6.2 Precup Damage and Replacement

[alanjd93]

Hey everyone,

Forgive me if I'm not following forum protocol, this is my first post and I'm a little technologically impaired.

I had a couple questions regarding precups. I noticed there are a few posts in the forum about this topic already, but most of them are from quite a few years back and it seems harder and harder to find replacements these days. I'm in the process of rebuilding a fairly low-mileage (80,000) '83 MY 6.2. Upon teardown I discovered in a couple cylinders it seems as though something broke off and went for a ride. The tops of two of the pistons were a bit hammered, nothing catastrophic, but enough to warrant a new set of pistons. The cylinder walls also showed some vertical scoring that required boring out to 0.030 over. The corresponding combustion chambers in the head showed similar signs of wear as pictured. The heads themselves are in good condition (crack free as far as I can tell, and flatness checks out within spec), but the two precups are pretty beaten up. I was wondering what would cause damage like this - Possibly part of a glow plug breaking off? I just want to make sure I address it at the root cause before it all goes back together. My other question was if the damage pictured warrants precup replacement? I have read in the manual that any cracks over 5mm require replacement. Most of the precups are cracked, about 6 out of 8 of them, and though some extend just over 5mm, none are too close to the head gasket sealing bead. From my research on the forum this is pretty commonplace, and I would probably feel comfortable running them as is, if it weren't for the other damage pictured. My last question, if precup replacement IS recommended, is where to source these damn things? I have heard conflicting reports on the quality of eBay precups, specifically imported parts. There's one eBay seller that seems legit as far as I can tell, but I really have to just take his word on it. They are listed as 6.5 precups, but I believe I read somewhere that they are all interchangeable but the size of the chamber on the diamond cups is larger and may reduce fuel economy. I will add that I will be installing a Banks turbo kit to the engine, and if the larger cups will be beneficial to performance or longevity, I'm not opposed to a slight drop in MPG. I've also heard of another user getting some through PTC Power. This site has the correct part number listed for the stock 6.2 3 notch cups. Unfortunately I was unable to get ahold of them and inquire about prices or part sourcing. I will post both links below. Does anyone have any experience with either of these sellers? This is my first diesel rebuild and though I feel fairly comfortable, I'm still in somewhat uncharted territory. I'm in the mindset of replacing anything within reason while I'm in it just FYI. This forum has been my best friend throughout this process, but until now I've just been a lurker. Any help would be greatly appreciated. Thank you in advance! :)

eBay Link
https://www.ebay.com/itm/6-5L-6-5-Diesel-Diamond-Pre-Cups-Precups-Pre-Combustion-Chamber-Cups-/163097660487

PTC Power
http://www.ptcpower.com/inventory/inventory_gmc.aspx

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[More Power]

I have read in the manual that any cracks over 5mm require replacement.

Welcome to our bulletin board!

Actually... the manual says that as long as the crack doesn't cross the sealing ring of the head gasket, the thermally cracked cup can be re-used. Many of the stainless-steel cups in use today by 6.2/6.5 owners around the world likely have cracks. It's not that big a deal, really.
Jim

[alanjd93]

Thank you! Happy to be finally part of the forum.
I definitely feel comfortable reusing the cups despite the cracks. I'm not sure if the pics I took do it justice, but what really concerns me is the damage from whatever foreign contaminant was in the cylinder. The first two pics show the worst of the set - not sure if I'm just overreacting, but it looks like the rounded edges of the fire slots are dented in a bit. Would this cause any driveability concerns or affect the integrity of the cups long term?

Thanks,

Alan

[More Power]

With a die grinder or Dremel tool you could dress the nicks and dings in the port. Smoothing the flow of gases should reduce the potential for a hotspot.

A loose broken-off glow plug tip can do a lot of damage. I spoke to an Army mechanic once who told me he saw other mechanics just break off the tip if one of them became swollen and wouldn't come out. I shudder....

[alanjd93]

Thanks for the advice, I think I'll do just that!
It's amazing what some people will do to cut corners... scary stuff. Who knows, could have been done to mine.

[Robyn]

I agree with Jim on the Issue.
Broken glow plugs.
Use only 60G plugs ....you may need to glow a tad longer in cold weather....
THE EARLY plugs were prone to swelling.....

Yes...lightly touch up the dented in areas in the precup port mouth.

Sounds like you're on the right track.

Fresh pistons/rings....Fresh bearings

FELPRO HEAD GASKETS AND NEW BOLTS (Bolts are a one time use....follow installation instructions with the bolts)
Clean threaded head bolt holes well ...wash with BRAKLEEN and blow dry just before installing the heads/bolts.....Install the bolts DRY into the holes....they have a dry sealer on them.

Use only hi temp silicone goop on the valve covers.
The pan was originally done with the goop in a tube as well.

I have had good success with the gaskets on the pan and timing cover....

