valve lapping

[Maki]

Hi Guy, please give me your view on valve lapping.
I bought paste, valve lapping tool and tried to do it. But now I'm not sure whether I'm doing it right. So the question would be how to do it right? How long, how hard, how fast should I do it? And what should I see in the end and how to check my work?

Thanks
M.

[Balidey]

I too have a question on lapping.

I have just recently finished rebuilding a head on my Fiat FIRE engine. As it was my first attempt at lapping in valves I think I did an adequate job as that car is only for road use, but my next build (and future plans) will be more ambitious and I was wondering if anyone had experience of these speed lapping tools as shown below.

Are they an aid to lapping or would I be better just concentrating on the traditional stick method?

[Guy Croft]

Since this is a race engine website I'm going to say this:

A well machined or ground valve-seat combo should lap in seconds with no more than fine lapping paste and a lapping tool like the one in Maki's photo - a rubber suction pad on a stick. You lap by rotating the valve back and forth 1/3 turn for a few seconds with a few dabs of past eon the valve and with application of downward pressure and then turning it thru 120 deg twice and repeating that procedure in each 120 deg position. Other rotation tools are definitely not required.

Lapping is - in my view - the best way to determine true contact face position and width. Some don't agree with it - I say - no problem. But I do it on every head I do. And we did it at Brian Hart's works on F1 engines - yes - even with Titanium valves and Hidural or Copper Beryllium seats. that's a lot of seats on a V10 and yes it was done by hand.

Now if it takes more than say, 20 seconds to get an evenly distributed contact face on valve and seat - there is a problem there. The valve may be bent (seat laps valve doesn't) or the contact may not be circular (valve laps seat doesn't). And an uneven width of seat can mean top and bottom of seat contact face is not concentric to guide axis or not perpendicular to it. If a light lap doesn't bring quite the result you wanted, try again and press down a bit harder to get the paste to work. These things matter, I have published enough photos on the site to emphasise the importance of accurate seat work.

I sometimes lap the valve directly after top grind and 45 deg contact face grind to show me exactly where to do the throat grinding. It's quicker than doing all the angles and then finding the contact face and valve don't match for width. In my exp, that said - you can get great results with a wide set contact face and narrow valve one but the valve contact face should never be wider than the seat - that doesn't work well at all on the flowbench.

You have to realise that in lapping, you are certainly wearing away metal. Even a light lap - like I am used to - will take in the order of 0.001" off the valve face. When I go to such extraordinary lengths to develop high flow precision seats (see forthcoming GC V/W CVH article) it makes no sense at all lapping them heavily. That might be fine for a garage, not for us, members!

Always use oil-based lapping paste. Some are water based - and the seats - if cast iron - and maybe valves too if not stainless - will corrode very quickly.

GC

[parrish]

As Guy says a light lapping should be all that is required, before you start lapping though you have to ask yourself why you are doing it, if its because the seats are pitted/burnt! look elsewhere for an answer. Thier is little point in lapping if for instance you have bad valve guide wear or badly pitted seats and valves, its only going to happen again.
If you dont know what you are doing go and find another head and practice, especially when using those tools that fit into a drill chuck, you can imagine the amount of metal that can be removed by an over zealous use of the tool and paste, whilst on the subject of paste also rememebr their are different grades available, you start with the coarse grade (if needed) and move onto the finner grades.
Remember to clean the valve, head and inlet/exhaust tracts completly of all traces of paste before reasembling.
And finaly as if to back up what Guy has just said, i have a small tin of 2 grade paste (Guy, i think you will know the one im talking about) and ive had it all my adult life (48 this year), and yes i do use it.
Happy lapping.
Steve

[DamirGTI]

Hello !

Just want to ask is it advisable to finish valve seats with paste after cutting 3 angle valve seats ?
I have practice this job only when i was cuttng single 45 degree seats to give some sort of finish for each valve and seat , but don't know if it's good thing to do in case of 3 angle seats .

Thanks !
Damir

[Guy Croft]

Damir, hi

sure you can lap them, use only fine paste though..

You'll find out how good your seat angle is. Read what I wrote above!

GC

[sumplug]

Lapping valves takes me back to the days when decoking heads was a very common thing. Times have moved on, but lapping the valves for that perfect seal is as important today as ever. And as Guy says, it also tells you if the seats are perpendicular.

Andy.

[Guy Croft]

Yes, why did we have to 'decoke' heads every so often and why has it gone out of fashion?

GC

[sumplug]

Yes, why did we have to 'decoke' heads every so often and why has it gone out of fashion?

GC

Proberly due to the "throwaway" trend today and the exchange head schemes which are popular. Fuel injection and the latest unleaded fuel has allowed a much cleaner exhaust, and so has cut carbon emissions down dramatically. So carboned up valves are not a common site anymore.
Engines used to need a rebuild at 40000 miles[Mini A series for example] due to the enormous mass weight the bottom end had and the large friction that mass caused. Because the SU carbs ran very very rich, the heads got carboned up quickly. Mind you, the old Austin Seven sidevalve engines needed a "decoke" every 1000 miles!!

Andy.