Hi,
As part of and A10 iron head overhaul I am fitting new valve springs. The engine has a 357 cam so the valves and therefore the springs are for SR.
New springs bought from SRM, although in a box marked MCA, so the should be right. However, SRM do suggest checking for enough space for the springs to compress by the required amount plus a bit to avoid the springs becoming coil bound. To do this would require the full assembly of the head and rocker box with the possible need to strip it all off again so some comparissons.
New spring vs old - free length 55mm vs 52mm, wire gauge 3.8mm vs 3.7mm, fitted gap between coils 2.36mm vs 3.1mm. I have fitted a new and an old for comparison. So will the new ones become coil bound in use and cause untold damage to the valve train, Should im just refit the old ones which work or but more springs?
The attached images show the new spring on the left and old on the right.
Dave.
I have fitted Road Rocket valves to my iron head A10, it has Eddie Dow type top collars and Goldie type valve springs from Ebor Motorcycles. Top quality items, springs have a slightly smaller OD than the original type Rocket springs and fitted perfectly. I found that the commonly supplied springs to be poor quality, did not match the BSA published lengths and lost length after around 12 months of light use.
Ebor also do a set of light springs for checking, I found these very useful when shimming the springs to length.
https://eborbikes.com/index.php?route=product/category&path=65_59
Hi Dave. I do not understand why you think a 357 cam needs rocket springs. Are you aware that rocket models had a modified rocker box to accomodate the longer springs. As a former parts dealer I can assure you that I would have been desparate before I used MCA parts in my engine. The modified rocker box had more clearance for the longer springs.
Terrys own workshop book gives these dimensions:.
set 451454 1950-61 a10 golde4n flash /a7 star twin shooting star 1947-61
outer spring wire dia .156 out dia 1.312 free length 1 7/8" total coils 5 1/4 left hand wound.
inner spring wire dia 0.98 out dia .843 free length 1 17/32" total coils 6 1/8 right hand wound.
67-439/67-440
set 451463 1953-62 a10 rocket
outer spring wire dia .144 out dia 1.375 free length 2 1/8" total coils 6 1/4 left hand wound
inner spring wire dia 1.20 out dia 1.145 free length 1 3/4" total coils6 1/2 right hand wound
67-883/ 67-884
best regards, Pat A.
Hi,
The head has SR valves and so has the SR top caps so as far as I know it needs SR springs.
Dave.
You will need SR springs or the Dow type conversion with Rocket valves. Flash springs are different and dont fit the Rocket collars etc.
The photo shows an Ebor collar/springs fitted to an alloy head, the outers variable rate.
Hi,
The head has been in use for many thousands of miles at this SR spec. so the rocker box etc. is all fine. My concern is with these specific springs. They look a bit crowded and have the potential to be compressed by about 0.465" before becoming coil bound ( the old springs have about 0.6" before being coil bound). I suppose what would help is the amount of valve opening that the 357 cam gives. I think it is only about 0.329 so if my measurements are close then it will probably be OK otherwise different springs needed. Who stocks genuine 'Terry's' these days or is there another good maker?
Dave.
Hi,
A few more measurements.
With no springs fitted the gap from the underside of the cap to the top of the valve guide can be measured and came out at 0.3754".
So with the total of the gaps in the fitted spring being 0.465", I have 0.465 - 0.3754 = 0.0815". This seems to fit SRM spec. of 0.080".
However, if the lift on the cam is 0.325 then there is only 0.464" clearance. This could improve as the collets settle a bit and there is the tappet clearance to consider.
I reckon I have over thought this and should have just stuck new springs in.
I must admit, an Ebor Bikes spring kit looks tempting.
Dave.
Dave.
hi dave ,
the ebor valve retainers look like they give more space for the valve springs, whereas your s/r caps look like longer valves are needed.
Hi Eddie,
The valves are exactly the same length as before. The inlets are new and the exhaust are being reused so everything should be the same.
The new springs are of a slightly heavier gauge so have less room to compress. Interestingly both the new and old springs weight the same at 74g. The material must being used must be less dense to get the same weight.
The new collars weigh 27g and the original BSA ones weigh 23g. It does not seem a lot but that 4g is an increase of 17% and it is increasing the inertia to get it moving and and then the momentum that needs to be overcome stopping it thousands of time a minute. So valve seats and cams etc will be worse off.
Incidentally the old springs have done 15,739 miles and lost 3mm in free length.
Dave.
i had the same issue with valve springs for an a65 and didnt use them in the end.
i m not sure 3mm wear length is excessive and wonder if they are just run in as you are using a top collar that gives less height than standard.
Hi,
I did think that the used springs may still be usable but as the head is off it seems the best idea to replace them and know they are good for over 15,000 miles.
The collar fitted is the right one for the RR/SR valve which is right for the RR/SR springs. The valves must be a bit longer to allow more space for the higher lift of the 357 cam I reckon.
If the new springs had the same gauge as the old ones there would not be a problem.
Dave.
Hi,
A bit of a coda to this opera. I have ordered the Ebor stuff as I could not find quality assured springs and the collets currently available have the wrong profile to fit properly in the groove in the valve stem.
So all I need now is a credit with SRM for what I will be sending back.
Dave.