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R Moulding
R Moulding
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B31 barrel for 6 15/32 conrod
#797789 02/07/20 7:04 pm
Joined: May 2012
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jimq Offline OP
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Hi. I have just received a b31 big fin cylinder I plan on using on a b31 race motor. The plan for the motor is to run a late gold star length rod. I was of the impression that that infolves machining the barrel to effecevly cut the top two fins off and finishing to the correct deck height. Looking at my barrel the material the head clamps down to around the studs is only cast between the top two fins. So my question here is how is this really done. Does it involve taking a bit off both the top and bottom of the cylinder, or welding material in at the top after the cut is taken, or is the extra material considered un necessary. Cheers. Jim.

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Re: B31 barrel for 6 15/32 conrod
jimq #797792 02/07/20 7:24 pm
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What Gold Star rod are you using, OEM? Is it new? I would not race with a used OEM steel rod. I would recommend a Carrillo rod and you can call them directly or use a vendor like Mikes Classic outside of Brisbane.


Bill B...


Boomer
Re: B31 barrel for 6 15/32 conrod
jimq #797794 02/07/20 7:41 pm
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I plan on using a carrillo rod. My question here concerns the machining of the cylinder to achieve the required deck height.

Re: B31 barrel for 6 15/32 conrod
jimq #797826 02/08/20 12:04 am
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Jim: I am curious as to which piston you are going to use. B31 or one of the 350cc GS versions. Your choice could help your barrel problems.

Gordo


The roadside repairs make for the best post ride stories.
Re: B31 barrel for 6 15/32 conrod
Gordo in Comox #797827 02/08/20 12:16 am
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jimq Offline OP
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I am currently thinking of using a DB350 gold star piston. Effectively building an iron 350 with gold star DB350 dimensions. Now I have but that in writing I realise I have assumed 350's got the 6 15/32 rod. I will look into that.

Last edited by jimq; 02/16/20 9:21 am.
Re: B31 barrel for 6 15/32 conrod
jimq #797828 02/08/20 12:25 am
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Jim: The rods were ZB 7 3/8, BB 6 7/8, CB and later 6 15/32. What I do not know is the barrel heights for those rods.

Gordo


The roadside repairs make for the best post ride stories.
Re: B31 barrel for 6 15/32 conrod
Gordo in Comox #797830 02/08/20 1:10 am
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jimq Offline OP
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Yeah I imagine the barrel height figure is available somewhere. I have read elsewhere the barrel is cut down by two fins but it appears to me that if the two top fins are taken away the material that makes up the barrel to head mating face are taken with them.

Re: B31 barrel for 6 15/32 conrod
jimq #797831 02/08/20 1:28 am
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Yeah I imagine the barrel height figure is available somewhere. I have read elsewhere the barrel is cut down by two fins but it appears to me that if the two top fins are taken away the material that makes up the barrel to head mating face are taken with them.

Re: B31 barrel for 6 15/32 conrod
jimq #797897 02/08/20 9:32 pm
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If you are using a Carrilo rod, why not use the longer one and not have to shorten the barrel. I know that Phil Pearson can supply them as I had one in my 400cc B31


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Re: B31 barrel for 6 15/32 conrod
Andy Higham #797942 02/09/20 5:36 am
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jimq Offline OP
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Yeah that's not a bad idea. I wasn't aware they were available. Is that in the B31 length or the short rod b33 length. I am hoping to build the motor to Db32 dimensions as I like the improved rod ratio and plan on going to an alloy top end down the track a bit. What is the Pearson crowd like to deal with, I plan on buying a few bits from them soon. I imagine they must supply more than what they advertise on the website.

Re: B31 barrel for 6 15/32 conrod
jimq #798042 02/10/20 4:35 pm
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Since we can talk about rods now, last year I had Carillo make me a 7.375" rod for my ZB Gold Star as Phil no longer supplies them. They will custom make a rod not in supply if given all the dimensions. It cost a little over $400 with shipping in the US.


Bill B...


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Re: B31 barrel for 6 15/32 conrod
jimq #798441 02/16/20 9:19 am
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Is there really no one here who knows anything about this topic.

i was under the impression it was a tried and tested method of building hot iron single motors.

Re: B31 barrel for 6 15/32 conrod
jimq #798475 02/16/20 5:54 pm
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Jimq: A B31 barrel is meant to use the 7 3/8 inch rod and a piston with a relatively high pin.

This is a B31 piston.

[Linked Image from i.postimg.cc]

If you use the 6 15/32 inch rod you will need to make up about 0.9 inches based on the rod difference.

This is a BB32 GS piston and it has a lower pin but not enough to make up all the difference and it was made for a 6 7/8 inch rod. I do not know the height of a BB32 GS barrel.

[Linked Image from i.postimg.cc]

I know you seem keen to run the short rod but it seems to be not all that easy. Maybe look at all the options. If you are using a later DB/DBD 32 GS piston you cannot use the 7 3/8 inch rod. The long rod will need the ZB32 GS piston. If you use the late DB/DBD piston and cannot shorten the barrel then you still have the custom rod as an option. Do you have either the piston or rod already bought?

My advice would be to first get the piston you want to use then calculate the rod you need to get made up or how much the barrel needs to be shortened. That is a straight forward calculation. You might find that you can shorten the barrel a little and still have metal to play with and make it work with a custom rod. You will end up with a compromise engine, somewhere between the long rod engine and the later short rod engines.

Maybe you could find out the pin height/deck of the various GS pistons as a starting point. I appreciate that the rod length can change the nature of the engine but the early long rod engines were winning races.

Gordo


The roadside repairs make for the best post ride stories.

Moderated by  Allan G, Jon W. Whitley 

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