Hitchcocks Forum

Guys

I need some advise. I have a 59 Bighead Bullet that I bought after a "build" into a 612 with sports cams etc etc. It had not ever been run or wired etc. After redoing lots of the "build" - I got it running and bent the inlet pushrod. I then found it had been fitted with the standard valve springs and was suffering spring bind when running (it all seems fine when turning over cold - but I have measured the gap between spring coils when compressed and it is below spec. I have tried asking our host for more info and this morning got the part number of some springs that may help. I have asked what the difference in the competition springs are relative to the standard - but I can not get an answer. Can anybody advise ?

Hi Citrus, there are a few possibilities here - first, the cam may have too much lift [very unlikely], the valve may be a little too short, again unlikely but possible if it is from something else for instance. More likely, the springs may be incorrect for the job, or someone may have shimmed them up at the spring seat and overdone it. If the valve seat has been changed, perhaps it has not been cut deep enough and needs cutting further. I don't use the alloy spring caps in our racers, as the centre ripped right out of one once, dropping a valve as a result - I am guessing your spring caps must be under considerable strain if you bent a pushrod, so you may want to fit new ones for peace of mind. I don't know off hand what the poundage of our host's competition springs are, but if you have access to a drill press, a set of scales and a ruler, you can work it out. I usually aim for a fitted length of around 35 - 36mm between spring seats and spring caps on the outer springs on our short stroked 350 Big Head racer, but the springs in that are sourced elsewhere, although their dimensions are very similar to those from our hosts, the valve spring caps are titanium replicas of the alloy ones. I usually aim for a little over 1 mm clear of coil bound at full lift and this engine revs up to 9500 rpm with no problems. Regards, Paul.

Great - thanks for your help Paul
I did some research yesterday with various springs out of my "stock" and found a significant difference in the way they compressed. I have fitted a NOS set to the head and will refit the head tomorrow and measure the compression and coil gap etc. These ones have a much more "even" compression
Off for a ride on a different Enfield today !!
Impressive results for your engines - well done

I had about a week away from the bike finishing a refurb of a couple of rooms in the house - but back to it now and thought I would close this out with the results.
The NOS springs I fitted bound (found on assembly) - so it was back to getting a set of sports springs from our host. They arrived and it was interesting to note that the bound spring length of the outer was the near the same as the standard (0.80" versus 0.77"). The inners were way different (0.70" to 0.50"). Given the step height of the base and cap for the inner - the inner was binding first on the standard by a long way (total step difference = 0.185") - so now the 2 springs bind at the same time. Makes sense.
BTW - I did check the collect to guide distance and there is heaps of room at full compression of the springs (about 0.30" gap)
So for anybody fitting the sports cams in a big head - it is important to at the very least change the inner springs. It is difficult to measure the inner spring clearance unless you assemble without the outers.
Anyway - that was not my complete problem. !!
Another issue with mine (and something to watch out for) is that the spring base was not sitting on the head - it was being held in the air by the guide. The guide was pressed in properly - so it appears that the guide is machined incorrectly (over length). Note - it was a new guide purchased from our host and fitted by the previous owner. To save removing the guide etc - I had a spring base that was the correct OD, but needed machining out to fit over the guide. I did not measure that - but it was about 0.5mm more spring length that I got
The bike was reassembled and run yesterday - now all good !
This is all part of picking up a project that has been 80% done by somebody else and finding all the things that were either done incorrectly - or incorrect parts installed !
It is a hobby and we all love our hobby

Glad to see it was fairly easily fixed once you'd discovered what was wrong. The joy of finding the stuff that ought to fit together but doesn't quite is all part of the fun, almost as much as making stuff that shouldn't fit together properly do just that.



Do you fancy posting some pictures of your bike?



A.

It is intended that the guide holds the spring base off the head on these bikes, this does two things, stops heat transfer to the spring and keeps the guide firmly in place.



You may have cured the symptom but not the actual problem?. It could of course run for many years like this without any problems?.

Revband - thanks for that info. Interesting. Most of my other bikes are CI heads and they all sit flat - so I had assumed the same. btw - the guide will always still be pushed down by the inner spring. Will run with what I have in. I am not racing it so should be fine as far as the heat goes.

Yes it is a bit unusual, there are other maker's who do this but many that don't so it should be fine?.

I only did it on the inlet side - as the exhaust was lower
Inlet running cooler - so I am happy

Appreciate your comments