Sunday, April 27, 2014

Spare Engine - Valve Lapping Techniques

In the last post I was investigating how to strip the head down and then said I was going to look into porting to get better flow. But after a little further investigation of the head, I have decided to delay the porting in favour of making sure that the valve seats are lapped properly. Once I know that they are air tight, then I will move onto porting.

I currently have two heads that I am working with, however I soon noticed that one had a rather nasty crack across one of the valve seats. A closer inspection confirmed that the crack extended into the valve port and across the gasket face. So that's one head declared dead, destined to be nothing more than scrap or maybe a door stopper. Before that though, I will use it to practice and hopefully perfect my porting techniques.

Cracked = DEAD.

So with one head totally knackered, I moved onto what I considered the better of the two heads. It was in a right state when it came off the block, but I was hopeful that it would clean up okay. In the previous post, I had started the cleaning process, but had not started lapping the valve seats yet.

So onto the reason for the post, learning how to lap the valves. The valve lapping tool I bought is basically a wooden stick with a suction cup on each end! The idea is that you stick the suction cup to the face of the valve and turn it back and forth with your hands. It comes with two pots of lapping compound, one coarse and the other fine.



The idea is that the lapping compound is gritty and is placed between the mating faces of the valve and the seat. As you rotate the valve, it grinds the two surfaces into a common smooth shape making them airtight.

A quick search on the internet will show how to lap valves far better that I ever could explain in this post. Here's one such link I found useful... http://www.myhonda650.com/cb650lapping.htm   This is just one, but there are literally hundreds.

Having used the valve lapping tool on the 'dead head', I found it did a pretty good job on some of the valves, bringing then up to scratch quite quickly. However they were in quite good condition to start with. Although I was now working on the 'better head', I could see that time had not been as kind and some of the seats were very badly pitted. As I started lapping these pitted seats, it was obvious that this method was simply not going to cut it..

Not a good start! The seat is pitted.
After - Still pitted
and here...
and here.

Also some of the valves were not in very good shape, but this wasn't too much of a headache as I could just pinch some of the better valves from the other head.

Oh dear!

Mechanising Things
If I were to do things properly, I would get the valve seats re-cut. And if I was going to do that, I would also have new valve guides put in. But, of course I won't be doing things properly, partially because I want to learn the skills, but mostly because it is much too expensive. So what to do? Clearly, lapping by hand was going to take all year and wear my hands out, so what if I mechanised the process to speed it up! Maybe I could grind out the pitting enough for what I need!

So roll out my 'Rubber Hose Assisted, Valve Lap Master 2000'! Not the catchiest of names I know, so maybe I'll just call it a drill with a spare bit of fuel hose stuck on the end. Just see the pics...

Screwdriver holder the wrong way around
Bit of old fuel hose.
(It pays never to throw anything away!)
Stick it on the end
Squirt of oil on the stem
Stick the other end on the valve stem.

I've read mixed stories about using a drill to lap the valves. Some good and some not so good. The one thing that is crucial is not to get any lapping compound on the valve stem as this would damage the guide. I also took the precaution of ensuring that there was a little oil in the guides to reduce friction further. The reason I went for the rubber hose method over mounting the drill directly to the end of the valve was two fold: The elasticity of the hose should apply even pressure to the valve seat as it rotates and secondly, I didn't want to damage the end of the valve with the drill chuck.



Tweeking
I used the knackered head to test out the technique and found it to be mostly successful after a few tweeks. I found that if the hose is too long, it just twists, so shorter was better. Also I found that the oil made the hose slip off the valve when rotating, so a few Jubilee Clips soon solved that problem.



Next I found that a slow rotation was better as it allowed for better control as I moved the valve in and out of the seat. If the drill was too fast, the hose just got twisted up. Also, I changed the rotation every now and then. I also found that if I didn't move it in and out, it would result in rings appearing in the seat, which are not good.

So after a little faffing about using a course compound and a lot of time, I managed to get the results I wanted. I was able to grind the seat past the pitting. Once past the pitting and there was a nice shiny unbroken ring of metal around the seat, I used the good old fashioned hand lapping tool with a fine compound to finish off.


From this...


to this...


Now I have the technique sorted, I will move onto the 'good head' and should be able to sort out the pitted seats without spending hours and hours of twirling a wooden stick backwards and forwards by hand, which would have driven me insane.

Like many of the things I do on this blog, this is experimental. Will it be a success: only time will tell...





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2 comments:

  1. Great post, you could have bought a cutting tool which would have put the seats back to 46 degrees then lapped the valve in which would have given you a 45 degree seat and valve for a really good seal, ive don't it a few times using something like this
    http://www.ebay.co.uk/itm/like/161653724888?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=108&chn=ps&device=c&rlsatarget=&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=108&ff19=0

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