Engine rebuild with no former experience........

Racediagnostics wrote:
Main bearing sizes, only four shown but there are five bearings?

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This are not the main bearing sizes. This are the size-index of the con-rod journals.

Ahh, thanks, that makes sense.

Some measurements today.



Micrometer set to 92mm.



Bore gauge set to zero in the micrometer.



These are the measured values at the top, middle and bottom of the bores in both the transverse and longitudinal directions. The measurements were taken at 13degC rather than the specifications which are probably taken at 20degC.

I have made an estimate of the for the likely values at 20degC but am not sure how accurate this is given the micrometre is also calibrated for 20degC.

Let me know if you think I should use a different value for cast iron.

The bore wear limit is 0.1mm just below the top ring, so it looks like I am well below this limit across all cylinders.



As for the pistons, the measurements are below. I was really surprised how much under the spec value they were, so I brought one into the house tonight to make a comparative measurement at 20degC and found the piston skirt on piston 1 was 0.025mm larger than at 13degC so used that value to adjust all the values. As the pistons are tapered, the bottom of the skirt measurements are the only ones that really make any sense.



So the bottom line of what I was wanting to check was the piston-bore clearance which has a spec limit of 0.08mm. From the table above you can see that piston 4 is the only one out of spec. However I am not really convinced that my measuring methodology is good enough.

It is also a PITA that the factory fitted the wrong pistons in positions 1 and 4, the marking on the block calls for piston code "2" and piston code "1" was actually fitted, that adds to the excess clearance.

Given standard size pistons in any group size are no longer available, fitting a larger group size piston is not an option.

Boring out the block to use the first repair stage pistons (92.5mm) would cost at least £1200 for the aftermarket pistons and machine work, for me I don't think it's worth it for one marginal cylinder.

What are your thoughts, please also offer any advice on my calculations.

Hi Jim,

how did you estimate the temperature correction value for the bore measurements?
Isn't it strange that piston 8 is bigger than it should be in the new condition? Taken the wear into account this piston should have (at least) fallen into group 2 when the engine was built together and I can hardly imagine that it was so much off size. That raises the question how accurate your measurements are. I am sure that you did your measurements thoroughly but there might be a cause of error that you (and me) didn't think about.
Would also be interesting to compare your measurements with somebody else who already did this job. But this is definitely not me .....

Regards,
Lutz

Jim, can be this an option. Sleeve a cylinder.
www.enginebuildermag.com/2018/09/not-sexy-but-necessary/

Rudiger

Hi Jim,

impressive work and documentation, thanks for all that!

Regarding your "Cyl.4"-problem: Because the space between the piston rings and the fire land area on piston No. 4 (and all others) are clean - no remains from burned oil, as well as on spark plug and combustion area in the cylinder head - the higher play not seem to lead to any problems. How it looks like If this cylinder would have burned oil, you can see on the parts of cylinder 1.

My opinion: No action required!

Even when the manual says, that 0,08mm is the spec limit, it's like in your refrigerator: If the "best before" date say's that your yoghurt is only best before today, normally it tastes perfect tomorrow as well.
Modern pistons has a Graphite coating on the piston shaft, that would be a perfect way to add some microns, but I don’t know how it’s applied. Above all, this material is extremely durable in this place.

Maybe you can contact this guys:
ftt-shop.franke-turbotechnik.com/Produkt...eitbeschichtung.html
They offer a recoating process with Polytetrafluorethylen (PTFE). I don’t know how it works, if it’s durable, what thickness or if these guy doing a good job, but maybe it’s a begging to ask them.
Alternative: www.kolbenbeschichtung.de/ Graphite coating up to 0,2mm, special offer to adjust play by coating.

Regards Martin

Chromix wrote: Hi Jim,

how did you estimate the temperature correction value for the bore measurements?

Regards,
Lutz

I used an online calculator which gave the thermal expansion for cast iron, however it is at best a rough estimate as it doesn't cater for iron with holes in it.

Chromix wrote: Hi Jim,

Isn't it strange that piston 8 is bigger than it should be in the new condition? Taken the wear into account this piston should have (at least) fallen into group 2 when the engine was built together and I can hardly imagine that it was so much off size. That raises the question how accurate your measurements are.

Regards,
Lutz

Yes, thats's odd, I will re-check the measurements on this piston when it is in the house at 20degC.
When taking the measurements I rounded up or down the thousandths to the nearest hundredth as I doubted that I was accurate enough to realistically record thousandths. That could account for an error of 0.005, lets add in another 0.01 for other errors and then my numbers could be +/- 0.015

Chromix wrote: Hi Jim,

Would also be interesting to compare your measurements with somebody else who already did this job. But this is definitely not me .....

