Valve lapping
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=209005
Printed Date: 25 Jan 2026 at 9:25am
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: Valve lapping
Posted By: 3rd generation
Subject: Valve lapping
Date Posted: 20 Nov 2025 at 8:25pm
| I’m sure this has been long discussed if lapping is needed. I just had hardened seats installed and valves ground. Personally I trust modern machining for a good seal. How many of you still lapped valves with hardened seats? |
Replies:
Posted By: steve(ill)
Date Posted: 20 Nov 2025 at 9:24pm
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always a good idea to get a finish lap.. if nothing else, to CONFIRM it is correct.. ------------- Like them all, but love the "B"s. |
Posted By: SteveM C/IL
Date Posted: 20 Nov 2025 at 10:22pm
| Can't hurt? When seats and valves are ground/cut correctly it's not needed. |
Posted By: DrAllis
Date Posted: 20 Nov 2025 at 11:35pm
| HELL NO !! Ask yourself this: do you think GM or FORD for example take the time to "lap" valves and seats after their modern and expensive valve grinding equipment has just mass produced hundreds of cylinder heads a day ?????? NOPE. |
Posted By: 3rd generation
Date Posted: 21 Nov 2025 at 4:52am
| Bluing would confirm a fit. I do think it’s a method appropriate many years ago and has stuck in the minds of some. My search has yet to convince me it’s needed. The hardened seats will do more for longevity than any lapping could provide. |
Posted By: Les Kerf
Date Posted: 21 Nov 2025 at 7:20am
When Grandson and I used my 100 year old Van Norman valve grinder and my Chinese-made seat cutters from Amazon to do a valve job on Mr Cato's Model C Allis-Chalmers head with the sloppy valve guides and cast seats, why yes, we most certainly felt it appropriate to hand lap the valves.  |
Posted By: AC7060IL
Date Posted: 21 Nov 2025 at 12:02pm
| You could easily have it vacuum tested to expose any possible valve/seat fit leaks. |
Posted By: steve(ill)
Date Posted: 21 Nov 2025 at 12:30pm
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lapping it for 30 seconds would give you an idea of the seat and verify that all is well.. Just a quick test. ------------- Like them all, but love the "B"s. |
Posted By: Les Kerf
Date Posted: 21 Nov 2025 at 2:36pm
Lapping valves after having been done correctly on modern equipment does no good and may reduce longevity by a small amount. I do not own such modern equipment  It would be difficult to prove the difference on a vintage Allis-Chalmers tractor.
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DrAllis wrote:Posted By: Sids
Date Posted: 21 Nov 2025 at 6:34pm
| I have always lapped valves. Why not? You can see the width and placement on the valve face and it shows on the seat as well. Just me I guess. Never had any problem. Now i wonder how many hundreds of valves and seats I have ground. |
Posted By: HudCo
Date Posted: 22 Nov 2025 at 2:31pm
| the onley time time to lap valve is to prolong the slow death of a wore out head if their is some reson to worry about it some blue ing will do even a sharpy magic marker will work if the guides were done incorrect or woren out lapping wont help |
Posted By: KJCHRIS
Date Posted: 22 Nov 2025 at 7:08pm
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I've always used Prussian bluing to double check on seats & valves, it just takes a few seconds per valve. ------------- AC 200, CAH, AC185D bareback, AC 180D bareback, D17 III, WF. D17 Blackbar grill, NF. D15 SFW. Case 1175 CAH, Bobcat 543B, |
Posted By: steve(ill)
Date Posted: 22 Nov 2025 at 7:41pm
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i dont think those that are saying "do a quick lap" are suggesting that it will FIX a BAD SEAT... A 30 second LAP would give you the same verification that you would get from Blueing or marking with a Marker.... Any will work.. All your trying to do is VERIFY the work was done right.. ------------- Like them all, but love the "B"s. |
Posted By: rw
Date Posted: 24 Nov 2025 at 11:00am
| I was taught not to lap valves with compound because compound introduces abrasives residue. I have been surprised at the number of folks that lap valves. We were also taught to put an interference angle of one degree. Not sure how prevalent that procedure is in the working world. I do remember checking with the prussian blue for the fine clean contact circle that provides the tight seal. |
Posted By: DrAllis
Date Posted: 24 Nov 2025 at 11:06am
| I was taught the same thing. Lapping compound wasn't getting cleaned off good enough. I always ground my seats 1 degree more than the valve (46 versus 45 for example) and once you were done, you could slap the valve on the seat 3 or 4 times and see whether you had a good seat or not. Bluing confirmed the width of the seat. |
Posted By: Les Kerf
Date Posted: 24 Nov 2025 at 12:19pm
Agreed  That truly is 'best practice' so long as you have good tooling and the skills to use it properly. I set my 100 year old valve grinder to 45 degrees (as best as I can eyeball it), and heaven only knows how close my chinese seat cutters are to 45 degrees. On my last job we measured stuff in microns, and I had access to optical measuring systems that I could have used to measure those angles in arc-seconds, but since retiring I no longer get to play with that equipment. These tractors were designed to be maintained by the average farmer with the help of the village blacksmith. If you feel the need to lap the valves, go for it! I challenge anyone to prove the difference after 1000 hours of run time
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