Author Topic: cb750DOHC rods and cb750SOHC rods.  (Read 2553 times)

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Offline Soos

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cb750DOHC rods and cb750SOHC rods.
« on: November 24, 2007, 03:48:08 AM »
I did a search on this... and pulled up very little, and none of it pertaining to what i was looking for...

Has anyone used DOHC conn. rods to lower their CR for a turbo?
Or for using high comp. pistons at a more mild compression ratio?

I measured the connecting rod journals on the 750 crankshafts I have(one '74 750, one '79 750) and the journals are all the same size.+- 0.0002

The DOHC rods are ... 1? 2? mm shorter.
Someone correct me, 'cause i'm eyeballing the length difference.

Or am I just the last man in line to find this out?

l8r
-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline JLeather

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #1 on: November 24, 2007, 04:51:33 AM »
http://www.satanicmechanic.org/conrods.shtml

The journals fit and the rods are 1.5 mm shorter.  Not sure how much of a crop in CR that is, might end up being too low...

Offline Soos

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #2 on: November 24, 2007, 09:40:37 PM »
Assuming the cb750 is 9:1 compression stock...
It would drop it to around 7.4:1 with the 1.5mm drop in rod length.

Too low possibly.
If you were changing rods, while the head was off, you could have it decked .5mm or so to get it around 7.8:1.


l8r

-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline 754

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #3 on: November 25, 2007, 09:30:46 AM »
Are rid widths the same? Do they use the same bearing shells?
Maker of the WELDLESS 750 Frame Kit
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My next bike will be a ..ANFOB.....

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73 836cc.. Green, had it for 3 decades!!
Lost quite a few CB 750's along the way

Offline JLeather

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #4 on: November 25, 2007, 10:17:06 AM »
I believe they do use the same bearing shells.

eldar

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #5 on: November 25, 2007, 10:58:44 AM »
Now are the dohc rods stronger? If not then you might be best off with dished pistons.

Offline JLeather

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #6 on: November 25, 2007, 11:10:48 AM »
They are stronger, especially the CB900 rods.  As to how much stronger, I'm not sure.  Presumably somewhere between a stock SOHC 750 rod and a Falicon.

eldar

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #7 on: November 25, 2007, 11:18:58 AM »
Do the 900 rods though fit into the 750 pistons. That is another question.

An Idea would be to take the 900 rods if they fit and shot-peen them to increase strength even more.
Connect them to a wiseco 836 kit and do a mild build thus saving the cost of new rods which really are not needed in a mild build.

sohc4

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #8 on: November 26, 2007, 12:49:50 AM »
The 900DOHC rods do fit the 750 pistons, but the rods are about 1.5mm longer than stock - not a 1:1 swap, unfortunately.

1100DOHC rods are also 1.5mm longer, and they have a 17mm wrist pin (stock is 15mm).

eldar

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #9 on: November 26, 2007, 06:49:11 AM »
Pretty sure 1.5 mm of material would be too much to shave off the piston too. I would guess the crank is not usable either to change the stroke, at least not without some modification.

So is there any idea how much stronger the dohc rods are compared to the sohc?

Offline rbmgf7

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #10 on: November 26, 2007, 12:05:55 PM »
Assuming the cb750 is 9:1 compression stock...
It would drop it to around 7.4:1 with the 1.5mm drop in rod length.

Too low possibly.
If you were changing rods, while the head was off, you could have it decked .5mm or so to get it around 7.8:1.


l8r



i don't know too much about engine performance but i was wondering about the compression issue.

would adding the wesico big bore kit help raise the ratio a little? also, soos mentioned decking the heads. what's stopping you from decking 1.5mm from the head and/or cylinder barrels? if the rods are 1.5mm shorter, why not just shave off another 1.5mm? then i was wondering if decking would change the length of the cam chain, therefore, altering the timing. but, couldn't that be compensated with an adjustable cam sprocket?

Offline cafebob

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #11 on: November 26, 2007, 12:44:59 PM »
removing 1.5mm from the head probably is not the best of ideas....you will probably change the size and shape of the combustion chamber significantly.  Taking more off the cylinders is probably doable, but if you do it may make it impossible to get the stock cam chain tight enough.

For my turbo set up I am going with 10.5:1 Arias forged pistons (this is the spec sheet for them http://www.satanicmechanic.de/images/Arias_Piston_Specs.pdf), 750 DOHC rods, and a 78 F head, megacycle 125-65 cam, 78F bottom end. By my calculations this set up will give me 7.6:1. I may deck my cylinders 0.5mm to get 8.0, but from my reading 7.6 isn't too low, and will allow me to run 8psi on pump gas safely. This set up allows you to run a stock, or new TIGHT cam chain with out having to degree the cam sprocket. My reasons for going with this set up are that turbo pistons are hard to find, and I want to run the F head.  I have not seen a set of turbo pistons that allow you to run the F head and get a decent CR.

