D3400 piston ring grooves

When i took my D3400 pistons out a 3J into the rebuild shop to get the ring grooves widen he said i couldnt be done because the top ring is a keystone ring and the ring groove is cast iron. Hopfully the experts chime in on this one.
Billy

When i took my D3400 pistons out a 3J into the rebuild shop to get the ring grooves widen he said i couldnt be done because the top ring is a keystone ring and the ring groove is cast iron. Hopfully the experts chime in on this one.
Billy

Looks like you have 2 options here-find a wider keystone ring in the correct bore size & cut to fit -- or cut the grove to a straight conventional ring grove & use a conv ring & spacer or 2 ea thin conventional rings in the top grove- haven't seen pistons but if they are already worn 25 - 5 more for the spacer should be OK for bal of groves.[side note- haven't done it much -but using 2 thin rings instead of 1 wide one in top grove seems to be able to get by with a little more clearance without making them an oil pump]

Depends on what you can get for rings. In the old days they would recut the ring grooves with a Manualath to 0.157 - 0.158 (square cut) and use over width rings (0.030 on the original 1/8 th) and have 0.003 to 0.004 clearance. I don't see why you couldn't do the same to all the compression rings if need be. SJ should be able to comment😊

I don't know if you can get wider rings for the 3-3/4 bore anymore or not. I think I would pass on using the 0.030 shims but that's just my opinion.

[QUOTE=. I think I would pass on using the 0.030 shims but that's just my opinion.[/QUOTE]

I'm qurious why you would pass?- some perkies come stock with spaced pistons???????I never had trouble w/ that setup..😕

The top groove seems to be a square not keystone type. I should take a closer look at it. I do not have the old rings as the previous owner threw them out. The new set from Hastings has a square top ring so I just assumed the piston was square type from that.

The previous owner found broken rings so another assumption is that is what caused the super worn groove at the top.

The original pistons from the 1950's had parallel-sided ring grooves. Keystone rings were a design improvement that appeared in the late 1960's, along with a myriad of other new design ideas for piston and ring design. You will find probably at least a dozen different designs and Part Numbers for pistons and rings over the decades, as Cat and aftermarket suppliers experimented with "improved" designs. Keystone rings were introduced to combat ring sticking, caused by carbon buildup. A keystone ring moves in the piston groove as the piston travels up and down, thus preventing carbon buildup. The bottom groove angle is critical with keystone rings, and you cannot install spacers in keystone rings.

The two piece Cat pistons of the late 1940's were a design failure, many of the pistons lost their tops when they came unscrewed in operation, and they weren't in use for very long.
The number of rings has reduced over the decades as well. Early Cat diesels used 6 ring pistons, then 5 ring pistons, then 4 ring. Each reduction in ring numbers was a "design improvement" that reduced friction .. and the new style pistons often had other changes such as smaller skirts, lighter weight, different piston crown design to improve combustion, and dissipate heat.

Cast-in iron bands in the pistons appeared in the 1940's and this improved piston life. Initially cast-in bands were only installed for the top ring. Then the design was altered so that the two top rings were in the cast-in iron band. The cast iron ring bands have changed in their design numerous times. Initially, a "wedge" or "key" shape band was used (larger on the inside than on the outside), then the design changed to an "arc" design, which had rounded corners on the inside of the band profile. The newer "arc" design reduces stress cracking.

Bolt-in heat plugs appeared in the 1960's. These were designed to allow for better heat dissipation in the piston crown, in precom style engines, particularly in turboed engines.
Wrist pin bore side relief was introduced in the 1960's. This is the design where the pin bore looks like it has been drilled twice in error, with a secondary groove each side of the wrist pin.
This design reduces piston cracking in the pin bore area. The piston pin effectively goes oval under the pressure of the combustion stroke, and the relief holes in the pin bore allow for this change in shape, to eliminate piston cracking. The piston bore in Cat pistons is also strengthened by a process called "roll burnishing" which improves the strength of the aluminum. Many aftermarket piston manufacturers don't add these features.

In the early 1960's "cam-ground" pistons appeared. These pistons are ground elliptical and tapered from top to bottom to better conform to the bore when at operating temperature.

If your pistons are in good shape, and all the rings are square cut (as I think they should be), just machine the grooves to accept thicker rings or spacers. A good ring supplier will be able to supply rings in a multitude of dimensions, so you should not have any problems in that dept.

[QUOTE=OzDozer;49186]

In the early 1960's "cam-ground" pistons appeared. --I guess you are talking cat pistons??- my Van Norman piston grinder came w/ wartime finish[OD & no smooth polished handles etc with a note in manual that this was to save time in wartime production but that all critical parts were not affected]- this grinder came with a cam for every cam grind that TRW offered + a few Van Norman specials--side note-- the weirdest piston setup I have run across is from the 3 cyl Ford eng I used in my cultivator tractor-- the iron - was a sleeve that was glued on the top part of piston from below the top ring to the top of piston- w/ the piston crown AL- till you get to the outside edge where it turns to iron- this piston had a KEYSTONE/HEADLAND ring--I thought that the dif in expansion would loosen the sleeve as the AL would expand it
when hot & shrink away when cold- & after a few years- sure enough- the iron & top ring ended up out in the corn field..😠

Hi tanker,
As best as I can recall I formed my opinion from an application that used a half keystone (flat on the bottom) ring with a spacer. The spacer wore a peculiar pattern in the bottom of the ring and looked to be actually interfering with the rings sealing function. The engine still operated (Hercules I think) but had oil burning problems and hard starting (direct electric) which indicated compression was also a problem.

Edit:
Don't want to give anybody the wrong idea here. Acceptable spacer installation (if you must) appears to be top of ring location, not bottom.