Pistons 280 SL

[declanobrien]

I am currently in the process of engine rebuild on my 280 SL Euro. Our forum has been of enormous help. I have now checked most specs, and also the valve to piston clearance, which seems to be at .9mm. I am thinking of shaving the tops of my pistons to help reinstate correct compression ratio (9.5 - 1), as my head is at or marginally below min spec. I am using a new cam chain, and timing marks seem good.
My pistons are 87.5 mm with stepped crown. It would be relative easy to shave .25mm (10thou) off the higher side of the crown, without shaving the stepped section. Would this removal affect the balance of the piston, or am I being obsessive?. If I am to shave the lower step, it would probably have to be done using a milling machine, whereas I can shave the high section with a lathe.
If all goes well, I will post all measurements etc on the forum, as I am trying to maintain all original parts where I can.
Declan

[George Des]

Others can weigh in, but my initial reaction is that this is not a good idea. Besides upsetting the balance of the pistons you are undoubtedly going to upset the heat transfer characteristics of each of the pistons possibly creating "hot" spots that may result in premature failure of the piston.

George Des

[JimVillers]

Declan ..... The only issue is to measure the thickness of the crown of the piston.  Some can be surprisingly thin.  I have shaved the top of the pistons on a 2.3-16 engine that I installed a supercharger on.  The purpose was to lower the compression to allow for more boost.  I can't see any reason why your would want to lower the compression on your engine.

[declanobrien]

Many thanks for replies.
Two issues have me concerned. First is that my head thickness is abour 83.75mm, ie .25 below min, and I am trying to correct the higher compression caused by this.
Second is that I am hoping to give myself a bit of safety on the valve to piston clearance.
The oversize pistons appear to have considerable metal on the crowns, certainly more than the modern pistons in newer, more modern cars.
Declan

[ja17]

Hello,

In most cases the compression is adjusted by measuring the volume of the combustion chambers in the head and carving some material out of the head. There are areas of the head where the combustion chambers can be carefully and safely carved out to lower compression and match up combustion chambers. This is a fairly common practice for many speed shops. When the volume of all six cylinders is exaclty the same the head is ready to install. Volume of the finished combustion chambers can be compared to the volume of the cylinders in the block to give a desired final compression ratio.

Of coarse The piston valve clearance should always be checked.  The steped pistons often resulted in a lower compression ratio in the first place. So it appears that your engine has been worked on before?  I believe all these cars came originally with "flat top postons"?  Do you have the compression figures from your engine before it was torn down?

You may have to have the volume of your combustion chambers measured first to  find out how unbalanced your combustion chambers are l and to find out if your compression  is too high.

Normally not much matierial in the combustion chambers will need to be removed to adjust the compression. I can relay information to you where  it can be sfely removed if you go this route. These measures are  extreme since in most cases your head woud just be scraped.  At this point you may want to look into a good used head as an alterntive.  

One other bad side effect of a head below minimum thickness, is that the timing chain life is shortened because as the thickness of the cylinder head decreases, the length of the timing chain remains the same and virtually becomes longer by twice the dimension (It has a shorter distance to travel).  

[JimVillers]

Declan .... You might want to consult the Technical Data Manual for the 280SL (mine is the 1966 version).  It has the specification for the amount that the piston can protrude above the flat of the block and it has a specification on the minimum distance that the intake valve must be below the flat of the head.

[Shvegel]

Also keep in mind most oversize pistons are already compensated for the larger bore by adding a deeper dish or ading a dish to a flat top piston.

[ctaylor738]

And there's always doubling the head gasket ...

[ja17]

Hello Chuck,

That's scary, I have never heard of anyone trying that on these engines. The head gasket is sooooooo skinny between cylinders !

Take care,

Joe

[declanobrien]

Thanks for all the very helpful comments. My pistons are absolutely level with the top of the block (the highest part of the stepped piston) when at TDC. My valves vary in set in depth between 0.5 and 0.6mm. This seems to give me a min of .9mm valve to piston clearance, checked with Plasticine (modelors clay) using an old head gasket.
I have the manual and will check the dims as suggested.
I will also do the maths on the Compression ratio, for the block and skimmed head that I have, and post the results, hopefully tomorrow. Don't know yet what I will find!!!!!!!!!!!!
It will mean a lot to me if I can get the engine together as it is all original, and the car is an original Irish car, bought by me from the 2nd owner who had it for 37 years.

[ja17]

Hello Brian,

Sounds like  you have plenty of valve clearance.

[declanobrien]

Have just done the maths. All as follows, based on my bores being 87.46mm, and travel of piston is 78.69mm: Combustion chamber vol (in the head) is 38cc.
Total vol of cylinder+gasket+head   524.42cc, and combustion vol is 51.917cc giving CR of 10.1:1.
I calculate that I would need to skim 0.5mm off total piston crown to bring CR to 9.6:1

[ja17]

Hello Brian,

Most likely the volume of the combustion chambers in the head is unequal.  You would have to measure each combustion chamber and adjust accordingly. When a head is cut, more aluminum is removed from the ends of the head causing the combustion chambers in the head tohave  less volume than the combustion chambers in the middle of the head.

[declanobrien]

Thanks a mill. I will check each vol and reply. If there is some differance, I can look at correcting by adjusting the skim off the pistons. I will now start to do some very accurate measurements to try and get best results.
I know this may seem extreme, but if I can save the original engine, I think it's worth it. I am also on a tight budget as things in Ireland are very difficult at the moment.
Regards
Declan

[ja17]

A performance shop measures combustion chamber volume by filling each combustion chamber with a liquid. The liquid is measured as it is used for each combustion chamber until it is full.   One way to determine if the combustion chambers is off before this is to measure the thickness of the head in the center and each end. If you have much variation you will have a difference in combustion chamber volume. 

[declanobrien]

Joe
I have now accurately checked the chambers, and the results give me a range of 39cc to 40.3cc. Based on all the good advice, I will NOT shave my pistons. You mentioned that you might be able to guide me if I wanted to enlarge the low reading chamber, which coinsides with the thinest measurement of the head thickness.
Many thanks
Declan

[ja17]

Hello Brian,

Could you let me know what the part number on the head is?

[declanobrien]

Thanks Joe
Head number is 130 016 1101 280SL 1970 European.
Declan

[ja17]

Hello Brian,

This is information is only for the later "oval combustion chamber head" like yours Brian.  The blue areas are where quite a bit of mass can be removed. Start by staying shallow go deeper only if required. Make the cavity smooth and free of ridges.

Take care,
Joe

[declanobrien]

Joe
This is very helpful. I have now had my guides checked, valve stems checked, and all are within spec. I will shim up the cam bearings, even out the chambers, and at last I can begin to rebuild  the engine.
Regards
Declan