Re: An Engine Rebuild Thread for Beginners
Posted: 18 Nov 2019, 15:42
Section 5: Lapping the Valves
From Wikipedia: Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them.
Definition of ‘Lap’ (about the 5th one down after running or cats drinking milk...) - Polish metal, glass or a gem.
As far as I can work out the idea is to ‘polish’ the valve and where it sits in the head (the valve seat....I think) so that the two faces mate together and therefore seal effectively. I’ve also read that a good contact with the two surfaces helps with heat dissipation between the valve and the head which probably improves something.
Tools Required: Power drill, Grinding Paste, Oil
Grinding Paste bought from eBay for approximately £5 I think......it was a while ago. You will see that the container has two ends - coarse and fine.
Firstly, I had to remind myself of the correct numbering sequence for the valves and that the Inlet Valves were the larger ones, exhaust the smaller ones.
I then had to select the correct head for cylinder 1 & 2 - mine are easily distinguished as the left hand head (3&4) has the screw still in that holds the oil cooler pipe work. Then, from my clearly labelled bags, take the valves for cylinder one. Apply some coarse grinding paste to the rim of the valve, dip the shaft in oil and drop it through the valve guide sleeve into the head.
Where the shaft of the valve emerges then lightly attach the drill chuck and tighten.
Pulling the drill towards myself so that the valve sat in the valve seat then apply some very slow rotations of the drill. I did two rotations clockwise then reversed the drill and two anti-clockwise. This process was repeated......very slowly until I could feel that the coarseness of the paste had gone - the ‘gritty-ness’ gets less and the drill rotates more smoothly.
Remove the drill and the valve from the head, wipe up the excess coarse paste and apply a thin layer of fine paste and repeat the process.
As you can see, this leaves a clear band round the valve where this has ‘polished’ against the head, hopefully improving the contact between the two surfaces.
One point to note is Cleanliness - I was extremely careful to only use my left hand to apply the grinding paste and then my right for handling the valve stems etc. You do not want to get and abrasive paste anywhere it shouldn’t be. I will completely clean the heads and valves before reassembling, however I didn’t see the point in making that process harder than it needs to be (frankly, there’s already enough cleaning to deal with).
Cleaning the valve up shows a clear ring around the valve that I measured.....don’t really know why but had read somewhere that it should be about 2mm.
The picture above shows the Exhaust Valve for cylinder one. The lapped ring was 3mm. The Inlet Valve for cylinder one had a 2.2mm ring - I have absolutely no idea if this is significant or not and shall await incoming wisdom.
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From Wikipedia: Lapping is a machining process in which two surfaces are rubbed together with an abrasive between them.
Definition of ‘Lap’ (about the 5th one down after running or cats drinking milk...) - Polish metal, glass or a gem.
As far as I can work out the idea is to ‘polish’ the valve and where it sits in the head (the valve seat....I think) so that the two faces mate together and therefore seal effectively. I’ve also read that a good contact with the two surfaces helps with heat dissipation between the valve and the head which probably improves something.
Tools Required: Power drill, Grinding Paste, Oil
Grinding Paste bought from eBay for approximately £5 I think......it was a while ago. You will see that the container has two ends - coarse and fine.
Firstly, I had to remind myself of the correct numbering sequence for the valves and that the Inlet Valves were the larger ones, exhaust the smaller ones.
I then had to select the correct head for cylinder 1 & 2 - mine are easily distinguished as the left hand head (3&4) has the screw still in that holds the oil cooler pipe work. Then, from my clearly labelled bags, take the valves for cylinder one. Apply some coarse grinding paste to the rim of the valve, dip the shaft in oil and drop it through the valve guide sleeve into the head.
Where the shaft of the valve emerges then lightly attach the drill chuck and tighten.
Pulling the drill towards myself so that the valve sat in the valve seat then apply some very slow rotations of the drill. I did two rotations clockwise then reversed the drill and two anti-clockwise. This process was repeated......very slowly until I could feel that the coarseness of the paste had gone - the ‘gritty-ness’ gets less and the drill rotates more smoothly.
Remove the drill and the valve from the head, wipe up the excess coarse paste and apply a thin layer of fine paste and repeat the process.
As you can see, this leaves a clear band round the valve where this has ‘polished’ against the head, hopefully improving the contact between the two surfaces.
One point to note is Cleanliness - I was extremely careful to only use my left hand to apply the grinding paste and then my right for handling the valve stems etc. You do not want to get and abrasive paste anywhere it shouldn’t be. I will completely clean the heads and valves before reassembling, however I didn’t see the point in making that process harder than it needs to be (frankly, there’s already enough cleaning to deal with).
Cleaning the valve up shows a clear ring around the valve that I measured.....don’t really know why but had read somewhere that it should be about 2mm.
The picture above shows the Exhaust Valve for cylinder one. The lapped ring was 3mm. The Inlet Valve for cylinder one had a 2.2mm ring - I have absolutely no idea if this is significant or not and shall await incoming wisdom.
Sent from my iPad using Tapatalk