Piston ring job

[drowe44]

Ok, after replacing the bad valve I did a wet dry compression test. results were:
cyl 1 2 3 4 5 6
dry 61 62 54 60 72 28
wet 90 85 72 89 92 50.

Pretty clear indication the rings need to be replaced. I have a couple of questions for the more experience M37 mechanics among us.
1. This engine was supposedly rebuilt 15 years ago. With less than 10k miles on it, should the rings be this worn out?
2. VPW has new rings that come in a variety of sizes (std, .010. .020, .030, .040). How should I decide on the correct ring size?
3. Same question for the connecting rod bearings.

Thanks

[rickf]

There is no way it should have worn out that quickly, either it was not rebuilt or it was a very shoddy job. Either way I would tear down the entire engine for a complete check.

The first thing you will need to do is to tear it down and see what size pistons are in it. Then you will need to measure the bore and see if it was bored correctly to fit the pistons. Also assess the cylinders to see if there is any crosshatching left on the cylinder walls. This is all needed info for rings. If the cylinders are worn beyond tolerance it will need to be bored for oversize pistons and rings. You can't just put bigger rings in. Same with the bearings, you need to find out if the crank was ever cut and see what size bearings are in it now. The first thing I would do is get some plastiguage and check the clearance of the bearings as they sit. You may be able to just replace them with what was there.
And that number 6 reading is so low it is probably more than worn rings.

[drowe44]

Is there a table in one of the TMs with the spec for piston/ring measurements? How about for the bearings? Or is it just general mechanic knowledge?

[just me]

Do yourself a favor. Use some Seafoam and see if you can't free up the rings. If the valves were problematic, the rings could be gummed up too.
Not to sound bitchy, but if you are asking these questions about rings, you have no business inside the engine. Find someone in your area who knows engines and get him/her to mentor you on the basics of engine work. Marginal or incorrect work costs hundreds of dollars for no good results and can completely ruin what were usable parts.
(Nothing makes me cringe more than people trying to start engines that have been sitting for years without opening them up and inspecting first. So many broken rings, pistons, valve trains and ruined bores from it. I've even seen broken camshafts from the practice.

[Kaegi]

I would soak them in AT fluid, marvel mystery oil or seafoam for a couple weeks rotating by hand a few times occasionally add a little more every 3 or 4 days. I agree with just me they might just be stuck from sitting. if an engine has been bored to oversize pistons 90% of the time they are stamped on top with the new size increase eg .010, .020, .030 etc. Same thing on rod bearings if crank has been machined undersized the back of the bearing shells will have .010 etc stamped in them

[rickf]

Do yourself a favor. Use some Seafoam and see if you can't free up the rings. If the valves were problematic, the rings could be gummed up too.
Not to sound bitchy, but if you are asking these questions about rings, you have no business inside the engine. Find someone in your area who knows engines and get him/her to mentor you on the basics of engine work. Marginal or incorrect work costs hundreds of dollars for no good results and can completely ruin what were usable parts.
(Nothing makes me cringe more than people trying to start engines that have been sitting for years without opening them up and inspecting first. So many broken rings, pistons, valve trains and ruined bores from it. I've even seen broken camshafts from the practice.

And to add to what Just Me has said, and I am not trying to be condescending but if you need to ask these questions you probably do not have all of the precision measuring tools you will need to find the information you will need to get the right parts. You are going to need a full set of standard micrometers, internal bore micrometers, precision straight edge and feeler gauges. And this is just to get the primary measurements. You can replace the rings and bearings based on the sizes marked on the pistons and old bearings but do you know they were right to begin with? Something made that engine wear out in a big hurry. There are tables with all of the information you will need but you have to have a pretty healthy dose of real world experience to interpret all of what you will see once inside the engine. Sizes are one thing but what you actually see on the bearing wear and crankshaft surface tell a big part of the story. If you have an old time mechanic that is willing to stop by and look over your shoulder once in a while and give advice then this is the perfect way to learn how to do this job. Just be aware that the tools involved are expensive, this is not the place for Harbor Freight specials.

[Elwood]

Is there a table in one of the TMs with the spec for piston/ring measurements? How about for the bearings? Or is it just general mechanic knowledge?

