Piston Height Above Deck of Block

[pgp001]

I have just been checking the cam timing on my engine rebuild using the two dial gauge and degree disc method, and noticed that on TDC the piston tops are actually proud of the top surface of the block by 0.0065". (six and a half thou).
This will be a result of having the deck skimmed to clean it up when the re-bore was done.

I am going to use a Minispares GUG702506HG gasket that in its uncompressed state measures around 0.060" thick, I do have another new gasket from an unknown source that only measures 0.040", but will not be using that one.

So what do the experts think, should I pull it to bits again and skim the piston tops flush with the block, or is 0.0065" not worth bothering about. I could do without the extra work involved, but if it needs it then I will have to do it.

Thanks
Phil

[philthehill]

In my opinion I would take the pistons out and skim them so that they are just below the top of the block.
I had the same problem with my 1380cc after the top of the block was skimmed. The engine was dismantled and the piston skimmed to just below the top of the block.
Con rod stretch must be taken into account - when the engine revs and the piston comes to TDC - stops and suddenly is on its way back to BDC - you must have sufficient clearance to take this into account.

[pgp001]

Looks like that's todays job taken care of then

I will need to make a turning fixture to hold the pistons as well. I have done it before on some of my model engines so that will be no problem. At least it is only the outer rim that needs some material removing, I was thinking of skimming 0.010" off, thus leaving the pistons below the top of the block by 0.0035".

Phil

[philthehill]

I would go for 0.015" off the top of the piston to be absolutely sure. Amended.

Phil

[pgp001]

Do you mean 0.015" ?

Phil

[philthehill]

Yes I do.
My excuse is that the heat is effecting my key board never mind my mind mind mind

[pgp001]

All the pistons are back out, and I am just finishing making the turning fixture.
Will post up some photo's later if i remember to take any.

Phil

[MattsGPOvan]

Dry building a engine is always a good idea to assess the work that’s needed.

Moving parts around on a NEW build could help you .
You are looking for all 4 pistons at the correct deck height.

[pgp001]

These are the parts for my piston crown turning fixture. It is a pot that the skirt sits into and is pulled in using the gudgeon pin and a small block with a cap screw from behind.


This shows the pulling block in position on the gudgeon pin.


This shows the piston retained onto the fixture.


This shows my ring compressor in position, to keep the rings in place while turning. It saves having to remove the rings again and risk any breakages.


This shows the actual turning taking place in the lathe.


This shows the ring compressor being used for its original intended purpose of getting the pistons back into the bores.
I am not a big fan of those wind up spring steel ring compressors, so made this one from an old cast iron cylinder liner.
Lots of Graphogen paste being used


The felt pen markings are left over from when I was matching the piston weights.
Phil

[philthehill]

Well done.

Brilliant piston holding tool and your piston ring clamp/sleeve is the way to go.

Phil

[oliver90owner]

Commendable adjustment technique.

I think I would have checked the clearances when the gasket is compressed and taken a decision on that. Can’t remember, off-hand, what the measurements were for my Pogo diesel engines, but they used different gasket thicknesses, dependent on piston protrusion. Given the compressed gasket thickness, I would add on bearing clearances (mains and big ends) and decide on that.

Re skimming the pistons, a dummy piston pin long enough to allow the piston to be secured to the face plate with shims would be my likely solution. But only if I felt that securing the piston in the 4 jaw chuck was likely to be risky.

Added: Factory piston protrusion, for fitting the thinnest of the gasket choice was 0.54-0.65mm. Maximum piston protrusion was quoted as 0.82mm for the thickest gasket.

[philthehill]

As the piston is alloy and you are only taking a smidgen off the top face of the piston I would not expect any problems though a two point back steady against the piston skirt would be an added protection against the piston holding block assy coming loose in the chuck.
Again well done.

Unfortunately the 'A' Series has not been afforded the luxury of having the availability of head gaskets of varying thickness.

As a guide - 3mm can be used as the thickness for a compressed 'A' Series head gasket.

Phil