1968CC Connecting Rod Length/Deck Height

[carguytroy]

Building a 1968CC engine for our Ghia. I’ve been waiting on parts for a few months and finally got everything together to move forward. Engine parts list 74mm crank, 92mm thick wall pistons A pin height, Webcams 163, 041 MOFOCO heads. The case has been decked and here’s a pic of that. I’m wondering about the rod length.

I have Unitech rods from CB 5.4 length and I’m wondering if they will work okay?

Any idea of shim(s) I would need?

Is this thing going to be too wide with the 5.4 rod?

And what deck height should I be shooting for?

If I need to buy a different set of rods or pistons I figure I’ve waited this long so what’s another week. I have the crank put together and set in the case checking for clearance at this point. Everything seems to clear the case okay. All help is appreciated.

[txoval]

Measure the distance from crank centerline to your case deck (do both case halves). Measure you cylinder length (sitting base to top). With 92mm thickwall cylinders they are probably 114mm. Then from there do the math.

Crank is 37mm
5.4 Rod would be 137.16mm
A-piston pin height is 39.6mm

Add these 3 together (213.76mm) and compare to the measurements listed first (added together).

This will tell you where you're at...(1mm = .04")

[Ohio Tom]

In theory, you would need 2.5mm of shim to get the same as stock deck height.
But, since you case is decked, more like 3.0mm.

Adjust as needed to fine tune deck/compression.

[txoval]

The other issue for him is that the thickwall 92 cylinders are taller than stock.

Standard 90.5/92 cylinders are ~112mm in length, while thickwalls are 114mm

Maybe with an A piston they are different...

Easy math though if he has the measuring tools

[txoval]

Also, based on the numbers on the case, I assume is was decked .06" or 1.5mm

[Teeroy]

Mock it up with what you have and see, then go from there. The ACN article has the info for this stroke, I believe he suggests 5.32 rods with A pistons, need to re-read to verify
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[Brian_e]

With the case cut that much, most suggestions of what other people have done will mean nothing for your situation.

Get your parts, mock it all up, and get a real dimension for YOUR parts and YOUR case. This is the only way to do it without a bunch of guess work and ordering the wrong parts.

You can mock it up with a old stock rod, and see where you are, then make the call on what rod length might fit better.

Brian
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[modok]

Or measure the case

[carguytroy]

Agreed I need to mock things up and measure. Working on that now. If I have to shim the thing 3mm like was said earlier I'm guessing that too much isn't it? I would think the exhaust, tins etc. wouldn't fit at some point. What's acceptable width of the engine for those things? Doing a little math based on a chart I found on the forum its going to need at least .090 shim.

[RickS]

[carguytroy]

Finally got a chance to mock it up a bit. Piston sticks out the top of the cylinder .0930 as per my cheap Harbor Freight caliper. If I need to be at .040 that would mean I need to shim it .133, the case has been decked .060 according to the numbers written on the side of the case. If I subtract that from the .133 that gives me .0733 over stock. Is this too wide/too much shim? And am I doing the math right? Here’s a couple pics of the situation.

[modok]

It's better to put a fat shim under the cylinder and measure in the middle, but yeah that seems in the ballpark.
Shoot for .050?
But if you want to run .040 I think you will need more precise measurements.


BTW, empi shims are better than bugpack/cb

[carguytroy]

[modok]

No big deal.
If you want to use 5.7 rods and stuff it all in a 40 horse engine compartment then you might be in trouble.
how much the case has been decked is making it seem worse than it is.

A lot of people be like...
"I built a 2275 Only used .020 shim", yeah well it's still .080 wider than stock they just don't know it, and that's the proof right there, that they don't notice you won't either.
The width of the engine is the distance the piston sticks out, or how far the heads do, not the shims used.
Tho I would prefer to stay away from shims beyond .150 because you start to get off the "pilot" section of the cylinder. Still works fine 95% of the time but just would prefer not to just so it makes sense.

[carguytroy]

Alright opted for the shorter rods. Because of that case being decked so much I would have had to shim well over .150 and I didn’t like that based on the info I got from you guys. My initial shim calculations were off so I bought the shorter rods. Using the engine calculator on CB’s web site and trying to get to 9:1 compression. If I plug in the 92 cyl, 74 stroke, 50cc heads(just checked them again to be sure.) it’s saying I need to be .064 deck height. Is that too far down in the cylinder or I’m I cool to roll with that?

[madmike]

Heck I'm running .120 with only 8:1 cr although my has a 'Hair dryer',,
even with a fk 10 for a cam it drives great off boost
don't sweat it on the width most headers have slip joints so they 'fit all' sizes
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[carguytroy]

General question sort of on topic. How much deck is too much?

Back totally on topic If I set The deck at .050 it will put the compression at 9.5:1 as per the calculator. If I run the thing at 9.5:1 will I have heat issues. The altitude is 4500-4600 where I live if that makes any difference.

[mark tucker]

[modok]

.040-.055
or >.120

Gaps in between should be avoided, because the mixture in the gap may get quenched but not squished, so it doesn't burn, and that's not helpful.

Really .040 is the ideal and you want to be as close as you can, but it is better to shoot higher because it's only going to get tighter over time, due to carbon build up, and due to the cylinder sinking into the case and heads, pistons rock, rods aren't perfectly straight nor the same length, stroke not exact, and AA pistons which the top isn't even square to the pin nor skirt, more tolerance will need to be allowed for such things