I started the tear down and got some plastigauge but before going further I decided to check some measurements to permit a calculation of my compression ratio.

So I took off the head. It's a 3.4375" bore.

I CCed the head at 70cc (4.2716 cu.in.) and I sorta guesstimated the bore space above the piston including the gasket to be 4cc (.245 cu.in.) and the swept volume is 532.124cc (32.4722 cu.in.).

 

I got 8.287 to 1 compression. I then checked the piston top to block top and got about .032". I wondered if my overbore pistons were higher compression too.

 

Today I pulled a piston they're M&W "add power" pistons and compared it's height to a 3.375" piston I had and to my amazement it measured about 0.060" BELOW the stock piston.

The only reason I could think of for the lower height was to compensate for the increased swept volume of the overbore and subsequent increase in compression that would result.

But heck if they were "more power" pistons they woulda been even more at a higher compression.

I'd appreciate anyones thoughts and possibly corrections of the 8.25 to 1 compression figure that my figuring produced.

So I'm now doubting my comp. ratio calcs.

 

I'll redo my calculations and check back.

Steve

 

 

Sent you a pm; my new Clevite stock pistons are "0.325 below the deck at TDC. I believe compression is determined by the distance between the wrist pin and the top of the piston. A longer dimension should have more compression than the shorter dimension. 

I've re-done my calculations and I get 6.41 to 1 as the CR which sounds more reasonable.

The lower height of the M&W 3.4375" pistons must be calculated to keep the stock 6.5-1 CR.

I'll continue checking for my knock and pondering how to increase the CR before re-assembly.

More to follow.

I can't really measure the  volume of the combustion chamber in a head milled, say .100" without actually doing it but I estimate the volume might decrease by,say, half the diameter of the bore or .464 cu.ins.

If  I subtract .464 cu. ins. from the chamber size my 6.41-1 goes up to 6.86-1 hardly worth the effort.

If i could machine the block I could get more but I'd have to pull the liners and then rebore the block for their lips.

 

But then I could also get the crank stroked  to bring my M&Ws up to the top of the block. Speaking of M&W pistons, I'd love to hear any more information about them.

I'll run the "even with the top of the block" numbers tomorrow when I'm a little sharper. At my age I start the day dull and it isn't long before I'm BLUNT.

Thanks all, for your thoughts and keep'em comin'.

Steve

 That done, pan back on I was ready to put the milled head on  it and in so doing I discovered that the long head bolts had to be shortened to compensate for the 100 thou removed from the thread. That done I checked the short head bolts and found their tapped holes in the block to be fine, no need to shorten them.

 

Dick L (et al) have you ever come accross the need to shorten the head bolts after milling a head?

 

I know this project seems to be dragging out but NOTHING gets done in a timely manner here. 

It just seems to me that when I worked 40+ hrs. a week I got more done at home than now that I'm retired.

Anybody else ever have to shorten head bolts after milling the head?

I guess I should have written down any and all of the numbers and identification.

 

Still, I can't figure how they could provide higher compression than the stock 3.375" pistons that came up higher in the bore??

Kinda tricky shortening the pushrods too.

Maybe a  shim under the the rocker brackets? 

You can use gr8 washers under the long head bolts to eliminate shortening them. I have "shimmed" rocker arm supports using machine washers (long story about why I needed to do this, suffice it to say that the old timers were very ingenious on keeping these machines running) with good results.

Thanks for the response.

Steve