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Jon W. Whitley
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#857360 08/31/21 7:37 am
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Greetings all.
I have a fairly standard 1971 A65 Thunderbolt. I've bought Kedo ball foot adjusters and SRM steel (chrome moly tube) pushrods. There's a lot of contradictory information out there - Much of which defies logic.
I'm thinking I can reduce clearances.
Standard - with alloy pushrods is 8 and 10 thou.
Megacycle seem to recommend 5 and 7 thou with their mild cams and (presumably) standard pushrods
SRM recommend staying with 8&10
Logic / material science suggests the pushrods expand about 4 thou less.
Has anyone experience running tighter clearances?
Its a touring bike not a racer - so doesn't run red hot or rev high.
Cheers
Steve


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I have some moly tube to make some pushrods and have been wondering what clearances to use when finally fitted. My plan is with the recommended steel clearances, if there are any, and run the engine until up to temp and then set the clearances to 1 thou inlet and 2 thou exhaust, then let the engine cool overnight and next day measure the cold clearance and record and use that from then on. I would do one further test which is to start it from cold and run up to running temp just to make sure at no point in the warm up cycle do the clearances go negative due to one part warming up faster than another.

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Hi. That sounds thorough - and would probably give the best real world answer. As far as I know there is no recommended steel tolerances as its not a standard configuration. I guess you could start with standard 8&10
Steve


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I run the chromolly pushrods and have used numerous cams (Spitfire cam, SRM Race cam and Megacycle X12). In this I have also messed about with different valve clearances. What I'd suggest is to stick with .008 and .010


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)

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I put mushroom headed adjusters in and with alloy cyl use virtually no clearance, because the alloy expands more than C/molly. Plenty of clearance hot. Measuring what it is hot and cold with stock iron cyl and alloy push rods for comparison would be interesting. Just the c/molly push rods is one thing but alloy cyls adds to that different expansion.


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My T'bolt is pretty standard and uses 6-8 cold with flash (CM) pushrods.
The extra 0.0001% power gained is incredible..........

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Nick - Thats interesting - is there any noticeable reduction in tappet noise?


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A little better, i hate rattly engines , wonder why i ride a beezer or triumph eh?
Most noise comes from not just the tappet clearances but the whole assembly.
Start with timing gears, cam followers then the rocker setups all add up.
You won't get them quiet, but you can get them acceptable, when you do the
tappets do the old 'exhaust just opening- do inlet, inlet just closing- do exhaust'.
Plus put a little tension on the back of the rocker arm when adjusting. I used to
do them hot and close the gaps up a bit but they come out very similar.

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Thanx Nick. Pushrods have just turned up so I can get going :-) I will follow your advice.


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Allan - That's covered pretty much all options! Did you get problems with tighter clearances? Also - as a matter of interest - whats the SRM 'full race' cam like?
Cheers
Steve


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Unit singles owners who have the Rupert Ratio Manual may have noticed that instead of the usual 8 to 10 thou, the recommendation is to adjust the tappets so that there is a small amount of perceptible play. I've tried this method and it does work, I haven't experienced any valves burning or other issues and it does quieten the valve train down.

Of course I don't recommend this technique on other bikes as the designs and materials may be completely different but I guess it does show that you can get away with smaller clearances in certain circumstances.


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Nick can also answer about the SRM cam. Gary from there raced with it and was very enthusiastic.

Quite a bit of rattle goes away with alloy nicasil cylinders and modern forged pistons that can run clearance in the range of .002-.0025". Timing gears are strong and dependable but do rattle, Honda and others added spring loaded split gears that replace rattle with whine, it sounds more precision but works the same. You can see backlash in timing gears with a strobe doing timing going on and off the throttle. The mark jumps. Some are better than others. But pistons are quite loud, unless the alternator nut is loose and it's rattling on the key. Then that wins.


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The SRM race type cam is overall very good, it's not radical and is quiet.
Engines will idle a little faster but evenly with it. It does give more mid/top range pull.
On a road bike i would spend the time on the head first though, as Mark knows,there
are huge gains to be had there before going to a hairy cam. The spitfire cam is very good.
A set of larger inlet valves and some work on the head would be my first step. PM make
larger inlets and they will go in without cutting the seats back too much. I have these in my
road bike and it is a very nice old thing to ride, it hasn't spoilt the flexibility and gives a nice
increase in mid range pull.
Just my 2c.

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I used the SRM race cam with one of my small port heads with small 38mm valves. also with other heads modified and standard. But it worked best with the really reduced small port head the best along with the 28mm carbs (bike is a 650). It also completely transformed the bike. Standard gearing it wheelied very easily (too easily). I used .010” lash on inlet and exhaust. I also used 3/4 radius tappets. From memory the timing was similar to the sifton 380.

The next time I had the engine apart I fitted the X12, the darn thing was a few thou over size on the timing side though I haven’t heard anyone else have the same issue. The cam isn’t anywhere near as good and it is supposed to be their road race cam. One I reamed the bushes put it meant I couldn’t re-fit the race cam without fitting new bushes too so it stayed in. It was more expensive and not worth it in my opinion. Performance is better than the standard 68-473 spitfire cam, but if you retard the spitfire cam a few degrees that then performs much better than the X12.

When I retarded the spitfire cam I did it a little too far and it developed a flat spot around 4000 rpm (not where you wanted it). If you knocked it down a gear and got it above 4500 rpm it went really well.

(I also used to have a lot more time on my hands)


Now let’s all have a beer beerchug

68’ A65 Lightning “clubman”
71’ A65 823 Thunderbolt (now rebuilt)
67’ D10 sportsman (undergoing restoration)
68’ D14 trials (undergoing transformation)


Moderated by  Allan G, Jon W. Whitley 

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