C2- Cylinder boring

[brumbach]

Brian:

With the advise you've given, I can now make an informed decision. I owe you. Thanks so much. Good luck with the studies.

Bill

Hey Bill,
Thanks for the compliment but I must say everything I know was pulled from kind people on internet forums like this.... and I did my first rebuild last year and didn't do all the HW I needed to do before hand.


This is what I'd do if I was in your situation.

#1 - CC the cylinder heads
#2 - do some research to see what the factory deck clearance should be and then what a typical deck job takes off. You can then get a estimate of what you'll be left to work with. .025 may be a good estimate

#3 - Select your pistons based on #1 and #2. You can play with the calculator using the data from #1 and the assumption from #2. You can then modulate the dome size and and pick a middle of the road head gasket like .029. You should then be able to determine whether you need flat tops or domed pistons. Buy them and send to builder

#4 - When all the stuff arrives at your door, assemble the short block and take piston/deck clearance measurements.

Step #1-#3 above should get you close to your target compression ratio.

#5 - select a head gasket anywhere from .015 (steel shim) to .039 composition to fine tune your compression ratio with your updated piston/deck clearnace. (may even need to use different head gaskets on each side)

IF - the assumptions were bad and you can't get where you need to be with head gasket selection, you can mill the heads or unshroud the intake/exhaust valves to adjust but this is unlikely.


I'm supposed to be studying right now so I'll illustrate in order to extend my procrastination:



Measured: 62 cc combustion chamber
assumption: deck clearance: .025 , bore =.030, head gasket = .029
known: stroke = 3.25
dome = 0 (flat top)

CR from calculator: 10.26


now let's say you assemble it and the deck height is .035 and you've already selected flat top pistons.
bore = .030
stroke = 3.25
dome = 0
head gasket = .015 (you can get away with this because your deck will be true - your heads should be true too - there is little tolerance for warpage with steel shim gaskets)

CR from calculator: 10.37







on the other hand... if your heads were 66cc
deck clearance = .025
bore .030
head gasket = .029
stroke = 3.25
dome = -5.33 (OEM SHP pistons)
CR = 10.44
if you changed to .039 head gasket CR = 10.17



Just play with the calc a little bit and you'll start to see how all these factors change compression ratio.

good luck! A wise man once told me MEASURE TWICE AND CUT ONCE. Some people might think it is overkill to take all these precautions but it will help you understand what's really going on inside that motor and if you ever have to make any changes in the future, you'll know exactly what you're dealing with.

Brian

 

[Tom Bryant]

The 355 I just finished cost $135. to bore .030 and fit pistons at my local NAPA machine shop. I used flat top hyper pistons with a .020 steel shim gasket and had the heads cut .020 also. After I polished and CC'c the chambers and figured in the deck height I had 9.36:1 compression ratio.

Tom

 

[brumbach]

Decking does a few things:

1) makes the deck surface "true" and flat
2) reduces the quench distance

A quench discussion is probably best saved for another thread but I'll try to explain the important things to consider when decking a block.

Piston/deck clearnance is the distance between top surface of the piston to the deck of the block. If the piston is a flat-top, this is just the distance between the top of the piston and the deck. If the pistons are domed or dished, the distance from the outer surface of the piston head to the deck.

What this means to you: if your block is decked, it INCREASES compression ratio holding all other factors constant. Many people prefer to deck the block, and use flat-tops to minimize quench for the same compression ratio. Let;s not get into quech though.....

check this out:
http://www.rbracing-rsr.com/compstaticcalc.html

I entered these parameters for you:
Bore: 4.030
Stoke: 3.25
Head gasket: .039 (typical fel-pro composition)
Deck Height: .020
Dome: 0 (flat tops)
Combustion chamber: 64

static comp. ratio: 9.89

Since your deck will be fresh and true, you could use steel shim factory-type gaskets (.015) and get to 10.5:1 or use thinner composition gaskets (.029) and get 10.15:1. You should be good on pump premium between 10 and 10.5. I'd shoot for just under 10.5 for max performance.



My point: there are several factors that one MUST consider when designing for compression ratio:

1. Cylinder head combustion chamber volume - can unshroud to increase or mill heads to reduce
2. Piston/deck clearance - can deck the block to reduce
3. Piston dome (or lack thereof) - can purchase with various domes or dishes. Can have the domes milled off to reduce.
4. Compressed head gasket thickness - can select with various compressed thicknesses.


1-3 need to selected carefully as modification is expensive - you have some inexpensive leeway in #4.

I don't know how one would estimate the piston/deck clearance without assembling the shortblock and taking measurements. Maybe someone could comment on what a typical decked block measures.

My factory block was .041 on one side and .031 on the other (.010 difference!) I should have decked it but I already had my pistons purchased and decking at that point would have put me way over the target compression ratio + would have required another trip to the machine shop. What I don't know is how a builder measures to know how much to take off!

I ended up using two different head gaskets to account for my .010 difference in deck height and my true static compression ratio was about 9.9. I wanted to be more towards 10.5 but the CC on my heads turned out to be ~66 instead of the original 62 as someone had unshrouded the intake valves.


Hopefully this makes sense.

Brian



p.s. I purchased a different set of heads that were supposed to 64CC and when I measured them they were 56!!! Need to measure everything if you really want to avoid pinging and low performance.

I'm back.

Well now I'm ready to get smart on "quench". Machine shop says block will clean up at .030. Not surprising, huh! Anyway, I need a little more help. From what I've read, quench height should fall between .035 and .045. Supposedly, the quench height is defined as the compressed thickness of the gasket plus piston-to-deck height. If this is true, then there's are limits to selecting the gasket when targeting a desired static compression ratio. For example, if the deck ht is .020, then I couldn't select a gasket thickness of more than .025 w/o exceeding the quench height. Is this right?

Thanks,

Bill

 

[Tom Bryant]

Sounds like you have a handle on it. My deck height was a little more than I wanted but it didn't need decked for any other reason. I used a thin steel shim gasket to get the quench that I wanted. Most gasket suppliers list the compressed thickness of their head gaskets.

Tom

 

[JohnZ]

Bill, what are you doing for connecting rods? Do NOT use pre-'67 stock rods - they're weak at the bolt seats. Either use '67 or later stock rods that have been Magnafluxed to check for cracks or aftermarket rods (Crower, Scat, etc.).

 

[Tom Bryant]

Good point made by John. Since you are building a L79 it must be a small journal. Small journal rods are getting harder to find but here are 2 options.

Scat Eagle

I'm planning on using one of these in the 3.00 in stroke engine I will be building for the '59.

Tom

 

[brumbach]

Bill, what are you doing for connecting rods? Do NOT use pre-'67 stock rods - they're weak at the bolt seats. Either use '67 or later stock rods that have been Magnafluxed to check for cracks or aftermarket rods (Crower, Scat, etc.).

I've got crower SP93200PF at the ready. Trying to find the right pistons at the moment. I'd like to buy forged pistons but can't seem to get to the optimum CR while staying inside the limits of quench.