The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: sprint on 20.05. 2014 14:17
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What is the difference between 67-1549 & 67-1571 Cyl Hd's?
It is my understanding that there was never a specific RGS head but just the large valve version of the SR engine, 67-1549 with 1.5" inlet valves instead of 1.455" fitted?
So what is a 67-1571 head, which is often referred to as a RGS head?
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My book notes 'em has 67-1122 a7ss,67-1126 road rocket, 67-1549 super rocket and 67-1571 rocket gold star .
The tech details have been lost to me ages ago but always assumed ( and we know what happens to assumptions ) that there was a difference in the RGS head and the super rocket but given BSA's management they could just have differentiated to add a few pounds/dollars to the price. I have heard 'em called big valve but just thought it was for the american market and hep etc, be interesting to get the full SP from more knowledgeable members , regards Bob.
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Bob I'm sure his has come up before (I had a big valve head) the difference in valve size is very small, someone will correct this but was it 3 or 4 thou, sounds not a lot but maybe did give room for a bit more gas flow.
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Bob I'm sure his has come up before (I had a big valve head) the difference in valve size is very small, someone will correct this but was it 3 or 4 thou, sounds not a lot but maybe did give room for a bit more gas flow.
Inlet valve on the 'big' valve head will be 1.5" instead of the std 1.455". But it seems that both 67-1549 and 67-1571 can be 'big' valve heads?
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The "big valve" is 37mm as against the "small valve" 36mm.
The 1-1/2" valve is an over size valve and to my best knowledge was never fitted as standard in any head.
The 1571 head was not exclusively fitted to the RGS, the RGS engine being a standard engine.
I have pondered this question for some time now and comparing the heads, there seems to be no noticable difference.
The best that I can come up with is that the 1571 head was used when a 1-3/16" carburettor was fitted.
My RGS has a 1549 head and my BVSR has a 1571.
Trev.
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Is the info here not correct then about the 'big' valve inlet and exh valve sizes? http://atlanticgreen.com/a10alloyhead.htm
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Thanks Trev and Bill for info will store away. Anyone have an ideaof the biggest valve poss on an iron head. I got SRM to do this to mine along with a spitfire cam plus follwers but can't remember how big they were. They are a standard size as I obtained a replacement easily when it was needed due to my idoticy . Have an idea that it wasn't much smaller than these mentioned, any ideas welcome regards BobH
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Sprint,
I don't know where the bloke on that site got his information but I can only go on what I have and that is 37mm inlet valves on both heads.
Trev.
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Hi Guys
Sorry. I know I don't post here much at all.
On my BSA web page for heads (http://atlanticgreen.com/a10alloyhead.htm) I simply documented what I had in the heads I own. I have at least a dozen and probably more than that, counting 3 different dual port heads.
I am in agreement with the position that RGS engines are the same as big valve super rocket engines. The only thing I do believe, is that the advertised dyno run in and testing proved it met or exceeded the minimum performance for the RGS. Therefore this added testing could account for the different engine part #42-0202, though the build components could have been 100% identical as the BVSR.
I have NOT seen or examined a 1571 head or it's markings. So you all have one up on me.
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Hi Dave,
What numbers are on the heads of your SR and RGS ?
Regards
John
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Hi Guys
Sorry. I know I don't post here much at all.
On my BSA web page for heads (http://atlanticgreen.com/a10alloyhead.htm) I simply documented what I had in the heads I own. I have at least a dozen and probably more than that, counting 3 different dual port heads.
I am in agreement with the position that RGS engines are the same as big valve super rocket engines. The only thing I do believe, is that the advertised dyno run in and testing proved it met or exceeded the minimum performance for the RGS. Therefore this added testing could account for the different engine part #42-0202, though the build components could have been 100% identical as the BVSR.
I have NOT seen or examined a 1571 head or it's markings. So you all have one up on me.
I now have both a 67-1571 and 67-1549 (cylinder head cast No's). The only difference I can see is that the 67-1549 uses the long shouldered nut to hold the rear of the rocker box and the corresponding studs will be shorter?
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Sprint,
All alloy heads use the sleeve nuts on the rocker boxes.
If you have one without, it must have been changed by someone in the past.
Trev.
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Sprint,
All alloy heads use the sleeve nuts on the rocker boxes.
If you have one without, it must have been changed by someone in the past.
Trev.
Thanks Trev
That leaves me a bit confused?
The 67-1549 that I have just obtained has the rear cly hd holes counterbored to take a sleeved nut, which I assume requires shorter studs in the inlet end of the rocker box, see photo.
My 67-1571, on bike at moment, simply has the same holes just parallel bored 5/16" clearance and uses std nuts and washers. and as a result the rockerbox inlet end studs are long, which is a pain as you have to remove them before you can remove or fit the rockerbox with the engine in the frame.
A friend has a 57 RR and 60 SR and they also have long studs at the inlet end and std nuts and washers, though I don't know what heads he has or if the rear holes are counterbored?
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Hi Dave,
What numbers are on the heads of your SR and RGS ?
