Posted: Fri Oct 25, 2019 7:29 am Post subject: Piston overbore sizes?
Hi all,
I've just got my block back from the machine shop.
The bores are now 3.128 I think the original bore size is 3.125
Please help this befuddled metric mind.
Which size overbore pistons should I be buying 3 thou over?
Ta, Dave.
(You can avoid this repeated question by adding the year and model jeep you have to your posting signature in your profile! I dare not make an answer based on a user ID that includes the M201/M38 models.)
I do not have any manuals for the Europe produced M201. If you are talking M38 then the limitations shown in TM 9-1804A page 123 & 124 and the factory OEM oversizes available are in ORD 9 SNL G740 (M38 IPL) pages 32 thru 38. Your smallest oversize is 0.020. So if your piston clearance exceeds the limits in TM 9-1804A then you must rebore the block to the next oversize. Your current assumed .003 increase in bore is checked relative to a piston by installing the piston in the bore and measuring skirt clearance which has a max limit of 0.0029 and a final wear limit of 0.0005.
I find it interesting that we Americans have been forced to convert to and learn the metric system for the last 25 years and now we can observe the originators of the metric system struggling with our inch system!!! _________________ Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
Hi Wes,
I've found my self in a 'cart before the horse' situation it's 1951 M38 block.
It popped a valve seat, taking the head off I found a crack so it had to go in to get the deck stictched.
They reground the deack and did the head at the same time.
They honed the cylinders too.
I was going to put new rings in, one ring fell to bits when the pistons came out.
She wasn't burning oil but I thought this is a good chance to fit new pistons same size as the old ones.
The old pistons don't have any clues to the fitted size.
It's raining cats and dogs here so the block is still sitting in the back of the car!
Cheers, Dave.
(assuming L-134, and that you don't have a letter in your ESN...)
[ETA: sorry, was typing when you posted]
TM 9 1804A page 121 re: the block itself says 3.125-3.127 bores when new, rebore to next oversize at 0.005 (so at 3.13-3.132)... It sounds like your machine shop found it STD and ran a hone through it, so you're looking at STD pistons.
I have to believe you could run STD rings at 3.128, what I don't know is if you could run them out to 3.132 without considering STD pistons with oversize rings.
Either way you should be checking skirts and end gaps... Do you have the pistons that came out?
ETA: Standard pistons sometimes have "STD" right on top, but can have no markings as well... anything oversize should conspicuously say so right on top. If you kept track of which piston came from which hole, I'd just re-ring it and put it back like she was (the skirts should be fine, but check ring end gaps )
Yes Wes, everything is sitting there, numbered and sorted. When it is light and dry again I’ll take some measurements. Re ringing looks like the plan given the annual mileage.
All the best,
Dave.
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