Tech Tip: BSA A50 & A65 Valve Springs That Coil Bind

Tech Tip: BSA A50 & A65 Valve Springs That Coil Bind

 

Rebuilding a BSA cylinder head is pretty straight forward once you have all the parts and tools at hand 

New valve guides, valves, retainers, springs (the whole she bang)

There are many critical aspects to consider when recondition a cylinder head such as...

  • Sizing valve guides
  • Installed spring height 
  • Rocker arm ratio
  • Valve stem protrusion  
  • etc

Although all of the points above are important, there is 1 major detail that shouldn't be overlooked..

If you fail to check this one step, it could wipe your cams out, bend push-rods and / or damage your engine 

If you haven't already guessed it, in today's Tech Tip blog post we will be covering valve springs specifically for 1966-1972 BSA A50 / A65 models

Before we begin this post is to merely make you aware (and educate you) of the problem and to prevent any future harm or damage to your vintage BSA engine 

Lets dig in..

Generic BSA 1966-1972 A50 / A65 valve springs

Generic BSA Valve Springs


 A few years back there where BSA A50 and A65 valve springs being sold on eBay by both retail and private sellers that caused some unfortunate issues such as engine failure and damage due to coil binding and extreme seat pressures 

The "generic" valve springs we are referring today are typically sold as 1966-1972 A50 & A65 springs under the part number 71-2222 and a few others

You may spot the springs in reference by a blank box with no labels or markings within the sellers photos...

 

Testing the generic valve springs 

Rimac Valve Spring Tester Testing BSA Valve Springs


If you have seen these springs before, you might have visually noticed that the generic springs have 6 thick coil wires with small spacing between each wire like the ones shown further above  

Since I happened to have a set of generic valve springs here at my shop, we will put these springs to the test 

The test I will be performing will cover both seat pressures; opened and closed 

I will be using a 1980's Rimac valve spring tester tool to calculate and record all figures 

I used the following figures taken out of a BSA factory workshop manual to produce my results

Here are some basic specs on the generic valve springs (all specs are approx)

 

  • Outer spring length @ 1.805" 
  • Inner spring length @ 1.407"
  • Outer coil wire thickness @ .160"
  • Innner coil wire thickness @ .119"
  • Outer Coil wire count @ 6
  • Inner coil wire count @ 6

 

Keep in mind that it is uncommon to find springs made today to the exact specs as original springs

BSA A50 / A65 models where through about 5-6 different valve spring assortments through the 10-year range 

Many manufactures have there own "blend" of springs that work but may be longer, shorter, progressively wound, etc that will work with a given application 

 

1966-1972 BSA A50 / A65 Springs ("generic" spring) testing results 

 Spring type Seat pressure Cam type Installed Height
Generic spring Open Rate @ (coil bound) .306" lift 1.300 or 1 5/16"
Generic spring Closed Rate @ 90# .306" lift 1.300 or 1 5/16"

 

Generic BSA Valve Spring Analysis


From the results above you can see a few problems that could arise during an engine break-in or a high speed run

Right off the bat at the open rate testing using a .306" lift cam, I get a coil bound condition on the Rimac valve spring tester 

I have seen a few installed height figures in various BSA workshop manuals that recommend 1.300" or 1 5/16" - using both of these figures during the test, I get a high (closed) seat pressure and coil binding before full lift  

The biggest issue I see with the generic valve springs is that the coil wire count is 6 - If the springs had 5 coils they would work, and in turn that would drop the seat pressure (open and closed) and would allow you to run slightly higher lift cams without coil binding..

 The only real practical way I could see these generic valve springs working is under a condition where you have worn valve seats and your valve is "sunk" exceeding the 1.300 or 1 5/16" installed height 

 

Springs that don't bind...

 

Comparison - Generic BSA Valve Springs & "Premium" Valve Springs


After reading the test results above, I'm sure you are now wondering, what springs should I use?

There are a few springs on the market sold by others (and us) that work and will provide satisfactory results for basic and mild road applications 

The springs that we use (and sell) here at Classic British Spares will fix your A50 and A65 coil binding issues when used with a stock or mild grind camshaft 

Malcolm and Kyle have been both tested and proven our "premium" springs in our own personal bikes and customers bikes including A65's and B40's

Our springs have been proven to be on par with original (now hard to find) valve springs with very minor differences 

If you are interested in learning more on the springs we sell (like the one shown above) or to purchase a set you can click on the links below

Purchase BSA "Premium" valve springs (A50, A65, etc) Click Here
BSA "Premium" valve springs blog / tech Click Here

 

Final thoughts 

 Final thoughts

I cannot stress enough to check every single part that you install in your old BSA engine

Unfortunately a small amount of defective parts or parts made to the wrong specs do exist on the market and shouldn't be used or installed without prior checking and examining 

Most folks don't think that valve springs would require a lot of attention, when in fact they do

It's the time when "you think" something is ok but in reality, its not..

 

Thanks for reading 

Thank you for reading todays Tech Tip post 

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See you on the next post!  


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