Regards
John
The SR is head is 1549 (just went and checked at the warehouse) , and the RGS (GA10-15xx) was a basket case and I have several 1549 heads to choose from (no 1571's) but one head came with the bigger valves. However...I may go for one of my late 650 twin carb heads and 389/689 monoblocs.
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Sprint,
I can't explain why your head is not counter bored.
I have all four Rocket heads and they are all bored.
Even the 1954 SS derelict head is bored.
Has someone put sleeves in the holes?
Trev.
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counterbored to take a sleeved nut, which I assume requires shorter studs in the inlet end of the rocker box,
only have one alloy head but the rocker studs are long (all four), do you have the correct sleeve nuts, there are/is different sleeve nut/s used elsewhere on the bike.
Also requires special washer to sit between sleeve nut and head, and as I recall it takes a bit of fiddling about to get the sleeve nut up into the counterbored bit and started.
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counterbored to take a sleeved nut, which I assume requires shorter studs in the inlet end of the rocker box,
only have one alloy head but the rocker studs are long (all four), do you have the correct sleeve nuts, there are/is different sleeve nut/s used elsewhere on the bike.
Also requires special washer to sit between sleeve nut and head, and as I recall it takes a bit of fiddling about to get the sleeve nut up into the counterbored bit and started.
Hi Bill
I don't have any sleeve nuts for the rocker box studs just plain washers and nuts.
You say that all of your rocker box studs are long? What length are they?
I can only estimate mine as they are on the bike, but the front one look to be about 1 3/4" - 2" and the rear ones about 2 1/2" - 2 3/4" long. If the front ones were any longer I would not be able to get the nuts on between the gap in the fins at the front.
From the photo you can see that the rear studs are long and have to be screwed in after the rocker box has been lifted over the push rods. There is also insufficient thread to take a sleeved nut. As previously indicated a friend has a 57 RR and 60 SR with the same arrangement with long rear studs and no sleeve nuts?
How long is the sleeve on the nuts which I assume go some way up the counterbore? Can you remember if the ends of the studs are hidden inside the counterbore?
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Hi Sprint
Not sure if I have a spare stud, I'll have a look tho
this page [urlhttp://www.draganfly.co.uk/index.php/bsa/bsa/a710--b3133--c101112--m202133/category/867-pre-unit-a-group][/url]
will show the numbers for the studs ( No 11 ) in the list ( note front and rear are different for rigid and plunger could this be a factor)
Think maybe the front are correct and the rear ones are wrong as mine goes on with the studs in the rocker box
Also here http://www.draganfly.co.uk/index.php/accessories-a-misc/product/13923- (http://www.draganfly.co.uk/index.php/accessories-a-misc/product/13923-)
picture of the rear extended nut, you might get an idea of the size from the pic (don't think I have a spare to measure but I'll look
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update
just been down the shed, took the extended nut off the r/box (it was only finger tight - phew)
total length including hex on end is 25mm special washer is 2mm thick and width just covers the nut head
Estimate of the stud length I judge to be 35mm from the base of the rocker box ( you can just feel the end of the stud with end of your pinkie - does not protrude through the head)
a word of warning I bought a set of long studs from Draganfly that turned out to be wrong (unusual for them I've always been happy with there stuff)
The threads were wrong, the whitworth bit that screws into the box was too long (result cracked box) so double check before fitting them.
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Thanks for the replies Bill and checking the length of your sleeved nut, looks like it was just as well that you have had a look your nuts!
It does seem strange as to why mine and my friends RR and SR, all with alloy heads, have been fitted with longer studs to the rear with std washers and nuts? Must have been a std BSA build at some stage as I can't imagine that all three could have just have been randomly converted by owners as it makes it impossible to remove and re-fit the rocker box with the engine in the frame without first removing them.
I guess one of those many mysteries, the answer to which is now long gone?
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All these aluminum heads I have are bored for the sleeve nuts... 5-1549, 3-1126, 1-1122, 2-1106dual port, 1-1102dual port.
Again, no knowledge of 1571 heads.
plus 3 late big fin iron heads, I should never have sold the early small fin head. *sad2*
The end of the rear studs are inside the bore of the head and do not stick out.
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Sprint,
I would be inclined to make it right and use the correct studs and nuts.
Years ago I put an alloy head on my Flash and not having the correct fasteners I removed the rear studs and used Allen screws.
I don't believe that BSA would have used sleeve nuts on the majority of alloy heads and not on a few.
Trev.
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Just a note on the washers for the sleeve nuts.
They are 7/16" x 11/32" x 0.175". A bit more than 2mm thick, Bill!
Trev.
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yep - picked up a head bolt washer I reckon, there in the same bag
the washer I bought with the extended nuts is 3mm - probably depends on supplier
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Just a note on the washers for the sleeve nuts.
They are 7/16" x 11/32" x 0.175". A bit more than 2mm thick, Bill!
Trev.
Correct parts now including 3mm thick small O/D washer.
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Just a thought; when you're opening up the inlet ports, one place you have to be real careful is around those sleeve-nut holes. Easy to break thru there. Sleeving those holes would make sense if the head has been ported.
Can you detect any sign of something having been pressed in there? or indeed if port work has been done?