The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Greybeard on 23.03. 2013 20:02
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The other day I took my GF for its first proper run since the rebuild. On the way home the engine suddenly weakened. I stopped and found that the left cylinder was not firing. The bike got me home.
I've now taken the rocker box off to find one of the exhaust push rods has broken, apparently after rubbing against the block or cylinder head. Valves all appear free. Finger in plug hole indicates some compression so I'm hopeful that my piston has not been holed!
The push rods are modern replacemements made of ally but presumably are ok for other engines.
During the rebuild I bought an original but reprofiled camshaft. I also replaced the cam followers with new ones. I'm wondering if the camshaft having a lower but correct profile may be affecting the point where the cam followers and hence the push rods sit inside the push rod riser causing them to touch the side. I'm going to take the head off so I can see where the push rod was touching. I can already feel that the holes in the head casting are narrower than the cylinder block tunnel so maybe some careful grinding will provide sufficient clearance.
What do you guys think about any of this?
I'm currently bidding for some original, steel pushrods on Ebay.
Neil Ives UK
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Hy neil I would never use alloy push rods. I had the same trouble and I could see where they were rubbing on the head
Casket. They were already in the bike when I bought it .when I took them out I noticed they were a lot thicker. They were also
Slightly bent.wardleybob
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Hi Neil
Found the same thing on my last project - perfect ring around the two rods, no noticeable wear though, my other Flash has no such marks
More to the point though my friends flash had one push rod that had a join, we have no way of knowing if it was like this from new, would seem rather strange if it was or if it was a repair in which case it would be even stranger as it was a perfect join and the rod was the correct length, it could of course have been made up from two broken rods, even stranger still.
I once bent n exhaust pushrod following a blast in third (indicated 94mph), similar symptom - bit down on power but firing ok, no further damage, straightened the rod in vice, put it back together and it was fine
(indicated 94mph)
I was young, light, strong and had an Avon full fairing on, these days my fillings would fall out and I wouldn't be able to straighten my hands for a week
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Just to agree with the others, I had the same problem which was detected by engine noise. I removed the head and found a marked ring around some of the ally push rods which where in line with the top of the tunnel. Apparently they flex under load so I fitted SRM steel ones.
All the best Goldy
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Just to be different I've used alloy push rods for years, on road and race track with not a hint of trouble.
They would bend a fair amount before breaking. The marking would need to be very deep to cause it to snap. Are they solid or hollow?
The only time I had them flex was after I put in some different springs that coil bound, still didn't break.
Cheers
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Update:
I've just been braving the zero degrees air temperature in my garage to remove the head of my GF. No damage to valve or piston thank goodness *smiley4*
The broken pushrod had been rubbing on part of the solid copper head gasket that was slightly protruding. I will trim the gasket when I reassemble.
I've won a set of original steel pushrods on Fleabay.
Did anybody have any concerns about using a reprofiled camshaft?
Hopefully by the time the weather improves I'll be ready to go.
Graybeard
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The push rods are modern replacemements made of ally but presumably are ok for other engines.
Neil Ives UK
I had another look at my original steel pushrods. They were pitted with rust but I've cleaned them up and used them again. They are probably still stronger than modern ally ones. They've completed another 30 mile run ok so fingers crossed!
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I do remember that the 'performance' ally pushrods available in the '70s were actually heavier than the standard steel ones!
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Neil, an interesting event! One of the few breakages that is failsafe in an engine. I have seen chafed alloy pushrods caused by using too large a washer under the short head bolt between the two cylinders, most commonly on A65s where alloy pushrods are standard. There is very little clearance here on both A10s and A65s.
Regarding alloy versus steel pushrods, there is a lot we can learn from the American high-performance V8 community, Which really has bottomed out this issue. Alloy pushrods can be a good bet if they are short, and if the spring loads are not colossal, creating a situation where whip is not an issue. The aluminium alloys have a much lower Young's modulus than steel, so pushrods of any length are in variably made of steel tube. American spec 4130 tube is a popular choice, and for anyone interested, small quantities of small diameter 4130 tube can be bought from the companies that supply components for home built light aircraft. Titanium is generally not thought of as a good choice either, because it's Young's modulus is similar to that of the aluminium alloys.
HTH, Jon