if possible because it avoids the risk of the valve spring compressor slipping and damaging the surface of the head when it's rebuilt, but what you say is correct, in a proper workshop environment i would skim, clean then rebuild

Surely in a "decent engine machine shop" you would use a proper press or a decent anti slip compressor?

Its what we do and I wouldn't do it any other way, suppose its preference at the end of the day.

yes i've used air powered valve spring compressors but if i'm honest it's actually far quicker to use a manual one. i garantee you 100% i could completely remove all the valves and re-install them in AFH head in under 10 minuites with a manual spring compressor

Edited 11 March, 201115 yr by SkodaBoy

again thanks for all your help, i had been worried about tackling a head rebuild myself as i've never done any thing like this before, but with the replies here, the pm's from max and some reading up on the net i think i'm gonna at least try and have a go at it.

so tonight i'm gonna get some beers in and retire to the 'workshop' with my socket set

out of interest where could i get lapping compound and the lapping tool from? while reading up on it apparently not many places stock it these days? i've found some on ebay but i prefer to buy stuff over the counter where i can and i'd rather not be waiting around for stuff to arrive when i'm eager to get stuck in

out of interest where could i get lapping compound and the lapping tool from?

Surprisingly Halfords sell it! Tool, course and fine compound for £3.99 if memory serves.

Good luck! Not too many beers and please post some pics

Edited 11 March, 201115 yr by cheys03

Yep halfords sold me mine and told me they were one of two places in Sheffield that sell them. Your gonna need a compressor guy.

I did a search on the halfords web site and it came up with nothing, but I was searching for lapping compound which would probably explain what I was doing wrong. Although I've managed to get the of some and the lapping tool over the weekend any way so all is good

What will I need a compressor for max?

How the hell do I get these valve springs off?

Cheys I do some of my best work while drunk, which is why I struggle to explain how I did stuff when people ask me

Few pics of where I am now

as you can see no obvious sign of damage on the valves (valve with maker pen on is the exhaust one which isn't seating properly)

how the head sits now

bottom end with pics of contact marks, this is after cleaning the crowns of the pistons up a bit

Edited 11 March, 201115 yr by McLOVIN

What will I need a compressor for max?

How the hell do I get these valve springs off?

this might help top pic shows afh head, bottom pic is mk3 2.0 8v head (same principle though) you use the compressor to hold the valve in place which pushing down on the top spring plate, then you can remove the split collets with a magnet

ps. if you can get you head as clean as the one in the bottom pic i'll be well impressed

one more for inspiration, done by me this one, nice and clean ready to fit... and yes it was skimmed with the valves in!!!

before- ebay purchase, £15 with 5 bent valves

after fully ported

Edited 12 March, 201115 yr by SkodaBoy

so i have a habit of buying the cheapest option possible which almost back fired this time, halfords had two spring compressors in stock, one was a big fucker for 40 some thing quid and the other was 21 quid so i went with the £21 one...

as you can tell the above isn't much good for an AFH as i discovered when i got home and tried to use it on it. never fear pikey tech tools and techniques are here...

this rude boy had the collets out in no time and worked out over £20 cheaper than buying the proper one, win :-D

head is currently stripped and ready for me to pick up a lapping stick from halfords after work tomorrow

strangely i can't wait to have a go at lapping the valves, would of done it today if it wasn't for unforeseen circumstances (read cracked sump on the daily )

any suggestions on how to clean that crap off the mounting face of the head? i tried white spirits but it didn't do a great deal

Edited 14 March, 201115 yr by McLOVIN

that's a compressor for doing OHV engines like Ford CVH, and A-series minis, and various others

the best way to get the crud off the head surface( if it's not being skimmed) is a nice flat block of steel with some 400 grit emery paper wrapped around it

edit: don't bother with the fine valve grinding paste, it's only really any good for freshly machined valve seats, use the coarse stuff, you will only need about a teaspoon full in total to do the whole job! don't use too much paste (either fine or coarse), use just enough to put round the seating surface of the valve, and DONT get any in the valve guides

Edited 14 March, 201115 yr by SkodaBoy