Michael Benno

I have some questions about piston / bore clearance specifications. The WSM has the specification for pistons (99.980mm) and bores (100.000) and when I subtract the two numbers and the piston to bore clearance is 0.020mm (0.010 each side). But what are the wear limits on this specification. How much variance is allowed before piston upsizing is required?

FredR

Michael,

The above sheet advises how to check clearances for new installations and has nothing to do with wear and retirement specs.

As can be seen from the data sheet the bore check and piston measurements are carried out at a depth of 50mm for the GTS models - this is shallower than the S4 model. Note this is on a new or rebored engine. The tolerance group mark should be visible on the pistons.

Unless something has changed since I last checked an engine bore [and that is a long time ago!] the bore wear is usually measured at the point of mid stroke- i.e. max piston velocity- as that is where most wear is expected. I used to check the wear at three positions stroke wise and two positions [axially and transversely] relative to the axis of the engine. Thus the next question logically is at what point is bore wear deemed unacceptable?

I suspect the clue to what you are looking for lies in section 10 of the WSM page 10-02. This tells one that if and when the installation has been completed in accordaance with the specs the piston skirt to bore clearance will be in the range of 0.024mm to 0.048mm and if/when wear has increased that dimension to 0.08mm it is retirement time for that set. Given that the pistons have a steel coating I would think that skirt wear is not likely given the loading is light [but it can be checked] and that bore wear from the action of the rings would be the obvious cause. What this suggests to me is that if you detect bore wear of 0.032mm then that may suggest thqat bore wear is of concern and needs further investigation.

Your might find this thread helpful-
https://rennlist.com/forums/928-foru...arities-3.html

Hopefully some kind soul can add some value [or correction] to the above.

Minor edit: To measure the bore one can of course use costly bore gauges but the easy and reliable method for DIY'ers like outselves is to insert a piston ring and measure the ring gap lower down in the bore where the piston rings do not contact assuming that to be the original bore dimension. Then use this as a base measure and compare to what you get top, mid and bottom range of piston ring travel envelope- doubtless you will probably be aware of such but...?

Rgds

Fred

Michael Benno

Fred, thanks for helping me find the information in the WSM, I could no remember where to look and you pointed me exactly to the information. I see it is also in the tech specs booklet. I feel a little silly for asking when the info is so plainly displayed. So thanks for helping me find it again.

The block is at the machine shop getting all the measurements done so I will have some information to share soon. Also thanks for the DYI tip, I did not know this but now I do!

FredR

Michael,

I struggle to remember what I had for breakfast so no need to feel silly in the slightest- this is the very reason why this forum exists- to help each other.

Polite reminder about photos of the heads post clean up- I will post in a separate threaad regarding the condition of your removed gaskets shortly. As expected they clearly show the kind of detail I was expecting to see and now i would like to correlate this with the attrition taken by the heads.