Posted: Sun Apr 12, 2020 9:10 am Post subject: Head and block decking/skimming
Hi. I have three L134 heads, one is a industrial from an RMC generator engine, one i believe is a CJ2 from a J prefix engine and one is an MC head. The industrial has a clearance from the squish to the gasket face of about 0.085”, the CJ is about 0.065” but the MC is flush. The squish and gasket face are level. Is this normal or has the head been decked/skimmed? I ask because I think I may need my block decked. The clearance from piston crown to gasket face is about 0.020”. The uncompressed head gasket is about 0.075” thick. So, a couple of questions: 1 - what is the minimum safe clearance from piston crown to head squish area and 2 - if the block was decked so the piston crown was flush what approximate compression ratio would that give? The bores have just been machined 0.030” oversized. _________________ Gary
M38 - June 1952
All the published clearance data that is available is published in the appropriate service manual for each engine. The important parts of all these manuals are here on this web site for you to use and download. Even if you lay all the manuals that cover your 3 heads in front of you, you will still be missing some very important clearance figures and original new measurements. The bottom line here is to place the machine work in the hands of a qualified and EXPERIENCED automotive machinist. For several critical dimensions there are no factory specs listed and the machinist will simply apply the industry standards to the measured clearances they find. These judgement calls are best left to the guys with a ton of experience who are very well versed in the art of measuring clearances and the industrial standards.
SHAVING HEADS relies a great deal on knowing how much meat is allowed to be removed. Problem here is Willys never shared the original thickness of each of their heads with us. Too much shaving leads to piston contact and spark plug to valve contact.
DECKING THE BLOCK has all the same issues with the lack of published original dimensions and the additional pain of valve seat and stem depths and lengths.
There are a ton of references out there on the web for these engines including manuals and service bulletins/letters. Try and find as many as you can and read them closely.
Our manuals downloads page is a good place to start as is my photo album and it's many detailed sub-albums:
Wes - I checked every one of those links and not one tells me what I need to know. I know it’s hard for you to just give a straight answer but please help me here.
1 - if my head squish area is flush with the gasket surface has it been skimmed?
2 - in your very extensive experience what is the factory clearance from squish area to gasket surface?
3 - again based on your experience what is the ‘accepted’ minimum clearance from piston to head squish area?
I don’t want to deck the block to the limit, I want to deck it the minimum we can but I do need to know whet3 the limit is. _________________ Gary
M38 - June 1952
4X4M38. I do have the engine serial number and will have it restampd if I have the block decked. RMC106676 just in case I lose all my data! _________________ Gary
M38 - June 1952
Wes - I checked every one of those links and not one tells me what I need to know. I know it’s hard for you to just give a straight answer but please help me here.
Gary,
That was the point I am trying to make. None tell you the answer to your question. I started asking that question in 1970 with my first jeep a GPW. Got the same answer you just did. I haven't found a good answer since then either. _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Thx Wes. Let me reword just one question. I’m asking for your experience not hard factory data. So, in your experience have you ever seen am MC type head that you knew to be in original form where the squish area was absolutely flush with the gasket face? _________________ Gary
M38 - June 1952
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