Just an FYI
Swollen plugs can be removed easily.
Remove the injector....Scare up a pice of heavy cloth/flanel and cut a piece that can be stuffed into the precup (Through injector hole) carefully cover the port from ther inside.
Break off the plug and tap the broken tip back into the cup..fish out with a magnet or a forceps type tool. BE SURE no small pieces of the broken plug are still in the cup....Remove the cloth...Reinstall the injector and a fresh plug.....GOOD TO GO

Good luck
Glad to have ya here at TDP

[alanjd93]

Thank you, I'll definitely pick up a set of 60g plugs before it goes back together! Hopefully I won't have to use your removal tips, but I'm glad to have it just in case - especially if it saves me from pulling a head (and my hair out) down the line.
I also picked up a set of ARP head studs a while back after doing some research on the forum. Lots of useful info here for sure.
I managed to dress the precups with a dremel today as suggested. Just have to check the bearing clearances and a few other odds and ends and the heads should be ready to go back on.
Thanks again for both of your help, definitely glad I reached out!

[Robyn]

In my opinion the best way to go on the 6.2/6.5 is the standard TTY bolts by Felpro.

These are TORQUE TO YIELD type...The are tightened up in several stages and then 1/4 turn on the final stage.

The bolt is then into the STRETCHY PHASE and acts to keep a constant clamping force on the heads/gaskets.

I have used a few different setups on these engines and have always come back to the factory type equipment. It works very well.....and lasts a long time....


Good luck

[alanjd93]

So upon further inspection it seems like one of the exhaust valves ate some glow plug debris as well... The head and the margin took most of it, but there are some concerning indentations in the face as well. Luckily the seat seems to have escaped any visual damage. I checked the valve stem to guide clearance with a dial indicator and they're all within spec. I would like to avoid taking it to the machine shop if possible, but wouldn't be opposed to it if you think it would be necessary - I want to do this right. Would I be able to get away with just replacing the valve in question and checking the seal with some lapping compound? If so, could you recommend any particular manufacturer to source new valves? Here's hoping this is the last surprise I get during this build.
Thank you in advance!
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[alanjd93]

Also, to your thoughts on the head studs - I did a little more research and I am considering getting a set of the Fel Pro TTY bolts as they are having a closeout sale at RockAuto at $10 a set. It seems like head studs are a good investment if you are trying to push max power out of these blocks, and I'm sure they work well if you follow the directions to a tee - just seems like the setup can be very finicky and I know it would be a PITA to pull the heads once it's in the vehicle if anything did happen. Personally, I'm not trying to hot rod this thing, I have a banks sidewinder that will be going on, but not planning on running any higher boost than specified. Seems like a good number of users have experienced coolant leaks among other issues. Knowing me, I would be paranoid I didn't set them up right and that would be in the back of my mind for a good long while after the rebuild. Thinkin about just keeping it simple and using the bolts that were intended with the Fel Pro gaskets I have. I can always sell the studs and make some of the money back to put towards other upgrades that are reasonable for my build.

[Robyn]

Lapping in a fresh valve can be done.....I have done it....

Just be sure that you use a thin film of Prussian blue fitting compound (Permatex makes it)
Set the valve gently in the seat and tap it lightly.

Lift the valve and make sure the contact is good all the way around the seat and the valve face.

Lapping all the valves would be OK
New seals on the guides..
Just the factory type will do.

The TTY bolts will serve you well.

These engines live the longest at boost levels in the 10-12 psi range
EXHAUST GAS TEMPS IN THE 1000 F AT THE PORT MAXIMUM AT HARD THROTTLE PULL.

The old glow plug through the "Chinery" trick is tough on things....
Be sure you get the injectors gone through and all set to the same POP pressure.

Keeping the pop even is paramount to get a smooth running engine.
Stick with stock pressures......do not be suckered into HIGH POPS.
Tougher on the IP and unnecessary..

Also a nice clean CONE of spray when the injector pops.

Slobbery injectors waste fuel and create high exhaust temps.

[alanjd93]

Thanks for the tips once again! That was more than I could have asked for. Got a new valve coming in and will definitely check out the seal with the Prussian blue after lapping. Noticed they don't have any positive type seals on either intake or exhaust - just the stem O-rings, which I will definitely be replacing. Would there be any benefit to positive seals on the exhaust side? I have a set of Mahle viton seals in the basement.

As for the injectors - I'm not sure of the condition or age of the set from this engine, but I have a couple sets of injectors in the basement - one of which came off of an engine that the PO claimed to have just replaced along with the IP. I have my eyes on a Bosch injector tester and have considered checking the pop pressures and spray patterns of the ones I have and choosing the best set out of the lot. I'm not opposed to taking them in to a shop either, I'll have to price out some local rebuilders. I would attempt to rebuild myself, but it seems like RockAuto doesn't sell the Bosch nozzles anymore... it's a shame, I had some in my cart earlier this year and didn't pull the trigger. Oh well.

[Robyn]

The factory seals have always been quite satisfactory on these engines.

There isn't any manifold vacuum on these engines....so the oil will not get sucked down the intake guides.

The O'ring seems to be quite fine

[alanjd93]

Good to know. Should have the new valve in by the end of the week, hopefully will get it back together soon... Thanks again for all your help!

