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1968CC Connecting Rod Length/Deck Height
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carguytroy
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PostPosted: Mon Feb 22, 2021 6:59 am    Post subject: 1968CC Connecting Rod Length/Deck Height Reply with quote

Building a 1968CC engine for our Ghia. Iíve 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 hereís a pic of that. Iím wondering about the rod length.

I have Unitech rods from CB 5.4 length and Iím 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 Iíve waited this long so whatís 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.

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txoval
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PostPosted: Mon Feb 22, 2021 7:20 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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")
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Ohio Tom
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PostPosted: Mon Feb 22, 2021 8:35 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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.
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txoval
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PostPosted: Mon Feb 22, 2021 9:09 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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
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txoval
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PostPosted: Mon Feb 22, 2021 9:12 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

Also, based on the numbers on the case, I assume is was decked .06" or 1.5mm
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PostPosted: Mon Feb 22, 2021 10:15 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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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PostPosted: Mon Feb 22, 2021 11:05 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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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PostPosted: Mon Feb 22, 2021 11:16 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

Brian_e wrote:
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
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PostPosted: Mon Feb 22, 2021 1:47 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

Or measure the case
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carguytroy
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PostPosted: Mon Feb 22, 2021 2:26 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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.
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PostPosted: Mon Feb 22, 2021 2:40 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

carguytroy wrote:
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.


If the case was decked .060, and you need a .090 shim, thatís a difference of .030 from stock. I donít see an issue.
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carguytroy
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PostPosted: Sat Feb 27, 2021 7:19 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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? Hereís a couple pics of the situation.
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modok
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PostPosted: Sat Feb 27, 2021 7:28 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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
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carguytroy
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PostPosted: Sat Feb 27, 2021 7:48 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

modok wrote:
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


Right on. So Iím not too wide for exhaust etc to fit? Thatís all Iím worried about. If you think .050 ish is better Iíll do that. Trying to be 9:1 compression. Still need to cc the heads. Canít remember exactly what I ordered on the heads. Took a while to get them. If I remember right they are 55cc
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modok
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PostPosted: Sat Feb 27, 2021 8:37 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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.
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carguytroy
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PostPosted: Sun Mar 07, 2021 10:58 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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 didnít 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 CBís 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.) itís saying I need to be .064 deck height. Is that too far down in the cylinder or Iím I cool to roll with that?
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madmike
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PostPosted: Sun Mar 07, 2021 11:20 am    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

Heck I'm running .120 Laughing with only 8:1 cr although my has a 'Hair dryer',,
even with a fk 10 for a cam it drives great off boost Laughing
don't sweat it on the width most headers have slip joints so they 'fit all' sizes
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carguytroy
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PostPosted: Sun Mar 07, 2021 3:33 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

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.
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PostPosted: Sun Mar 07, 2021 4:16 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

[email protected] wrote:
Brian_e wrote:
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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x4. dont be stupid and think "this will work" when in reality it probably wont. the engine should be 5mm wider as thats how much stroke you added. but REALITY WILL USUALLY BE DIFFERENT THAT WHAT MATH SAYS. Stop wasting time guessing, just mock it up, and then you will know whats what ,like hmm should of got 76 stroke 5.5 rods&B pistons...or something else. a shot in the dark is like a shit in the dark in a ally...you have a very good possibility of stepping in something. look before you leap and order somethen you dont need and have to then get what you do need and who wins?? the UPS guy!!!! and they make great $$$ for a unskilled job.
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modok
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PostPosted: Sun Mar 07, 2021 5:23 pm    Post subject: Re: 1968CC Connecting Rod Length/Deck Height Reply with quote

.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
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