Are the weisco piston's forged, if so it would be easy to do the same with their kit. Only real problem is you have to basically tear the whole motor down to change the rods, but it is a good idea to inspect everything if you are building out a turbo motor. There is a set of forged rods on ebay right now if you want something a little different.


http://cgi.ebay.com/ebaymotors/Forged-Connecting-Rods-Rod-NEW-Honda-CB750-CB-750-K-F-A_W0QQitemZ170159030199QQihZ007QQcategoryZ35595QQrdZ1QQssPageNameZWD1VQQcmdZViewItem


eldar

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #12 on: November 26, 2007, 01:41:36 PM »
Yes the wisecos are forged. It is the cyclex pistons that are cast.

Offline Soos

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #13 on: November 26, 2007, 04:19:06 PM »

For my turbo set up I am going with 10.5:1 Arias forged pistons (this is the spec sheet for them http://www.satanicmechanic.de/images/Arias_Piston_Specs.pdf), 750 DOHC rods, and a 78 F head, megacycle 125-65 cam, 78F bottom end. By my calculations this set up will give me 7.6:1. I may deck my cylinders 0.5mm to get 8.0, but from my reading 7.6 isn't too low, and will allow me to run 8psi on pump gas safely. This set up allows you to run a stock, or new TIGHT cam chain with out having to degree the cam sprocket. My reasons for going with this set up are that turbo pistons are hard to find, and I want to run the F head.  I have not seen a set of turbo pistons that allow you to run the F head and get a decent CR.




Well, the 7.6CR was assuming 9:1 CR(with stock pistons)
With a 10.5:1 CR set of pistons, with the shorter rtods, you end up more like 8.39:1 CR.
That would be a perfect turbo setup IMO. That is assuming a 61mm bore.

With a 65mm bore, the CR would be more like 8.17:1 CR. almost perfect.

l8r
-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline cafebob

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #14 on: November 26, 2007, 04:53:26 PM »
how do you figure, my spread sheet gives me 8.7 w/ 65mm bore, .05 head gasket, DOHC rods and K head....

bob

Offline Soos

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #15 on: November 26, 2007, 05:50:44 PM »
Most CR's are figured with the gasket already in place.
And I was wrong on the 8.17:1..
836/4/10.5=19.90
cc's lost due to the 1.5mm shorter rods comes to 4.977cc's(at 65mm)
19.9+4.977=24.877     
209(cc's per cyl) /24.877=8.4:1

and if you were to throw in an extra .5mm due to head hasket the CR would be around 7.8, not 8.7.

Unless I am using fuzzy math.... :) I think I am in the ballpark.
Please correct me as to where I am wrong.

l8r


-=≡ Soos ≡=-
Just think to yourself what would Alowishus Devander Abercrombie do?
"Brix will be shat by your neighbors." - schwebel
(61mm)652cc 1979 cb650

Offline cafebob

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Re: cb750DOHC rods and cb750SOHC rods.
« Reply #16 on: November 26, 2007, 06:41:59 PM »
i use this little program to calculate CR's

http://www.sophtwarekodewerks.com/Packages/EngineCR/EngineCR.exe
Stock K

Bore 2.401
Stroke 2.480
Combustion CC = 22 (from Hondaman's measurments)
Piston type domed
Piston volume 6.25 cc
Deck clearance 0.039 (1mm) (guess)
Head gasket diameter 2.402 (guess)
Head Gasket thickness = .051
 
gives CR of 9.2 (exactly what my book says)

inches for my pistons (10.5:1) with K head

Bore 2.559
Stroke 2.480
Combustion CC = 22 (from Hondaman's measurments)
Piston type domed
Piston volume 7.08 cc
Deck clearance 0.039
Head gasket diameter 2.402 (guess)
Head Gasket thickness = .051

gives me 10.50:1

 my pistons (10.5:1) with K head dohc rods

Bore 2.559
Stroke 2.480
Combustion CC = 22 (from Hondaman's measurments)
Piston type domed
Piston volume 7.08 cc
Deck clearance 0.098
Head gasket diameter 2.402 (guess)
Head Gasket thickness = .051

gives me 8.75:1

my pistons (10.5:1) with K head dohc rods, with F head

Bore 2.559
Stroke 2.480
Combustion CC = 27 (from Hondaman's measurments)
Piston type domed
Piston volume 7.08 cc
Deck clearance 0.098
Head gasket diameter 2.402 (guess)
Head Gasket thickness = .051

gives me 7.539:1


but you might get diffrent numbers...

bob