17 replies to this topic

[crn1969]

hi am in process of stripping head down for clean re-lap valves etc.this is my first attempt when taking out valves don't seem to have oil seals on all of them? why is this? and should I replace on all of them when rebuilding ? many thanks 998 1989 a+ unleaded head.

[MaxDempsey]

When I did my head I put stem seals on all the valves as the kit came with 8 and therefore thought it shouldn't do any harm to, however mine also only had four when I took it off aswell

Edited by MaxDempsey, 24 January 2016 - 02:13 PM.

[MaxDempsey]

I believe the reason that they only had four (on the inlets) from factory is that the inlet vacuum can suck oil down the valve stem, causing it to burn

Edited by MaxDempsey, 24 January 2016 - 02:14 PM.

[crn1969]

many thanks max think I will replace on all .not a cost issue just didn't know if they were not installed originally for an important reason? also any idea on valve spring life expectancy was thinking of replacing while head in bits but not sure if the old rule of " if it aint broke don't fix it " applies?

[MaxDempsey]

I have not replaced mine and do holiday work in an engine builders and haven't replaced any springs so yeah, 'if it ain't broke don't fix it' applies here 

[dklawson]

Max's explanation is correct.  The seals were originally placed on the intakes only to prevent oil from being sucked down into the combustion chambers.  The exhaust valves are always under pressure.

 

Care must be taken when fitting seals to the exhaust valves.  IF the head was recently rebuilt with new guides and the clearance between the valve stems and guides is minimal, you may not want to fit seals on the exhaust.  The little bit of oil entering the gap can help keep the exhaust valves working freely.  If there is sufficient clearance on the exhaust valves, then there is typically no problem fitting seals to them.

[crn1969]

thanks doug most helpful.will take all comments on board.

[Spider]

If the stem to guide clearance isn't correct, don't reassemble it until it's corrected!

 

Always fit seals to all valves, the inlets have been explained above, however, oil going down the exhausts will cause the seats to in the head to pit and quite quickly, even hardened ones.

[crn1969]

many thanks.much appreciated.

[tiger99]

You should ALWAYS replace valve springs at an overhaul. They are not that expensive compared to the other parts. Weak springs can contribute to rapid erosion of valve seats, and wear of the rockers, tappets etc, if the valves bounce, which also causes serious power loss.

 

Some people compare the length of a new spring to an old one and wrongly conclude that because the old one is perhaps 90% of the length of the new one, it is still exerting 90% of the force. Not so, it is the difference between FREE length and INSTALLED length that determines the closing pressure. And, the used springs have probably lost some of their modulus of elasticity too.

 

There is also the possibility of fatigue fracture, possibly resulting in a valve dropping down the cylinder at full revs....

 

If I found that an engine reconditioners were as incompetent and penny-pinching as to re-use valve springs, I would never, ever use them for anything. It is even worse than fitting oversills.

Edited by tiger99, 24 January 2016 - 07:47 PM.

[carbon]

If the stem to guide clearance isn't correct, don't reassemble it until it's corrected!

 

Always fit seals to all valves, the inlets have been explained above, however, oil going down the exhausts will cause the seats to in the head to pit and quite quickly, even hardened ones.

+1 for fitting valve stem seals to exhaust valves.

- it reduces oil consumption

- and prevents coke build-up on the exhaust valve which leads to pitting of the valve seat in the cylinder head

[crn1969]

Thanks guys some great knowledge on here.very useful.

[Cooperman]

Yes, fit all 8.

[crn1969]

Also should I remove valve guides before thoroughly cleaning head? Is that even possible? Will they be okay left in situ? Whilst cleaning.they seem in okay condition.

[Cooperman]

If you remove the valve guides and fit new ones, you will have to get the valve seats re-cut and re-lap all the valves. You cannot remove and re-fit existing guides as removal will damage them. New ones won't necessarily re-centre on the existing valve seat centre.