[alanjd93]

Well I'm glad I took the time to properly check the valves... I tested the contact area of the two in question and wouldn't ya know! The compound only made a half moon shape on the valve face... They didn't look bent on initial inspection, but I chucked one up in a drill and spun it. Sure enough, I could see the head wobble ever so slightly. Luckily the seats were still in good shape and I was able to get a nice uniform contact area on the two replacements! I'll check the seal with some mineral spirits this weekend and if all looks good the heads are finally going back on!

[ronniejoe]

General Engine Products builds the new 6.5 Diesel heads with positive seals on the exhaust valves and with o-rings on both. I just picked up two brand new sets from the GEP factory in Franklin, OH today.

For the record, TTY bolts in this application are a compromise to get higher initial clamp load out of a lower strength fastener. If you have ever looked at a stress-strain diagram for a ductile material, you will understand why it was done. I'm not a fan of Wikipedia, but they have a good example of a stress-strain diagram for a ductile material like steel here: https://en.wikipedia.org/wiki/Stress%E2%80%93strain_curve

Stress is the force per unit area, so for a bolt, it is the clamp load divided by the cross sectional area of the shank (or minor diameter of the thread if it is smaller). Strain is the change in length per unit length when under load. When you apply a load (force) to a ductile material, it will initially stretch so that its strain is directly linearly proportional to the stress applied. The slope is known as Young's Modulus, or the modulus of elasticity, of the material. This is true up to a stress-strain level know as the yield point. Below the yield point, the material is elastic so that any strain induced by the applied load will relax when the load is removed and the material will return to its original shape (or length). Once yield occurs, plastic deformation is happening and the material will never return to its original shape (or length) after the load is removed. Between the yield strength and the ultimate strength of the material, as the diagram shows, the material can take higher loads (with higher stress) and plastic deformation continues. This means that in a TTY bolt, it is intentionally stretched beyond yield to gain a higher clamp load than can be obtained in the elastic region. The problem is, if anything causes any further strain to occur (such as thermal excursions, excessive peak cylinder pressure from advanced timing, or anything else that could momentarily increase the load in the fastener) it will cause a loss of clamp after the load causing the strain is removed. Normal engineering practice is to design for stresses below the yield point, in fact, it is usually three standard deviations below yield. The use of TTY bolts is actually a cheap-out crutch that GM used to cheat and increase clamp load without paying for a better fastener material. It's a shortcut, not a beneficial feature.

The head studs are made from higher strength material (SAE 8740 processed to provide 180,000 psi ultimate tensile strength). They are never torqued above yield, so they are reusable and they won't lose their clamp if some load excursion occurs. The use of proper sealer on the stud end threads prevents coolant leakage.

After all the 6.2/6.5 engines that I have worked on over the last 30 years, there are two things that I insist a new overhaul gets: splayed main caps and head studs -- no matter whether you are talking about a stock rebuild or performance build. I know. That is not a popular opinion here. However, I have two examples of naturally aspirated 6.2s in my shop right now, both VIN C out of half ton trucks, that have all three main webs cracked. Obviously, these were not hot rods or used for heavy towing and you can't exactly say that a VIN C 6.2 was a powerhouse. One has a 1982 casting date code... I used to think the web cracking disease only affected the 6.5. Not anymore. Here is a photo of a 6.2 from a 1987 R1500 pickup. You can see two of the cracked webs. All three center webs were cracked like this -- on both sides.

6020

I am building an NA 6.5 to replace this engine. It is getting splayed man caps and head studs. Oh, and I use Cometic Multi-Layer Steel head gaskets. I have experienced several failures of the FelPro gaskets. Again, not a popular opinion here. You can get by with the TTY bolts. That's what they were built with from the factory. However, I prefer to build margin into my engines and not rely on gimmicks or shortcuts, especially on something as critical as head fasteners.

Your results may vary. Others will likely say that I don't know what I am talking about. That's OK. I just build these and make them run. I do things with them that certain others say cannot be done. Occasionally, I sit down at the keyboard to share my experience with others. I don't live on the forum.

[alanjd93]

Thanks for the input, I'm open to hearing differences in opinion. I'm here to learn, as I have very little experience and trial and error time into these engines. I have looked into splayed mains before, I know that Unique Diesel (Peninsular?) sells the caps. Just not exactly in my budget right now. Is this something that could be done by most machine shops or is it a more specialized procedure? The block I currently have is an '82 660 casting. It is relatively low mileage with no main cracks, so I put a light chamfer on the bolt holes and cleaned up the sharp edges in the bottom end. I will also be running a main girdle and fluidampr. Down the line I'd love to find a one piece rear block and build it up to be bullet proof - splayed mains, forged crank, etc... but right now it's not exactly in the cards. Still kind of on the fence with using the studs. My head and block have not been prepped with the correct surface finish to run an MLS gasket, and I already have the rotating assembly together. I know the Fel Pro gaskets I have are meant to be used with TTY bolts, so not sure if I want to use them with the studs. My main concern is coolant leaks though the bolt holes in the deck. Have you experienced this? What type of sealant do you use on the threads, high temp PTFE?