The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: emilios on 06.02. 2010 16:37

Title: A10 1956 valve clearance
Post by: emilios on 06.02. 2010 16:37
I set my 56 A10 iron head valve clearance to .10 inlet and .16 exsaust...its what haynes say...
I have hi compression pistons but i dont know what cam i use...
Is that valve clearance ok?
Title: Re: A10 1956 valve clearance
Post by: MG on 06.02. 2010 17:39
Hello emilios!

If you don't intend to thrash it, .008inch (0.2mm) inlet and 0.010inch (0.25mm) exhaust is enough. There's much less tappet noise with these settings.
Title: Re: A10 1956 valve clearance
Post by: trevinoz on 06.02. 2010 20:33
               It depends on which camshaft you have.
If you have 67-334, .010"/.016" is correct.
The other clearances are for the sporting camshafts.
Title: Re: A10 1956 valve clearance
Post by: MG on 06.02. 2010 21:36
trev - the .010"/.016" are stated for all iron head A7/A10 engines after no. AA7 101 (1952) afaik. Later models of the GF used the 67-356 cams too. I don't see the sense in using larger valve clearances with a milder cam. I think BSA recommended the larger valve clearance for the iron head models due to the higher temperatures than with the alloy heads, but like I said before, I never had a problem with .008/.010 when not thrashing the bike unreasonably.
Title: Re: A10 1956 valve clearance
Post by: trevinoz on 06.02. 2010 23:22
          You are applying cock-eyed logic here.
The iron head machines use exactly the same clearances as the alloy heads when the same camshaft is used.
The 67-334 cam runs quietly with the recommended clearances.
The most likely explanation for the large clearance is that the 334 has quietening ramps on the exhaust lobes.
Title: Re: A10 1956 valve clearance
Post by: MG on 07.02. 2010 09:33

You planted the seed  *smile*
Here is data taken from Roy Bacon's book "BSA Twins and Triples" (appendix). I sorted the valve clearances by model and year and stated which camshaft is fitted (this data being taken from Bacon's book "BSA Twin Restoration):

1946-50: .015/.015 (67-690/67-695)
1951-54: .010/.010 (67-334)
1955-59: .010/.016 (67-334)
1960-61: .008/.010 (67-356)

A7 Star Twin
1949-50: .015/.015 (67-695)
1951-54: .008/.012 (67-356)

A7 Shooting Star
1955-59: .008/.012 (67-356)
1960-61: .008/.010 (67-356)

1950 only: 015/.015 (67-334)
1951-54: .010/.010 (67-334)
1955-59: .010/.016 (67-334)
1960-62: .008/.010 (67-356)

A10 Super Flash
1953-54: .008/.008 (67-356)

A10 Road Rocket
1955-57: .008/.008 (67-356)

A10 Super Rocket
1958-59: .008/.008 (67-356)
1960-63: .008/.010 (67-357 probably export only in 60-61, 67-356 for home, used for all from 62)

A10 Rocket Gold Star
1962-63: .008/.010 (67-357)

You can see that there are slightly different settings for one and the same camshaft used in different models. To make things worse, there are also yearly changes.

Here's another quote from "BSA Twin Restorations" (p. 57, Camshafts):
"In general it is best to run what is specified for your model and set it to the correct timing for the year. If you have a sports camshaft in an iron engine use the original gaps, which are set by expansion rates and not cam timing, but remember to keep an eye on any non-standard set-up while it is settling down."

I personally would not consider that to be so crucial. The original settings for a 356 cam in an iron head engine don't differ much the alloy head settings (bit more clearance on the ex. valve). And experience shows that the smaller clearances are okay when riding at a reasonable pace.
But Trev, you are basically right, the cam makes the main difference. Thanks for putting my thoughts right.  *ex*

So it would be a good idea for Emilios to check the valve timing to figure out which cam is fitted.
for the 67-334 this would be: i.o. 30, i.c. 70, e.o. 65, e.c. 25
for the 67-356: i.o. 42, i.c. 62, e.o. 67, e.c. 37
(data taken from "BSA Twin Restorations", p. 209)

