Deck Height Mystery

[Chickenstrips]

I am working on reassembling my 73.5 911T engine. Cylinders were a little corroded so had them bored to 85mm. Had 85mm moritz style pistons made by JE, and fitted to cylinders by Millenium. The case spigot were faced .25mm so would decrease DH, but I needed 1mm gasket at cyl base to get DH of 1.3mmm so DH net increase of .75mm from original. DH now was 1.3mm measuring from the flat rim on JEs to cyl top. So all is good. The mystery to me is that my net increase in DH should be increase of .5mm. If I am at 1.3mm now this implies I was at .8mm with factory set up. Does that seem too low? Or is it entirely possible that I am not accounting for other factors. Conrods were rebushed/resized with no offset. Same crank. New shells. Case not align bored. What could account for that? Maybe factory targeted 1.0 DH and at implied former .8mm itÂ’s not far enough off to be an interesting question? Thanks

[tom1394racing]

Was your case resized and line bored back to STD size? Removing material from the case mating surfaces, as is done when resizing and reboring the case, will reduce deck ht by the amount of material that was removed from the case mating surfaces.

[Chickenstrips]

Spigots faced, but two experts thought I did not need align bore, so did not. And to make this all the more puzzling the JE's have slightly less compression height.

[tom1394racing]

You might try the solder method for determining your deck ht.

[edmayo]

did you measure the original deck height?

[Richy]

If you do the math , the engineered deck height will be between .7 and 1mm , so what's the mystery ?

[Chickenstrips]

I did not have the good sense to measure, the deck height when I took it apart. I thought I saw that factory target was as low as 1.0, and .8mm seemed too low based on the 1.25mm to 1.5mm that seems to be the new min, but what do I know. Glad that does not seem odd to those that know more. And I did the clay test today for piston to cylinder clearance, instead of solder because i wanted a visual image over a wide area of the piston, and it was kind of cool to see the exhaust valve make a smile shaped impression to see where it would line up with the piston relief. Lowest clearance was 1.4mm at the rim over the wrist pins, and then >4mm under the valves.

I will do the piston to valve test, but I do not think I will preassemble without sealing up. I am thinking that with 8.0-8.5 CR, CIS T Cam, and these measurements that it would not be a bad bet to assemble with sealant and all, betting that I will pass piston to valve test turning valave adjuster, and only have to assemble once. Seem reasonable?

[edmayo]

I do a test assemble by putting clay on piston #2, then installing cam housing, timing cam on cyl.#1, then installing rocker arms for #2 and rotating engine to get the valve impressions in the clay. In this manner I don't chance getting false impressions while timing the camshaft. I would rather time the cam rather than just using the valve adjusters. I'm sure with T cams your clearance will be plenty adequate.

[Chickenstrips]

I like that idea, if I do preassemble. I am feeling emboldened to seal it up on first go and then test to be safe. Might be more cautious if I was pushing it. Thanks!