Joined: Nov 05, 2009 Posts: 174 Location: Florianopolis, Brazil
Posted: Fri Oct 29, 2021 4:35 pm Post subject: M38 L-head engine
Hello guys,
I have a 42 GPW Engine and the engine head is supposedly at its lifetime end.
This question maybe applies to the M38 engine head as well. If not, please excuse me.
My 42 GPW engine head has currently a thickness of 49,5mm . A couple of friends told me this has been shaved / plain-machined too much along the years, after many engine overhauls.
So questions:
1- what would be the limit of engine head thickness for the proper engine operation? Reduction of performance? Overheating?
2- what are the problems caused by the decrease of combustion chamber volume, resulted from this engine head thickness shortening?
3- have you experienced the use of a thicker engine head gasket? Is this available for sale in the USA?
4- has the mb/Gpw engine the same compression ratio of the m38 MC engine?
Help!!!!!!!
Thank you, all best!
Claudio
I am not aware of a life time limit on the L134 engine heads. There are limits for wear, damage and etc. Since you are talking MB/GPW then TM 9-1803A is your bible. For the M38 the TM 9-1804A is your bible. I have read them both front to back as I have also read SM-1002, SM-1046, Chilton 6817 (45-84 CJ), Moses Ludel's 46-71 CJ rebuilders manual, the 1969 Form IS-1009 Jeep 4 cyl industrial engine service manual & IPL, and the 1953 & 1961 Motor's Auto Repair manuals. Not in any of these manuals is a minimum cylinder head thickness listed. Nor is a maximum limit on material removal when shaving the heads listed. The only assumption to make here is that head shaving should be limited to a material removal amount that will not cause valve contact with the head and will not raise compressions so high they cannot be managed properly. It will be wise to check any piston showing higher than the top of the block and if they do protude above the top surface of the block then check their clearance with the shaved head also.
Quote:
1- what would be the limit of engine head thickness for the proper engine operation? Reduction of performance? Overheating?
As I said above without any published base figures it is impossible to do any accurate dimensional computations.
2- what are the problems caused by the decrease of combustion chamber volume, resulted from this engine head thickness shortening?
Higher operating temps and possible knocking
3- have you experienced the use of a thicker engine head gasket? Is this available for sale in the USA?
The use of double head gaskets or any other head gasket thickening measure is only a temp and unreliable band aid.
4- has the mb/Gpw engine the same compression ratio of the m38 MC engine? They are both originally 6:48 to 1
There is useful info in those posts however the bulk of the offered statistics are derived for putzing with used heads. The only true factory data I saw presented was in the 2A posting:
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F Bill
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Quote Reply Posted: 03 Feb. 2013 at 5:11am
Oooh I see a sleeper industrial head conversion in your future....How thick overall is the industrial head?
It is interesting that the shelf seen in my stock head is not visible in the industrial one.
Here's some head thickness specs from Willys Motors post a while ago. I will try and find the specs for the 640161 and Supersonic heads later.
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Quote Reply Posted: 02 Dec. 2010 at 9:16pm
800376 head thickness = 2-9/64 = 6.48 compression ratio
640035 head thickness = 2-7/64 = 7.0:1 compression ratio
The 800376 is a M38 head and the 640035 is a high altitude optional head.
Edited by F Bill - 03 Feb. 2013 at 5:21am
I am going to assume that Willys Motors AKA Keith Buckly offered info from his stash of factory blueprints for those two heads he mentions. But even with that data we need to see the blueprint to confirm exactly where on the head that measurement was illustrated. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
The G503 link to Jim Allen's post shares the best all around reply and I'll bet Jim got his measurements from Keith Buckley as well!
Quote:
Compression Ratio Calculations Post by ArmySailor » Wed Aug 13, 2008 11:03 am
I did this research for my "Classic Mode" column in the now defunct J-Rations Magazine. Rather than have it languish and never be seen, I though I'd post it into a place where it might do some good.