Regards, Markus
Title: Re: A10 1956 valve clearance
Post by: emilios on 07.02. 2010 14:12
Thanks guys..
Im waiting my magneto to come from england(i send it for overhauling).
As soon as i get magneto i will set ignition and i will check what cam im using...
I have to tell you that my bike is 56 iron head(plunger) but i thought it was 53 model so i set clearance 0.10 both valves when i
bougth bike from england...
Bike was working fine for 2 years...???
Title: Re: A10 1956 valve clearance
Post by: MG on 07.02. 2010 15:58
My guess is that there were problems, maybe cases of burnt exhaust valves, when the bikes were stressed (thrashing, sidecar use, etc.), so they increased the ex valve clearance. Power output and therefore thermal stress also rose during the years.
Maybe that's the reason why they changed the settings for the later models with the same cams, but this doesn't necessarilly mean that the smaller gaps WILL cause problems. It might be just fine when the bike is not pushed too hard.
Title: Re: A10 1956 valve clearance
Post by: RichardL on 07.02. 2010 20:26
I wouldn't dare get in the middle of the debate over which settings to use for the valve clearances, because I have way more to learn about it than to add to it. However, based on this discussion, I did conjure up (without trying it) a way to determine which camshaft is in place. Here it is:

Review the cam lifts shown at

Notice that there are rather large differences in lift between the 334 and 356. For example, exhausts at approximately 0.28" (334) vs. 0.305".

Loosen one of the valve adjusters (let's say on an exhaust) so it comes very close to touching the valve when the cam is at maximum lift. Using your feeler gauge, measure the clearance at this point, for discussion, let's say it's 0.005"

Now, turn the engine over until the cam is at minimum lift and the adjuster is at its farthest from the top of the valve. Measure this. If you get something less than, say, 0.290?, it would seem certain that the cam is a 334. If you get something over 0.305? you probably have a 356, unless you are over 0.340?, in which case it would be a 357 (not likely, I suppose). If you get something between 0.290? and 0.0305?, you probably need to refine and redo your measurements. (Remember, I?ve based these ranges on starting with a clearance of 0.005?, if you start with something different, you would adjust the arithmetic.)  You will probably need to use a stack of feeler gauge blades to measure the minimum lift value. I thought about trying to come up with a spreadsheet to choose a range of sequential feeler gauges to put together to get a desired thickness result, but I think the number of possibilities is 2exp24, so I decided against it.

Anyway, I hope this either helps or is amusing. I also hope it?s not boring from being too obvious.  

Now, I will watch football.

Richard L.
Title: Re: A10 1956 valve clearance
Post by: MG on 07.02. 2010 21:00
basically a good idea, but the diagrams on the site show cam lift, not valve lift, so you have to include the lever transmission caused by the rocker arms into your arithmetic!
But would be okay for a comparitive measurements.

Cheers, Markus
Title: Re: A10 1956 valve clearance
Post by: RichardL on 07.02. 2010 23:04
You're so right, while I am wearing an egg facial. Anyway, I did confess my ignorance in advance. Having gone this far, and if it matters, if someone can provide the rocker geometry, I would be happy to do the math.

Richard L.
Title: Re: A10 1956 valve clearance
Post by: Brian on 08.02. 2010 00:11
While there are obviously different recommended valve clearances I run all  mine at .010", inlet and exhaust, regardless of camshaft. The one exception being the Star Twin which runs .015" being the longstroke motor.

Now while I dont consider that I "thrash" my bikes they do work hard. Last week I went for a ride on the 51' A7, when it came time to come home, a 150k run, I had  a strong headwind and it was 38 degrees C. I sat on 60mph all the way, when I stopped you couldnt touch the oil tank, I reckon I could have deep fried chips in the oil !

The thing is I have done this many times on different bikes and never had a problem, they will tolerate a lot more than most think.

I am fairly confident that if your clearances are somewhere between 8 and 12 thou you will be ok.

What I am getting at here is by all means set the clearances to the recommended settings but dont loose any sleep if you think they may be a thou or two out.
Title: Re: A10 1956 valve clearance
Post by: MG on 08.02. 2010 09:14
Brian, absolutely agree. Like I said, I run mine on smaller clearances too and never had a problem. Esp. with the alloy heads a larger clearance is well audible, and I prefer my engines to be mechanically quiet.
And if this is okay for Australian temperatures, I probably never will experience any probs here in Europe.

***story-mode on***
In my days as a student I once purchased an old Volvo 440 (with a 1.9l Renault Diesel engine, these don't have hydraulic valve lifters, but tappets need to be adjusted with shims). Obviously the PO never had the valve clearance checked (or adjusted) during the 180.000 km the car had covered. Two of the ex valves were down to a clearance of 2thou  *ex* (with engine being cold), and still it ran without any problems. Nevertheless it was a good purchase, cost me 100 Euros because the defective electric power-steering pump had caused a cable fire. Was an easy fix, though, like the valve clearance. Just grinded the shims down to fit with fine abrasive paper on a flat surface  *smile*
***story-mode off***

Cheers, MG
Title: Re: A10 1956 valve clearance
Post by: BSA_54A10 on 10.02. 2010 10:48
I too like a quiet engine so I tighten the tappets till they are "just able to be rotated" when hot and the cylinder is on firing stroke . Works for me.