The BSA A7-A10 Forum

Bikes, Pictures, Stories & more => Chat, Offtopic, Meetings & Everything Else => Topic started by: muskrat on 14.09. 2016 10:18

Title: Con rod length
Post by: muskrat on 14.09. 2016 10:18
Mention was made of short rods in an A10 in Richard A's topic http://www.a7a10.net/forum/index.php?topic=11051.0;topicseen.
Not wanting to get away from his original post I thought I'd start this one.
Rocker Racer has put 6" rods in his racing A10 outfit shortening his barrels by 1/2".
I went the other way with my racing A7SS. I used A75 pistons that had a shorter crown height so needed longer rods. I shortened A10 rods by 2.5mm to give me a 14:1 comp ratio to run on methanol.
Just wondering if any other members have tried any other combinations. Briz, Danmar, Dan ???
Found this article and thought it interesting reading. http://victorylibrary.com/mopar/rod-tech-c.htm
Cheers
Title: Re: Con rod length
Post by: Rocket Racer on 14.09. 2016 22:34
http://victorylibrary.com/brit/A10-c.htm (http://victorylibrary.com/brit/A10-c.htm)

Aplogies the attachment is poor quality have misplaced the original
Title: Re: Con rod length
Post by: RichardL on 14.09. 2016 23:04
RR,

Thanks for posting the article. I think it would be a great read if it was readable. If you have a scanner you could make a pdf Then, if you are familiar with the technique, you could put it on GoogleDrive and give us the link. If too much trouble, still thanks.

Richard L.
Title: Re: Con rod length
Post by: Rocket Racer on 14.09. 2016 23:28
http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=275394 (http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=275394)

Title: Re: Con rod length
Post by: RichardL on 15.09. 2016 01:32
Read it. Love it. Can't do it.

As a side note, being from California, all the places named are familiar. I have a feeling I won't be able to stop in at Hap Alzina's, which is where my own bike shipped to in '55.

Richard L.
Title: Re: Con rod length
Post by: Rocket Racer on 15.09. 2016 02:37
Those places sound so exotic...   We have the "American" factor to thank for driving BSA to develop the sports twin models. Particularly the American love of flat tracking.
My favourites being the factory daytona racers of '54
the barrel you get if you follow the instructions  *dribble*
also featured in http://www.a7a10.net/forum/index.php?topic=1998.90 (http://www.a7a10.net/forum/index.php?topic=1998.90)
(bottom of page 7)
Short rod motors also improve ease of fitting rocker boxes as the top end is 1/2" lower than a stock motor, also improved space around the carb's.
Title: Re: Con rod length
Post by: RichardL on 15.09. 2016 14:34
RR,

Like I said, I can't do it, as I lack the machinery and, maybe, courage. I am, however, curious about one detail from the article and how you did it. They say to build up a welded lip at the back edge of the pushrod tunnel. I assume this is because the pointy part recedes as material is removed. Is yours built up? I can't tell.

Richard L.
Title: Re: Con rod length
Post by: Rocket Racer on 23.09. 2020 04:49
RR,

Like I said, I can't do it, as I lack the machinery and, maybe, courage. I am, however, curious about one detail from the article and how you did it. They say to build up a welded lip at the back edge of the pushrod tunnel. I assume this is because the pointy part recedes as material is removed. Is yours built up? I can't tell.

Richard L.

a mere 4 years late but to answer Richards question yes it is.

Am currently back to pondering rod lengths, gudgeon heights, bores and strokes to build a 500 based around  a7ss cases, https://www.a7a10.net/forum/index.php?topic=15075.0 (https://www.a7a10.net/forum/index.php?topic=15075.0)
Title: Re: Con rod length
Post by: berger on 23.09. 2020 14:17
ello RR I presume they are A10 barrels shaved down, so just out of interest how much thread depth do you get left for the head bolts? I am interested to see how the thread depth compares to my A7 barrels cheers
Title: Re: Con rod length
Post by: RDfella on 23.09. 2020 16:54
The article reproduced appears to suggest that fitting shorter rods will lead to increased horsepower. That is not necessarily so. Many factors are involved and to me the advantages (less reciprocating weight) are likely outweighed by increased friction, higher piston acceleration (potentially meaning lower rev limit) etc. There is, however, a sweet spot when it comes to choosing bore / stroke ratio. BSA did a fair bit of experimentation on this, and rather than re-invent the wheel I relied heavily on their conclusions when designing my present vee twin.
Title: Re: Con rod length
Post by: Rocket Racer on 23.09. 2020 21:54
ello RR I presume they are A10 barrels shaved down, so just out of interest how much thread depth do you get left for the head bolts? I am interested to see how the thread depth compares to my A7 barrels cheers
Yes thats an a10 barrel, its currently all still assembled but I am hoping to pull it apart soon and can check. We did have to check the bolt depths to make sure it would all pull down tight. I do have an A7 head here so can measure that too. When looking at a7 heads I was surprised how many I came across that had failed, either the flanges or across the back of the pushrod section. The short rod option while increasing piston speed does shorten the duration of the piston being stationary and the engine happily revs to 7500 if allowed to.
On my A7 thread I will be revisiting deck heights, rod lengths and bore and stroke to get an engine that will rev harder than I allow my 650 to (I try to keep in under 6500 as a general rule).