If you don't have the basic complement of TM manuals for your truck, you need to start with them. Try this topic: http://www.g741.org/PHPBB/viewtopic.php ... als#p69041

TM 9-1840A will have many of the specs for rebuilding your T245 engine, including piston ring gab and clearance in the piston grooves. However, that manual is a basic rebuild, and a good rebuild will include more work and measurements that what is in that book.

I'd also suggest a general engine rebuilding book, since you're asking about some general knowledge. Something like this: https://www.amazon.com/Engine-Builders- ... 1557882452. Even if you don't do the work yourself (and unless you're going to acquire a shop full of specialized tools, you're likely to be taking your engine to a professional for some or all of the work - so an understanding of what questions to ask, what work should and must be done and to what tolerances is essential).

Don't be afraid to read up as much as you can, and then get started yourself. If you follow the manuals, you can generally disassemble the engine without damaging anything (take lots of photos, make notes, and tag and bag everything separately). You can leave the machine work and maybe the reassembly to a competent professional, but we all had to start somewhere on engine work, so don't worry about your initial lack of knowledge.

As has been posted, you may have stuck rings and/or valves. You did a compression test, but did you also do a leak down test? Or even just remove the valve covers and see if any of the valves are visibly stuck? As this is a flathead, you can have a stuck open valve without piston interference.

[drowe44]

I've rebuilt 3 engines in my life. The first was my 70 Landcruiser 35 years ago. Still driving it on that rebuild. Did a 4 cylinder Toyota in 90 and a Saturn in 2008. They are all still running strong. I just need to refresh my knowledge and get some specifics on the M-37 engine. First thing I looked was the valves. They are all OK. I'll get the pistons out soon and we'll see what that shows. Since the engine was professionally rebuiltI'm hoping for stuck less than 5k miles ago I'm hoping for stuck rings and not a complete rebuild.

[hbb]

Do yourself a favor. Use some Seafoam and see if you can't free up the rings. If the valves were problematic, the rings could be gummed up too.
Not to sound bitchy, but if you are asking these questions about rings, you have no business inside the engine. Find someone in your area who knows engines and get him/her to mentor you on the basics of engine work. Marginal or incorrect work costs hundreds of dollars for no good results and can completely ruin what were usable parts.
(Nothing makes me cringe more than people trying to start engines that have been sitting for years without opening them up and inspecting first. So many broken rings, pistons, valve trains and ruined bores from it. I've even seen broken camshafts from the practice.

Not trying to stir the pot, but isn't this form designed to help the The new G 741 owners , when someone is asking a question none of us should assume anything about that individual as99.9% of the the people that own a M series vehicle had not much knowledge of them when they were first introduced to them. GIs that worked on these vehicles in the field and motor pools pretty much knew nothing about them either. They all had instructors and they all asked questions, true they all had TMs to follow. When I first got involved with a M37 I was not aware of the TMs available, slowly as time went on I had become introduced to those publications. Being a heavy line technician in GM dealerships for over 30 years I continually learn something about these vehicles. I agree they are very primitive in the way they were constructed but they have their own personalities and idiosyncrasies. So try not to embarrass yourself by appearing to know everything about everything because you do not! Asking a question doesn't prove ignorance.
Just an opinion!

[Cal_Gary]

Hello All, monitoring the thread for a while now and feel that some friendly reminders are in order.. Numerous valid points have been made throughout, and although I err on the side of "assume no malice" when reviewing commentary, our forum exists to be supportive of all who find their way here-everyone is welcome, and the last thing we want to do is drive away fellow enthusiasts. HB makes a valid point that nobody knows everything about the intricacies of these vehicles, even if you worked the line on Mound Road when they were built. In today's politically correct world, commentary and guidance considered "straight and to the point" 30 years ago is not often received the same way today so we should be sensitive to the times we live in, and be thankful for those less experienced who are trying to learn (just like the rest of us in the near or distant past). I can speak for everyone here who has over-stretched their mechanical capabilities at some point-it is a learning experience, and that knowledge can and should be shared with others in a positive way without an air of negativity.

In closing, at some point I'll need to do the job that DROWE is contemplating, and hope that everyone of you who posted here will be willing to share your expertise with me when I reach out!
Thanks!
Gary

[sturmtyger380]

YouTube can teach you a lot: https://www.youtube.com/watch?v=qLzMQtJAljg