The high compression head for the flathead Willys (part # 640035) was a standard head with an extra 0.035-inch milled off. It was stamped “7.0” on the front of the thermostat housing and delivered a 7:1 compression ratio, the same as the Kaiser Supersonic engine, a rebadged Willys engine used in the Kaiser Henry-J. The Henry-J made 68 hp at sea level, five more ponies than the Willys. At 6K feet, 7:1 should gives back most of what you lose at that altitude. In milling the head, you increase the compression ratio by reducing combustion chamber volume. To get a 7:1 ratio, you want the volume of the dome in the head to be about 82 cc, as measured via “CC-ing” the head with the spark plug installed. The standard 6.48:1 head dome volume was about 90.5 cc. It’s important to know if your head has already been milled (for reasons I outline later) before you have it milled. You can tell by measuring the thickness of the head from the mating surface to one of the machined head stud bosses on the corner of the head. A standard head will be around 2.1406 inches (plus or minus a few thousanths). If you want to go for the 7:1, then you want the total thickness of the head to be around 2.1094 in., which should not be exceeded unless you know what you are doing. The thickness of the head deck and combustion chamber roof is about a quarter-inch, so you have a little room to play, but don’t be careless. Verify the chamber volume by CC’ing. There are some dangers in removing too much material, strictly from the performance level. When you skim material from the head, it moves the roof of the combustion chamber closer to the valves. Taken to excess, that can create a situation called valve shrouding, which reduces airflow in and out of the combustion chamber. In some cases, you can gain from the CR boost and then lose what you gained due to shrouding. From what I can glean from very old magazine articles and tech manuals, where I got much of the information for this answer, that point may come around 8:1 for the Willys. For an engine with a stock cam at sea level, 7-7.5:1 may be the practical limit. Also remember there are two compression ratios to consider, static and effective. Static is the often quoted number calculated on the total volume of the air in the cylinder with the piston at the bottom of stroke compressed into the much smaller combustion chamber. The effective ratio takes into account the closing point of the intake valve. The compression stroke doesn’t truly start until the intake valve is fully closed, which happens at 20 degrees After Top Dead Center on the Go Devil. The effective ratio is always lower than the static. The rough effective ratio of a stock Willys flathead with a 6.48:1 static ratio is 5.17:1. With a 7:1 static ratio, the effective ratio is 5.57:1. How much compression an engine will tolerate is highly variable. The old-time Jeep “Hotrodding” info isn’t of too much use today, because the fuels have changed so much. The only people I could speak with directly have only gone to a verified 7:1 at sea level. Finally, if you removed too much head material (about 0.090-in) the valves will hit the roof of the combustion chamber when they open.Last edited by ArmySailor on Wed Aug 13, 2008 6:10 pm, edited 2 times in total.
Jim Allen Keeping the Good 'Ol Days of Four Wheeling Alive
Joined: Sep 18, 2012 Posts: 592 Location: South East Saskatchewan
Posted: Sat Oct 30, 2021 9:07 am Post subject:
Depends on how brave he is, I have an old renault and the factory manual actually gives specs for letting out the chamber in the head if it has been milled past an acceptable point. Now obviously great care is needed here as you are thinning the roof of the cylinder so you would need an accurate measurement of the thickness there to be sure it won't break through under pressure or cause collision with the spark plug. _________________ Carievale Saskatchewan Canada
http://www.willysmjeeps.com/v2/modules.php?set_albumName=album210&op=modload&name=gallery&file=index&include=view_album.php
If Claudio's head were either of the two that Keith listed the factory spec for. Which his head is not. So that remains a guess until we hear from Keith of the factory new dimension of the head that Claudio has. This is where Claudio must provides us with the casting number off his head before we can even ask Keith for it's spec.
I have had a lot of flatheads ground to make them flat and remove pits. My rule is, if the valves don't hit it, I use it.
Not too scientific, but has always worked for me. Jeep and Dodge flathead. _________________ M38 23100 June 51
M38 34523 Sep 51
M606/CJ3B 1964 Kaiser 57348 118458
M37B1 1963
Willys MB 1944
Ford GPW 1942
Dodge WC63 1943
Dodge Power Wagon WM300 1967
Plymouth Barracuda, 1973, 340 4 barrel
All ground up restored by me.
Lots of others.
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