Pistons and rings. Replace, rebore, reuse?


(Eric Feron) #1

Hello all,

I have finished dismantling the XK engine in my 75 XJC.

I will now take the block to hot tub wash and magnaflux (the last block I have checked showed cracks bewten every single bore! Scrap).

There is no ridge at the top of the bores, the walls still show signs of the crisscross pattern, so I am tempted to only replace the rings. But looking at the pistons, I am not fully confident and would like the opinion of experts.

This will not be a racing car, just a nice weekend driver.

Here are some photos, what do you think?

Thank you.




(Eric Feron) #2

I read from others threads that the pistons business is not straight forward.
Cast, forged, silicon etc.
Mahle, AE, Venolia, Wiseco…
I have a lot to learn…


(Robin O'Connor) #3

Hi Eric.
I am slowly putting on my flame proof underware as I write this :slight_smile:
If the car is to be a nice occasional driver then I would be inclined to check if the ring grooves are within cooee of standard then I would just re-ring, a little piston slap will only add to the symphony of the XK engine
I am in the process of refurbishing a 3.4 for my ‘S’ type and had to re-groove the top rings as they had way too much clearance they are now runnng with 2.5mm (IIRC) wide top rings, and I just honed the bores.
Others will chime in possible.
Cheers


(Lovell) #4

Greetings All,

I would say got the amount of labor expended, new!

You only mention visual inspection on the bores, any measurements done?


(Lee140FHC) #5

Where is the dome on those pistons? Even the 8:1 engines have a pronounced dome…is that engine only 7:1 compression? Not very familiar with '75 4.2 engines but the combustion chambers are huge on all XKs so without a domed piston, compression ratio is very low. IF that is a 7:1 engine, I would buy 8:1 pistons for it.


(Eric Feron) #6

Yes the pistons have a dome.
This is a standard UK specs engine, the stamped number starts with an S. Not sure what that this translates into in terms of compression rate.


(Robin O'Connor) #7

IIRC the S will be the grade of piston


First picture is my 8:1 3.4 the second is from the manual


(Lee140FHC) #8

IIRC, the size code(F, G, H, etc) do not go all the way “up” to S, at least not that I’ve seen…lemme check my manual…Yep, the codes are F thru K.


(David Ahlers) #9

Eric you need to measure a few things before you take a decision:

  1. what is piston skirt to cylinder clearance? Check with a new standard piston if you have access to one.
  2. what is the ring land clearance with existing & new rings? If in spec you may be able to reuse piston if #1 is within spec
  3. what is the ring end gap with new (& old) rings?
  4. how pronounced is the ridge at the top of the cylinder?

You have a lot of shiny area on the piston skirt that may indicate rocking from wear. If you do not have the measuring tools needed to measure see if you can locate someone who does and can help.
Good luck!


(Eric Feron) #10

S is the marking on the engine block, there is no reason why the pistons would not be its original ones.
I will check the markings on the pistons and take measurements tomorrow.
Thank you!


(Terry Sturgeon) #11

Hi Eric Your pistons are 9:1 or maybe 8.7:1. Because of the larger bore, but unchanged combustion chamber size from the 3.4 and 3.8 engines the dome is smaller to maintain the same c.r. I don’t see any cross hatch marks in the photos. Back in 1975 ( maybe Napoleon was still alive??) I pulled the pistons on my '68 that had 75k miles, because it was going through a quart of oil every 150 -200 miles (been doing it for as long as I owned the car). The bores were glazed from a too gentle break-in and you could see very clearly the cross hatching under the glaze. I used a ball hone to break the glaze, installed new piston rings, and ran the pistons until 10 years age (Nappy was definitely dead by then). One of the things I notice about your piston is that they are worn on the thrust side, so I don’t think you have a glaze problem. You also don’t mention an excess oil consumption.

Changing the pistons is your choice. You don’t talk about any piston noise so maybe it’s okay. You should probably have the bores and pistons mikked to be sure, including ring lands. You can also have the piston knurled to slightly expand their size. You need to change rings and hone the bores.
Be very careful about buying piston rings. You must carefully measure them to determine that they fit. In particular back spacing of the rings is critical. If not enough you can have catastrophic engine failure.


