The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Pecon on 27.12. 2013 15:30
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Hello All
I had a question under the heading "Timing slip" in the electrical section. Well now that the turkey and ham are long gone I decided to go through all the suggestions received but it was only when I came to adjust the tappets that I found that one of the gaps was 1/4 inch on the exhaust valve..!!!. I pulled off the rocker box and found that one of the pushrods was melted onto the cup of the rocker. The rod had shortened by 1/4 " compared with the other. I'm still puzzled as to what might have happened as the bike has done about 700 miles since a rebuild without missing a beat. I did find getting the pushrods into place a fiddly job but I double checked to make sure that they were all seated properly.
I'll place an order for a new set from SRM in the near future but I am concerned about one thing. There appears to be a hole through the pushrods and also in the rocker itself. Is this an oil feed?. The hole in the rocker appears clear but is there an oil feed through this to the valves or will the oil feed to the rocker box be enough.
Peter
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one of the pushrods was melted onto the cup of the rocker. The rod had shortened by 1/4 "
!&! Any pictures?
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This may be helpful, or not.
http://www.bsawiki.co.uk/index.php?title=Chapter01
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A few photos, apologies for the quality but you should be able to make out the melted alloy on the rocker and the damage to the pushrod.
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Wow. I always thought pushrods where supposed to have tips with special hardened metal.
Sure it's melted? (if you ever reached that kind of temp, lots more would probably have gone bad as well), looks more like worn down and 'disfigured'. And the missing metal must have gone somewhere, check the oilways.
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I'm thinkin' the hardened end must be in the sump or, maybe, the camshaft trough (though, not sure it could live there with the cam turning).
Richard L.
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Hi Peter.
Not sure if you are aware the push rods are of different lengths, one pair for inlet and one pair for the exhaust.
The two long rods are the inner ones and the two short the outer ones.
The oil feed to the rockers changed during the lifetime of the A10, the latest was an external feed to both rocker shafts then oil mist to the valve gear.
There should be no hole in the push rods.
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Believe it or not they started off the same length *sad2* I forgot to mention that the bike is a 1955 plunger and while I know that there were changes over the years I'm not sure if these are the correct pushrods. Who knows what has been done over the years.
Peter
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Hi Peter,
My opinion is that alloy pushrods are a problem waiting to happen !!!!!
Theres no advantage using them in a road bike
At least you have found the problem,
Regards
John
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Hello John
I was under the impression that they were all alloy, is there a better alternative and where is the best place to get them?. I had a look at the SRM site and was going to buy just one exhaust pushrod as all the others seem fine or is that a false economy ?.
Peter
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Peter,
I have seen that same problem with alloy push rods.
Find a set of original steel rods and eliminate an ongoing headache.
Trev.
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HI Peter,
I would buy a new set,
SRM's are steel and I have used a few sets without issue
John
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Ok steel it is then. Does anybody have any thoughts on why this might have happened in the first place?. As I mentioned I was particularly careful to ensure that the rods were correctly positioned during assembly and the bike ran perfectly for about 700 miles up to the time I started to notice flames from the carb at about 3000 rpm. I take it that the fact that the exhaust valve on the left cylinder was barely opening might have somehow forced the hot gas back through the inlet?.
Peter
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I would suspect insufficient tappet clearance in conjunction with a too soft alloy.
You mention the bikes a plunger, so iron head and barrel. The original hollow steel push rods would be more stable in length than after market alloy ones which will expand taking up your tappet clearances.
once you had some alloy bonded in the top cap area it would have worn against the balance of the rod resulting in rapid wear.
If the pushrod was a good rotating fit into the rocker it is an unusual problem.
On my alloy headed road rocket I do run alloy push rods (they came with the bike) and have no issues with them but personally wouldnt fit them in an iron engine.
What cam are you running and what tappet clearances?
Find some reputable pushrods, preferably some steel ones, fit them carefully and make sure your cam followers and rocker cups are free of debris and keep an eye on you tappets until you have some confidence in them.
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Hi RR
The engine was overhauled by SRM and the cam that they fitted is described in on the receipt as A7/A10 Super Rocket cam part number 67.0356. I honestly can't recall what clearances I used but I do remember that the tappets were very noisy at 10 and 16 thou so I reduced the exhaust by just a couple of thou to reduce the noise.
What you say makes sense if one of the rods got stuck in the cup. I noticed that there are no steel caps on the rods just all alloy, is this normal?. I removed the sump filter and it was full of slivers of alloy, presumably from the pushrod. Hopefully these were soft enough to be chewed up without causing damage to the rest of the engine.
Peter
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Hi pecon. Ive seen alloy rods which have too large a radius on the ends so stick in the rockers. This was a wel known issud for a while. Maybe you had some of these?
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I've had alloy push rods in mine for over 20 years. On average they wear less than a thou a year. The ends are polished.
I have seen on flebay alloy rods with steel ends, but if your not racing, std rods will out last most of us.
Cheers
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Your tappet clearances were too high for a 356 cam, those clearances are for the 334 cam.
Yours should be .008" & .010"
Trev.
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This thread got me thinking about weights of the items:
standard steel pushrods: 171.5 grams for a set
parallel alloy pushrods: 127.5 grams for a set
my shorter wide section alloy pushrods (off bender) with relieved ends are 89 grams the set.
so not all alloy pushrods are the same, and a steel pushrod is about 10 grams heavier than its normal alloy equivelent
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Thanks Trev, perhaps the incorrect clearances did have an affect on the rods, I'll rectify that when I get the replacements.
RR I was not aware that there were so many variations on the push rod types. I assume that the lighter set are for more sporty work?
Peter
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To add to RR's rod types, I have tapered alloy (duralium) rods in the cafe. About 1/8" thicker in the middle.
Re Trevs clearances. I have to set them at 10 & 12 on the cafe, if I use 8 & 10 she props the valves open when HOT (35degrees 75mph in 3rd up hill).
The recommended settings are just that, recommended. If there are other things/mods at play it's suck n see.
Cheers
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Musky,
The Gurus recommended .010" & .012" for the 357 cam for maximum power.
Trev.
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(35degrees) Never experienced that here..!!!
(75mph in 3rd up hill) Never will. *smiley4*
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Thanks Trev, I always find out the hard way. I like to set them as close as possible so not to waste any of the cam *eek*.
Cheers
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Hey Richard,....?
I'm thinkin' the hardened end must be in the sump or, maybe, the camshaft trough (though, not sure it could live there with the cam turning).
Richard L.
Am I missing out on something..?.... how will the ends make it to be out of the pushrod tunnel...??... *conf*
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I did pull of the sump filter to check and found that it was full of slivers of alloy but nothing bigger and none were found under the gauze. I'm hoping that if any did go further that damage was avoided. I've drained the oil but so far no sign of anything amiss but to be sure I'll check the tank filter before I refill.
Peter
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Musky,
The Gurus recommended .010" & .012" for the 357 cam for maximum power.
Trev.
Worth noting those clearances relate to that cam fitted with an alloy head.
If using alloy push rods in an all iron motor bigger clearances would make sense. Ideally use steel push rods with an iron head to avoid the disproportionate expansion rates
Have just been helping a mate with an all iron GF and even with the 334 that supposed to have 16 thou clearances on the exhaust. In fact as it had lesser clearances had burnt at least one of the exhaust seats, so heads currently off for attention.