Valve Guides

[alanp]

This a question which probably Muskrat can advise on due to his racing tuning experience but there may be others - I've cleared up the excess casting inside the inlet and exhaust ports, particularly the exhaust which had a severe casting ridge tranversely across the ports just immediately behind the valve guides hindering smooth flow BUT the valve guides form quite a barrier to good flow, so I was wondering if anyone has shortened them a bit? I notice that my guides have a short counterbore (0.22" deep) at the port ends, presumably to avoid carbon buildup from jamming the valves in the guides. Now I can see some sense in this applying to the exhaust valves but not the inlets. Therefore, at least, I should be able to reduce the length of the inlet guides by 0.22" without detriment to valve stability since the counterbore isn't supporting the inlet valves anyhow. Any thoughts/experience?

[dpaddock]

What year and model BSA, Alan?

David

[alanp]

A10/RGS big valve head.

[MikeN]

I wouldnt want to shorten them at all on a road bike. If it were mine i would re-make the inlet guides the same length but without the countrebore to get the maximum bearing surface .
   In the past, I have done what Phil Irving describes in "tuning for speed" ( it wasnt on a BSA) and that is to streamline the inlet guides by filing them to an edge on the incoming charge side. also reducing the thickness on the sides a bit . I have no idea if it made any difference but it made me feel better.Not recommended for ex guides as thinning them down will make the valve  run hotter.
Mike

[chaterlea25]

Hi Alan
I have never seen a .22 counter bore in BSA guides, (nearly 1/4in??)
perhaps this was done by a previous owner who found pattern valves were fouling the guides
I have come across this before!!!
I have followed the info in Tuning for speed like Mike and have never encountered a problem
HTH
John O R

[Rusty nuts]

I have never seen a .22 counter bore in BSA guides, (nearly 1/4in??)

All the available guides made by Alpha I believe, have this very feature.

Rusty

[muskrat]

G'day Alan.
               you can shorten the inlet guides and slim them as Mike says but don't get too greedy in doing so. I prefer bronze guides over cast. Ex ports can be polished but inlet should be smooth with an etched finish to help atomization. Get a copy of Tuning for Speed, the more you can do from this book the quicker the motor will be. It showed on my lap times.
Cheers

[alanp]

Thanks all!
Muskrat, I can't do much work on the inlets due to its S shape single carb arrangement. It's crying out for resurfacing but.... I don't want to take the guides back out of the head to get further in, anyhow, it's only for beating the Triumphs on the A38, shouldn't really need to do much!
Alan

[muskrat]

G'day again Alan,
                       theres lots of other ways to improve things. Reciprocating mass (tappets, push rods, pistons) can be reduced eg alloy push rods, allen key type tappet adjusters, chamfered skirt pistons etc. 357 cam (new ones on ebay). But a well tuned standard with a few less teeth on the back will take them with a run up.
Cheers