any help PLEASE appreciated on lapping in stainless valves

[sidewaysbo]

i have a pair of descent used staged v8 developments heads to to fit on my 4.6 bottom end before i fit the heads i fluid tested the valves with petrol to check if all seat were seating perfectly and a few of the valves were letting petrol past when fluid is put in inlet & exhaust ports on checking there stainless valves and two angle seats i believe, i have spent a descent amount of time on one head lapping them in, i have dummy built them with out oil seals fitted and fluid tested them again to find there still letting some fluid past is there a special paste for stainless valves or do they require a lot more time lapping stainless valves than normal steel valves i am using one of those lapping in devises that fit in an electric drill jobbies any help appreciated thanks bo

[sidecar]

V8 Dev tend to use Real Steel's DW500 valves which I did not think were stainless, are you sure that they are SS?

[kiwicar]

Hi
this sounds a little odd to me, I have stainless valves in hardened seats and using standard 2 stage paste method with medium then fine paste they lapped in as I would expect, about 20 seconds with a stick with a sucker on the end, rotating about a 1/6 turn every 3 to 4 seconds while twizzling the stick backward and forward, then about 10 seconds with fine paste same technique then engineers blue to check them.
Have the seats been cut properly with a tool centred using the valve guide? becayse badly cut seats that are not properly centred will not lap in. Are the valve seat faces heavily pitted? has the engine been run on LPG? ate the valves bent?
I always recon a valve should lap in in about the time stated above, total about 90 seconds/valve if you are doing one chamber at a time by you have wiped off the paste and cleaned the blue off with meths. If you are spending longer then you have an issue that needs sorting.
Best regards
Mike

[sidewaysbo]

thanks guys for replying when looking over the valves i took it for granted because i checked a magnet never attatched to them that they were stainless valves so i lapped them in for 5 to 10 mins each valve we looked with magnifying glass at the seats and valves could not see any pits on either and the heads have definitly not been run on gas seems really strange as the heads looked perfect when i picked them up they came off a well known hillclimb mgb and as he was uprating his complete engine he sold the parts off the engine out the car ibut this still leaking with the petrol test seems strange there not completely sealing ?

[sidecar]

I believe that some valves are made of a steel that is not magnetic but is still not stainless.

Of the top of my head they are made from Austenitic or Nimonic steel. Not sure if either or both of these are magnetic. You can also get valves which are two different steels friction welded together, the heads are made of the better non-magnetic stuff whilst the stems are made from a cheaper material which is magnetic.

[stevieturbo]

Seats cut properly along with matching valves should seal pretty well even without grinding.

Are you testing them as installed with springs, or the valves just sitting there ?

Does both seat and valve have a good uniform finish after lapping in ? ie both making good contact

[Darkspeed]

A grey line on the seat and valve does not indicate a decent seal because the thickness of the paste will always make a clear line on each the only way is to blue the seat with the thinnest smear of blue as this will immediately show where the low spot is.

[sidewaysbo]

thanks again guys, springs and collets fitted when i fluid tested i think i will have to get hold of some of this engineers blue and check the way you suggested thing is its only one head that like it other head valves seal fine my mate works in a little engineering shop can he check the valves are definitly true in a lathe or would it reqiure an engine building engineering shop to check the valves sorry to be a bit vague

[Darkspeed]

This probably relates to half a dozen other threads but will put it here for now

OK

A lapped in seat that produced a nice grey line all the way around which has now been blued where you can see a low spot - bear in mind that the blue is only microns thick.



Other side of seat also the blue is thinning indicating another low.



And after further checking and lapping in to get a perfect seal all around and uniform seat thickness - It will start at around 1mm but after a few miles will increase in width and seal



Having a perfect seal means no loss of pressure no hot spots etc etc. And it does not take that much extra effort.

[sidewaysbo]

as above been a great help pictures brilliant thanks again