(tony) #12

from what I have read

  1. measure the gap between ring and piston land.
    If this is too high, new pistons & rings are called for

  2. If piston to land gap is ok, measure ring in bore for gap

  3. measure everything with outside micrometers & inside micrometers, (or T-gauges)


(Terry Sturgeon) #13

Hi Eric Because you are going to be using new rings that did not come matched to your pistons these measurements are critical, but most critical of all is measuring back clearance of the rings. The Jaguar manual I’ve got (for an E Type) doesn’t mention this, I suspect because they thought you’d use factory purchased rings?

Take a look at this site https://www.hastingspistonrings.com/tech-tips-faqs/ring-groove-depths-of-piston-rings and it will explain how to do it. Having said that I’m going to tell you I know more than Hasting about doing this - lol.

They show the ring inserted on the thrust side of the piston. Not on your pistons - sorry. Measure it there but more critically measure it over the pin bosses - that is 90 degrees from where you see it in their diagram. They also say it’s ok if the ring, once inserted is flush with the surface of the piston. Maybe but I’d be a lot happier if the ring went further into the groove - say by .005 to .007 - that is more back space. This additional space is needed to ensure good compression by permitting combustion gases to get behind the ring and push it into the wall. Why all the concern of checking over the bosses. Most piston are cam ground - that is, when cold they are a very slight oval, the narrow part of the oval being over the pin bosses, where most of the aluminum is. When the piston heats up the pin boss area expands more than the thrust sides, and the piston becomes a circle. Most manufactures however cut the ring grooves on a lathe so they are circular when cold - when the piston heats up and becomes circular the grooves become slightly oval, the widest part being over the bosses, pushing the ring towards the wall of the bore. It there is hard contact between the back of the groove and the ring and the wall you won’t be a happy guy! That’s why this area is the most important to check.

As to how to measure - if you have a reasonable set of calipers you can measure both the inside of the groove and the thickness of the ring.


(Eric Feron) #14

Thank you for all this great, detailed and precise info!!


(Eric Feron) #15

After magnaflux, it turned out that one on the liners is cracked!! Not the block (as I first thought), the liner.
I wonder if this would the right opportunity to replace all liners with flanged ones.
Any expert advice?
Thanks,


(Paul Wigton) #16

I thought I answered on another thread…YES.


(Puddinhead) #17

That’s what I had done to my '66 fhc etype, and I went with 8:1 to extinguish any possibility of pinging. It was reasonable $ as I yanked the engine myself (underneath) and took bare block to machine shop with top hat liners and companion fitted pistons. I used a racing shop that was familiar with removing liners and grinding grove for new top hat style pistons. My '66 block had all 6 pistons frozen and walls were scored but no cracks between liners…

Patrick
'66 fhc


(tony) #18

I hope you dont mind me asking how much $ they charged for the liners to be replaced?

Our local Jag engine rebuilder/machinist wont fully warranty unless he does this, and I believe he may have said the price was $A1200 = $US800

That was a few years ago, and may have included the price of boring the new liners to take .030" over pistons


(Puddinhead) #19

If your machinist does it for 800$ that’s a bargain. ! Why 30 thou pistons? That makes no sense, my top hat liner/pistons are not oversized. Is he saying the crack requires 30thou to fix, wouldn’t the top hat itself remove crack ?

Patrick
'66 fhc


(tony) #20

The only reason was I had a set of .030" new pistons & rings, so it was largely a question of cost.

The cost of a set of new standard pistons & rings was more than the cost of an overbore

I did not proceed with that rebuild

Strangely enough, I luckily obtained a rebuilt engine, and fortunately was able to have its rebuild sheets forwarded to me.

It had also had new tophat liners, bored .030" over, and new 9:1 pistons, cant really figure why they would do that…unless maybe he had a set of 9:1 .030" laying around too lol