About coating of 2.7S MFI Pistons

[HKZ Bob]

Dear Team,
I have these pistons and due to the fact porsche changed a lot on the coating I like to ask the team some questions. The books I have are not in detail reflecting the 1973 mfi piston.

Is this a Nikasil coated pistion ?

Is is possible to have them re-coated ? Where ?

How thick is the coating layer ?

Piston no 1 has 89.955mm. It is a 0 piston. What is the tolerance limit.




Piston no 5 has 89.975mm. There is a value of 89.98 stamped on top . Sp is 0.03. Thee is no 89.98 on the 73 chart.

What is the tolerance limit on this piston. Is this a service piston.



The point is I guess I do not have the excact data sheet for it. The one table looks to me as an 2.7 CIS pistons table.

[Peterfrans]

As far as I know its not the piston thats nikasil coated, but the cilinder wall

[Eagledriver]

Peter is correct. The pistons are not coated except in Alusil cylinder applications. 73 RS pistons (what you have) are not coated and are meant to run in Nikasil cylinders. The clearances are as specified in your charts. The size of the pistons is not important except for it's relavence to the clearance of the cylinder to piston. New clearances are between .001 and .0025 inches. This clearance is only at the skirt and at the 12/6 o'clock postition. The wear limit is .004 inches.

There are also limits to the taper and "ovalness" of the cylinders but Nikasil cylinders usually have minimal wear.

-Andy

[HKZ Bob]

Ęagle but what do I have.
They donīt match. On the upper chart Beginning with 73 models
a 89.98 is not listed.
And a 74 can not be because I have an MFI. & with an CIS the design changed.

bob

[911pcars]

The pistons/cylinders were probably added at a later date. Measure the piston and its cylinder and compare with the factory specs (oil clearance, ring side clearance, piston pin-to-bore clearance) as Andy (Eagledriver) suggests.

Sherwood

[HKZ Bob]

Team,
it means the 0 piston with 89.955 is not in spec if it had run in Nikasil caoted lines but for Alusil it is still OK.
The upper 73 Porsche chart says it in spec. Donīt know what to believe.

What is this than ?
A "2" piston with beginning of 74 ?

Piston no 5 has 89.975mm. There is a value of 89.98 stamped on top . Sp is 0.03. Thee is no 89.98 on the 73 chart.

This Nikasil table says:but mine are not coated
0 is 89.97 +-0.005
1 is 89.98 +-0.005
2 is 89.99 +-0.005

See attached chart.

The upper porsche chart for Type Carrera says:
for 0 piston 89.950 +- 0.005
for 1 piston 89.960 +- 0.005
for 2 piston 89.970 +- 0.005

donīt know what right or wrong.

the 74 chart tolerances is I assume also wrong
+- 0.0005

[davidppp]

Hello there.

I see your question, and cannot tell you whether the factory actually documented the existence of an 89.98mm RS piston...my spec book is on "loan".

But I can tell you:

A. What you have is an RS poston, mde for a Nikasil barrel.

B. They look worn out to me..there is clear evidence of damage, and from bitter experience, its unusual for the side clearance and visual condition (use magnification) of the top ring groove on a forged Mahle to be good enough for them to be worth reringing.

Good replacements are readily available and worth it.
You can and IMHO should raise the CR moderately.

Kind regards
David

[HKZ Bob]

David. What is an IMHO ?
If I sent you a picture of the ring groove can you evaluated them.

I might have the opertunity to coat the pistons with Ferrosstan by 20ĩ.

Greetings
Bob

[davidppp]

Hello Bob.

No, you need to measure it..aas well as look for damage.

I would have much less worry about a slightly loose bore/piston clearance than a worn ring groove, or a collapsing skirt.

By the time you measure all the relevant dimensins, you will I am sure conclude its not worth playing about with them..they are worn and tired, and new are worth it..

In My Humble Opinion..

kind regards
david

[911pcars]

I only see two photos, and that of the tops only. Not sure how the ring lands and side clearance and overall condition can be evaluated from these photos.

Look only to the spec charts for suggested factory clearances and wear limits. Use measuring tools and visual inspection to determine the condition of your pistons/cylinders.

Sherwood

[HKZ Bob]

Hello Team,
I havenīt got an answer to my question. It looks tricky due to the variance of pistons & source of information.

Thank you again

[tom1394racing]

Bob

As everyone has been saying....You need to carefully measure the piston and cylinders using a precision micrometer and a bore gage to determine the diameter of each. This will give you the piston to cylinder clearance. The chart below is from a 2.7 RS set I had earlier this year. D1 and D2 (in green and blue) are the cylinder bore diameters measured at 90 deg increments.
D1 and D2 (in purple) are the piston skirt diameters measured in two different places along the skirt. The difference between these is the piston to cylinder clearance and should be less than 0.1mm.



You also need to check the ring lands to ensure that there is no damaged and that they have not worn too wide. I use feeler stock to measure the ring land widths. The top ring is the most critical as it will be the one that tend to wear out first.

[HKZ Bob]

Tom nice chart. So piston 1,2,3 are above spec. But what about the values from the Porsche Spec. Have you replaced these pistons ?