Regards,
Lutz

There will not be many of these people around.

Hi Martin,

I tend to agree with you, as the piston and bore both look good visually. Probably no action necessary. I will take the block to be cleaned and honed then I will remeasure both the block and pistons at room temp as a final double check.

Hi Jim,

get it honed - why that? You may rethink, the bore diameter will not shrink by that!

Regards Martin

Jim, ask the machine shop to measure piston and sleeve. go from there.
Rudiger

Hi Martin,

Just a light home to recreate the cross hatch pattern where it has worn. I will discuss with the machine shop.

Hi Rudiger,

I will ask the machine shop if they think it is necessary.

Chromix wrote: Hi Jim,

Isn't it strange that piston 8 is bigger than it should be in the new condition?

Regards,
Lutz

I brought piston 8 into the house this evening the room temp is ~22 degC.

Here is the measurement, 91.993mm, it seems to correlate with my thermal expansion estimate which is good news for my methodology but doesn't explain why it is larger than marked.







Once the block has been cleaned and if my wife goes out, I may try to retake all the measurements with the block in the conservatory.

Well, then it is the way it is. Maybe somebody put the piston in the wrong box.

Regards, Lutz

Racediagnostics wrote: Hi Martin,

Just a light home to recreate the cross hatch pattern where it has worn. I will discuss with the machine shop.

Hi Jim,

please think twice, from a performance point of view: Is there anything that can or must be improved by doing this?
2 things will definitely happen:
• Bore diameter will increase (not good)
• Piston rings need to be renewed (expensive)

The machine shop of course sees a business case and probably will recommend this action.

Regards Martin

Hi Martin,

I actually managed to buy two full sets of piston rings cheaply from Autodoc (I can return the unused set) one set is iron, the other chrome plated. I think Autodoc had an error in their pricing , the full set of rings was only ~ £35.

Hello,

I would be not too concerned about the piston diameter measurement.
Unless liner, piston and rings show any unusual/ bad running pattern.
Your pictures show good running pattern obviously they lived very well with each other together.

Not convincing to see the mix of pistons, it's just telling us that not everybody at Mercedes was as focused about Quality as they could and should have been.

Piston can be "deformed" very easily and you just "hit" it hard on the table and you will measure something different.
Not to hit the piston, but we did this during school/ apprentice ship just to see how easy they can be deformed.

Anyhow, alloy like cast iron/ steel as well has internal forces that can/ will relax during operation. On small parts that's not dramatic but you are measuring 1/100 millimetre with the calliper and that's might be one more confusing factor after many 1000th miles time to relax internal forces.

Did you did a pressure loss check of each cylinder before you opened the engine? Would be interesting to see how pressure went away on Inlett/ Exhaust Valve and piston rings.
This should done each time before an engine is opened.
Presse loss test not a compression test only.
The test tells you where pressure is lost and thats what's to be avoided.
Replacing all parts will improve pressure loss for sure but why changing parts that are good/ keeping the pressure ?

To do a professional 3 angle valve seat job now is mandatory. Thats most likely a good job for a machine shop unless you got a Hunger or comparable valve seat machine available.

Pistons and rings are now out, so it's a good time to change piston rings.
As somebody mentioned before the machine shop is might be a little hesitant to just hon/ prepare your liners for new piston rings.
They want to make money without the danger to receive a claim later and honing used liners owns more risk then honing fresh machined liners.

I would look for a honing brush and prep my liners myself. Just a slight brush to prep the honing pattern for the cast iron rings.
Chromed rings are harder and might be better suited for fresh machined/ oversized liners with out any previous wear pattern.

If you want be 100% safe, pistons need to be renewed and liner need to be machined.

Just my 0.02$

Kind regards

JP

No eBay APP ID and/or Cert ID defined in Kunena configurationJim, get a set of extractor sockets, I found them to be very handy on stripped inhex bolts.
BTW, great work and even better documentation! this will help a lot of people.

Racediagnostics wrote: Hi Martin,

I actually managed to buy two full sets of piston rings cheaply from Autodoc (I can return the unused set) one set is iron, the other chrome plated. I think Autodoc had an error in their pricing , the full set of rings was only ~ £35.

Wow!

Catch up for the last couple of days.

I gently cleaned up the decks.



Checking the decks for flatness with a straight edge.



The thinnest feeler gauge I have is 0.04mm and that would not fit under anywhere.



The same for the main bearings with the old shells.....



.... and without the shells.



Moving on to measure the crank pins and main bearings.



Then finally heading off the the machine shop.