Pushrod problem

[Pecon]

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

[a10 gf]

one of the pushrods was melted onto the cup of the rocker. The rod had shortened by 1/4 "

!&!   Any pictures?

[RichardL]

This may be helpful, or not.

http://www.bsawiki.co.uk/index.php?title=Chapter01

[Pecon]

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.

[a10 gf]

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.

[RichardL]

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.

[Billybream]

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.

[Pecon]

Believe it or not they started off the same length  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

[chaterlea25]

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

[Pecon]

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

[trevinoz]

Peter,
                  I have seen that same problem with alloy push rods.
Find a set of original steel rods and eliminate an ongoing headache.

Trev.

[chaterlea25]

HI Peter,
I would buy a new set,
SRM's are steel and I have used a few sets without issue

John

[Pecon]

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

[Rocket Racer]

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.

[Pecon]

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