Author Topic: Valve Lapping Question.  (Read 867 times)

Offline rick41

  • Full Member
  • ****
  • Posts: 33
Valve Lapping Question.
« on: October 16, 2021, 03:57:27 PM »
I am building small Engine (Upshur).  I am using Timesaver lapping compound.  The valve head is steel and the valve seat is brass.  Which type of compound do I use?  Green for hard metals or yellow for soft metals or alternate using both?  Thoughts? Rick

Offline rklopp

  • Full Member
  • ****
  • Posts: 57
Re: Valve Lapping Question.
« Reply #1 on: October 16, 2021, 04:34:33 PM »
Is it confirmed that you need to lap the valves? I built six modified Upshur Twin engines and did not lap the valves. Compression is excellent and the engines run well. I just made sure to have good surface finish on the valves and seats, and good concentricity between all surfaces:  the valve stems and its sealing surfaces and the valve guides and seats. I left perhaps 0.001" or so clearance between the valve stems and guides to accommodate any residual eccentricities. I just did the same exercise on a 1/2-scale Olds hit-n-miss, and it has superb compression.

Offline stevehuckss396

  • Full Member
  • ****
  • Posts: 1503
  • Sterling Heights, MI USA
    • Steve's Miniature Sparkplugs
Re: Valve Lapping Question.
« Reply #2 on: October 16, 2021, 07:29:11 PM »
I'm with rklopp! Don't lapp if you don't need to. Machine with a sharp tool and get that bright finish. Lapping compound will destroy your seats. If you need to do something use 2 or 3000 grit or car polish (not wax) so you get a good finish. Not fine enough grit and the brass will scratch and make things worse.
Do not be like the cat who wanted a fish but was afraid to get his paws wet.

Offline rick41

  • Full Member
  • ****
  • Posts: 33
Re: Valve Lapping Question.
« Reply #3 on: October 17, 2021, 05:23:24 PM »
Thanks for the responses to my question.  I am a bits surprised/confused as when following build threads, most of them seem to talk about lapping valve seats.  Guess I will have to work harder on the machining of the valves and seats.

Offline Craig DeShong

  • Full Member
  • ****
  • Posts: 1245
  • Raleigh, NC. USA
Re: Valve Lapping Question.
« Reply #4 on: October 17, 2021, 06:52:11 PM »
Your problem is that you have a fairly hard material with the valves (steel) and a relatively soft material with the seats (brass).  Lapping will not seat the valves as only the brass will be affected by the lapping.
Craig
The destination motivates us toward excellence, the journey entertains us, and along the way we meet so many interesting people.

Offline Laurentic

  • Full Member
  • ****
  • Posts: 204
  • Nr Yeovil, Somerset, England
Re: Valve Lapping Question.
« Reply #5 on: October 17, 2021, 07:13:47 PM »
Actually Craig, the opposite may occur.  Generally speaking, the softer material holds the abrasive grinding material and wears away the harder material.  That is why, for example, when lapping very hard fuel valve seats on big sea going diesels the lapping blocks were Cast Iron, but they did need refacing from time to time!

Otherwise, I agree with replys #1 & 2.

Chris

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 6975
  • Barrie, Ontario Canada
Re: Valve Lapping Question.
« Reply #6 on: October 17, 2021, 09:11:52 PM »
If you made ANY kind of i.c. engine without lapping the valves, and it ran---You're a better man than I Gunga din!!!! I have always lapped the valves and seats on engines I have built. Sometimes two or three times before I got them to seal well enough to run. That could be because the very first i.c. engine I built many years ago had instructions on "lapping the valves" and I've never seen reason to do it differently. Valve sealing is still the biggest issue I have with getting my engines to run properly. I do know that too much lapping will end up making the seat too large and become impossible to make the valves seat properly---I just experienced that on my latest engine build, the "T-head engine". I ended up having to remove the valve cages and replace them with new ones before I got the engine to run at a slow idle.---Brian Rupnow

Offline crueby

  • Full Member
  • *****
  • Posts: 14474
  • Rochester NY
Re: Valve Lapping Question.
« Reply #7 on: October 17, 2021, 10:08:58 PM »
I do not totally agree with the statement that the grit will stick in the softer metal and only abrade the harder one. True with some compounds, especially diamond paste, but not as true with ones like Timesavers yellow.


I just the other day lapped in the stainless steel valve rods into the bronze tubes in my engine. I did all four tubes with one steel rod, slightly tapered at the end to aid in starting and getting compound in, using the very fine grit yellow Timesavers. Most of the change was in the bronze tubes, a little bit in the steel rod. When I started, the rest of the rods would barely start into the tubes. After lapping they all fit nicely. Total change was less than a thou, mostly in the bronze.
In the past I have also used the green compound, intended for harder metals. It acted more like you described where it abraded the harder metal more.
Just my experience, hope it helps.


Chris

Offline Laurentic

  • Full Member
  • ****
  • Posts: 204
  • Nr Yeovil, Somerset, England
Re: Valve Lapping Question.
« Reply #8 on: October 17, 2021, 11:03:27 PM »
Hi Chris - I have no experience with Timesaver of any colour, in that I bow to your superior knowledge.  All my grinding/lapping operations have been with carborundum abrasives, and with those, they very definitely do embed themselves into the softer material to abrade the harder material.

From there came the well known engineering latin maxim "non-illegitimus carborundum" commonly quoted by my generational colleages, which roughly translates as "don't let the b******s grind you down"

:cheers:

Chris

Offline crueby

  • Full Member
  • *****
  • Posts: 14474
  • Rochester NY
Re: Valve Lapping Question.
« Reply #9 on: October 17, 2021, 11:27:14 PM »
Hi Chris - I have no experience with Timesaver of any colour, in that I bow to your superior knowledge.  All my grinding/lapping operations have been with carborundum abrasives, and with those, they very definitely do embed themselves into the softer material to abrade the harder material.

From there came the well known engineering latin maxim "non-illegitimus carborundum" commonly quoted by my generational colleages, which roughly translates as "don't let the b******s grind you down"

 :cheers:

Chris
:ThumbsUp:


The timesaver green version seems to be more like the usual abrasive grit, while the yellow acts different. It breaks down during use to finer particles, then blends into the oil and washes out. Not leaving active grit behind is a plus since it stops cutting. They sell trial kits, little tins of each grit. For a model engine maker, one trial kit is a lifetime supply.

Offline rick41

  • Full Member
  • ****
  • Posts: 33
Re: Valve Lapping Question.
« Reply #10 on: October 19, 2021, 04:12:46 PM »
Thanks to all who commented on my post.  Informative, but did not answer my original question. My question has been answered by accident on another board in a discussion about lapping bushings..  The comment was according to Timesaver, one should never use Green (for hard metals) compound on bronze or brass, so my idea of alternating (mixing) between green for the steel valve and yellow for the brass seat is a no-no.  I plan to use only yellow to lap my valves.  Rick