Was wondering if anyone with experience working on the Cummins has any advice. I have my engine apart (I first was just going to pull ithe motor out of the engine bay to replace the leaking oil pan gasket and front and rear seals, then decided I might as well throw in new rings and bearings as long as its out, but now...) and have measured the bores. At top dead center they measure about 1 to 2 thousandths more than spec. I think the top limit is 4.0203, and mine are measuring about 4.0215 to 4.0225. It's just in a very small band at TDC, about 1/4 inch, that they're worn that badly. And its really only one bore (#5 I think) that was 4.0225, with the other five closer to 4.0215. Other than the band at TDC, the remainder of the bores are right in the middle of the spec. Also, the spec for out of round is 0.0015, and I'm measuring about 0.0020 at the top of the bore. So the question is, how necessary is it to rebore under these circumstances? I called CUmmins this morning about the cost of a new set of oversized pistons, but there computer was down. I'm not sure I want to know how much a new set of pistons would cost. Before pulling things apart, it burned a bit of oil but nothing unreasonable. And with the injection timing advanced pretty far, I was getting about 22-24 mpg under light loads. any thoughts would be appreciated.

(btw, it's a 93 D250 5 speed. I'm also putting all new bearings in the Getrag at the same time.)

 

Welcome, I'm not a machinest but I would not be too concerned. how many miles if I may ask?

Fill out your signiture so we know what you have everytime

Be patient someone with much more experiance will chime in, usually at night.

Good luck,
Michael

 

I just found out from Cummins that 6 new pistons cost $1044 plus $135 in shipping if I want them in less than a week.

 

it only has 225,000 miles on it.

 

Thats outrageous for pistons!! At that price I think I would just rehone the cylinders cross hatch and everything then put new rings in and be done with it. Just clean the old pistons up good, sand bast them if youcan especially in the piston ring grooves.

 

I would also check the bottom of the cylinder, it's more critical at the bottom than the top. I'm thinking a set of over sized rings and hone the cylinder, 225 miles isn't that many miles for the ctd.

 

From what gather from your discription ,what you are mearsuring is the ridge , that the point at the top of the bore that the rings do'nt reach so a ridge is the area that does not get worn down by the rings, now if my reading of your discription is correct , then the next thing is unless there is some other issue these like other diesel eng. should go 1/2 mill. miles before they reach wear limits in the bore , then with the old school and diesels, rebuilding the head twice to the bottom end is tipacal. but before we get ahead of our selfs lets look at where your going , it sounds like you want to keep this one for some time . why are you doing bearings ? they should last longer depending on use, also how much moding are you going to do , is your buget better now then you expect it to be by the time 500,000 miles go by? I'm anticipating the bubble to burst , so if able do the rebuild now , do'nt forget to ring and port the the head , bigger studs[ boy I've got a lot of time on my hands this morrning] the thing is you've got it apart this far now your commited.

 

Do NOT do this!!! The Shape of the ring groves is important to proper ring seal. Sand blasting the pistons will remove material and distroy the ring lands. Use a good solvent and steel wool /Scotch brite pad being careful not to modify the ring lands. Also a ring groove tool will help remove deposits deep in the grooves.

 

Yup, you can bead blast the tops of the pistons but tape off the ring lands, if you alter the ring land, ring side clearance will go out the window...along with your ring sealing.

 

Thats what some old engine rebuilders said to do and I wondered about that but who am I to argue with someone with more experience than me. I definately won't do that or tell anybody to now.

What about having them hot washed at a machine shop? like the blocks?

 

Sure you could have them hot tanked provided the cleaning solution is safe for aluminum, sure would save alot of cleaning Back in the day when I was at Mitsu I`d replace pistons when doing a re-ring (warrany repairs only), just because it was easier than cleaning all that darned carbon.

 

Get slightly oversized rings and file them to fit as close to the minimum gap as you can, unless you are running boat loads of nitrous. In that case cut them to the middle of the spec. This will cut your blow by significantly. Clean the ring grooves carefully with a clean ring for that groove. The chromed outside surface of the ring can really get the crud out, but be careful not to damage the land. You can't JB weld those!

 

Came across this post and became curious on what you did?
The suggestion to sandblast blew me away! I certainly hope you ignored that. I am wondering (especially with the price you mentioned for pistons!!!!) if anyone ever knurls the pistons to regain desired clearance. Also, what is the desired ring gap? Realizing I need to get my hands on some factory service manuals.

 

If the top ring ridge is very deep you should get a ridge reamer to take it out. What happens is that with new rings the top edge is square while the worn edge next to the ridge is round. The new ring hits the rounded edge of the ridge and it breaks the separator between the top and second rings. Then things go downhill from there. If the difference is only .001 then it's probably not a problem. I've rebuilt engines with a .010 wear in the cylinders by knurling the piston skirts and then reaming the ridge and honing. It works fine for gassers but I'm sure the Cummins is a bit more critical but if you're only .0015 oversize then I don't even think knurling is necessary just hone it, ring it and slap it back together.

Edwin

 

If they were cast iron pistons you could do this, but its a DON'T do it to an aluminum piston and keep it away from wire wheels too.

I bought a piston knurling machine just because most of the engines we work on at home are wore out and getting pistons on certain engines is difficult. I would not do it on an engine thats going to be working hard like a cummins in a truck will.