Machine Shop Help...

So I’m a little confused, I took my block in to have it cleaned, measured, milled, bored/honed. They finally got back to me tonight and said the crank was good but it’s going to be 10/10 (I assume .010 under for each rod and main journals.

He said the block needed to be bored over 20 thousands but needed to get a set of pistons next week (Silv-O-Lite 1120) for final measurements and hone. He said he would take it to 19, then measure the pistons and make the adjustment for final bore. He also said it would give me a final CR of 9.2:1 (by the Silv-O-Lite book).

Through all my reading and research (many hours on CI and the Falcon Handbook) the CR he is quoting does not sound right. The stock pistons (dish) are 8.7:1, I had planned on going with flat top pistons to boost my CR but had not planned on coming up with any final numbers until I had all the details from the machine shop (deck height after milled).

Running the CR calculators, I’m not seeing how he’s coming up with 9.2:1. I don’t do this kind of work every day, any insights you all may have would be appreciated.

While piston style and deck height play a part in final CR. You cant throw a number up without knowing chamber size and head gasket thickness.

redxm":t0w7zeg4 said:

While piston style and deck height play a part in final CR. You cant throw a number up without knowing chamber size and head gasket thickness.

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Exactly.... How can he quote a CR without asking anything else. Why does the manual state 9.2:1? I kind of get the feeling it's a hard upsell for pistons but I might just be hyper sensitive.

kboldin":1agyl61b said:

redxm":1agyl61b said:

While piston style and deck height play a part in final CR. You cant throw a number up without knowing chamber size and head gasket thickness.

Click to expand...

Exactly.... How can he quote a CR without asking anything else. Why does the manual state 9.2:1? I kind of get the feeling it's a hard upsell for pistons but I might just be hyper sensitive.

Click to expand...

.. machine shop typically may start with year-of-design specs to project CR, arguably unless head chamber volume is actually measured, too many variables are possible for accurate prediction.

My machinist took @ .070 off a D7xx head and projected CR from OEM specs but stock steel gasket spec and chamber measurement differences changed things...

kboldin":1hr47h07 said:

So I’m a little confused, I took my block in to have it cleaned, measured, milled, bored/honed. They finally got back to me tonight and said the crank was good but it’s going to be 10/10 (I assume .010 under for each rod and main journals.

Click to expand...

You are correct crankshaft is ground .010 under size on both rods and mains, that's great that it will clean up to 10/10.

Yes the CR in the piston catalog listed at 9.2 would only be right if block and head measured to the specs they used. It maybe somewhere in the ball park yet until you measure everything its only a close guess. Have you CC'ed your head yet? Most Ford heads when new had bigger chambers to start with so depends if or how many times it was machined. Good luck on your build.

Yep, went over and talked with him this morning. A book measurement is just that, I'm sticking with the plan.