The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: bsaketcase650 on 20.06. 2012 20:05
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Gents,
Is there anyone out there who can give me advice or guidance to end my 'suffering'!!! Recently completed a top to tail rebuild on my 1953 A7 Star Twin. This included new valves, springs and guides (all supplied courtesy of SRM). On completion of rebuild soon had engine running nicely. However, after short while the engine began running erratically, backfiring and strange 'twanging' noises coming from the rocker box and eventually coming to a stop. Allowed to cool and attempted to restart without any success. Removed rocker inspection covers and could see that inlet push rod had become disengaged from its rocker cup and the left side inlet valve apparently seized partially open. Cylinder head off which confirmed the diagnosis. Removed the offending valve and thinking that the stem clearance may have been a little tight gently ran a reamer down the guide to clean it up. Refitted the valve which ran very smoothly up and down with negligible play. Re-assembled and very quickly had the engine running smoothly. But hold on - what's that squeaking noise from the rocker box that's developing as the engine warms? Stop engine and removed spark plugs. Peering into the left had pot can see the same inlet valve is stuck wide open.
Much cursing and swearing!! Will have to pull the damned thing down again. What's occurring here? Bronze guides no good? Clearance too tight? Oil starvation? (As is my habit on a new build I always put my finger over the oil return to get the amber nectar into the valve train). Oil feed via the banjos seems OK and unobstructed.
Boys, with your vast collective knowledge can someone give me a clue?
Regards to all
Simon *sad2*
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G'day Simon,
iron head should have steel not bronze guides. I'd guess that the first time the rod may not have been home properly and the valve has kissed the piston and bent slightly.
Cheers
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I had this problem and it was caused by new banjo bolts that I fitted to the rocker box, the small hole for the oil feed was too small think it's 3/64 you will find lots of posts about it. I opened them up and I also used thinner oil for the first few miles and no more problems.
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when you you put your valve back in the guide did you lube it with anything over than engine oil? i only ask as i once heard of someone using chain lube and this caused the same problem you describe...........cheers baz
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Gents,
Thanks for the replies. Pulled the head off today. Yet to remove valves and springs. In answer to a couple of suggestions raised - the banjo bolts are the originals. I retained these for the very reason that Goldy points out. In response to baz I lubed the valve stems on assembly with plenty of 40w oil.
I am of a mind to agree with muskrat that perhaps steel guides are the way to go rather than bronze although I am still curious as to why it has only affected one valve. That said, I have yet to remove the offending valve and check to see if it is slightly bent. There is no marking on the piston crown to suggest that it has come into contact with the valve.
Cheers,
Simon
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G'day Simon,
have a look here, http://www.a7a10.net/forum/index.php/topic,2422.msg15653.html#msg15653 due to the different expansion rates the bronze guide has grabbed the valve. Often when removing guides some head material is lost so the hole has to be reamed and over size guides fitted. That 1 troublesome guide hole might not have lost any and the guide is tight and as it expands (can't go out [iron head]so goes in) grabs.
I have had the opposite happen with iron guides in an alloy head.
Cheers
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Guys,
Problem now fixed! Hoorah!! As Muskrat said the way to go was iron guides. Took the cylinder head to T & L Engineering in Bedford (who I thoroughly recommend). They agreed that phosphor bronze guides guides in a cast iron head were not suitable. Consequently they removed those guides and replaced with the cast iron variety. All valves were fine and received a light polish to the stems prior to refitting. Reassembled today and now running as sweet as a nut. Fingers crossed it continues that way. Just need to get the old girl out on the road and get some miles under her belt as part of the running in process. On discussing this problem with T & L, I was told that bronze guides can be used in iron heads provided that the guide is manganese/bronze mix. However given that the old A7 Star Twin is a low output engine cast iron is absolutely fine and as fitted originally at the factory.
Many thanks for your input chaps.
Simon
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Sorry for dredging up an old post, but I have been experiencing exactly the same problem on my A7ss following my rebuild. It has brand new valves and guides (bronze guides and alloy head). So far I have had the head off twice now due to one of the inlet valves nipping up in the guide. On each occasion I have had the guide reamed slightly and the valve stem polished, and each time it has run perfectly for the first mile or so before the valve seizes open and the push rod drops out of the rocker cup. I am still using the original banjo bolts and the oil flow to the rockers appears to be good. I am at a loss. Even if you can't offer any practical solutions please just say something kind. *sad2*
Mosin
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G'day Mosin.
Are the guides std or oversize? What cam are you running? When you take the valve out, where are the rub marks, top or bottom of the guide? Are there marks on the top of the guide to indicate the cotters touch it?
Look on the bright side. You'll be a wiz at the rockerbox.
Cheers
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The guides are standard and I am running a 356 cam. I haven't actually removed the valves myself because I took it back to the engineering shop who did the work on the head to get it sorted so I didn't see where the rub marks were. The guy who did the work said that he did not want to over-ream the guides and risk having oil run down and produce a smoky cylinder which makes sense to me. Is it just a case of reaming the guide out a little at a time until they stop seizing? It is the refitting of the rockerbox which is really getting me down. Not least because I had one of the bolts shear on me the first time as I was tightening it down so I had to take the whole thing off to get the snapped bit of bolt out. Then start all over again. *sad2* *sad2* *sad2*
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G'day Mosin.
The running clearance between valve and guide is 0.001" to 0.0015'. Not a lot to play with. Guides shouldn't be reamed. A ball hone should be used. https://tinyurl.com/y23823dz
Is that side getting hotter than the other? Is there difference in the plug colours?
Cheers
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Mosin On the bright side it runs OK on one cylinder....some bikes this week don't even cough.
Oilflow is easiest check. Try swapping the banjo bolts in case there is some hidden difference. Next, with the box off, check the rocker shaft is clear, warm the box to easily remove the shaft.
If all in order, you need to have a look at the valve. Check for bending and/or bow in the stem, and witness marks of higher loading along the length. Piston crown....has it touched? Maybe worth swapping the inlet valves over, it may just be a case of on the limit tolerances for guide and stem. If you can not find anything simple amiss then bite the bullet and poke a bit more from the guide, like musky says, it will only need a light internal polish, honing will be fine.
Swarfy.
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Me again.
The tolerance I quoted may be a little tight. That's what I give the rHonda (and my SS when running methanol). Your SS should be 0.002" to 0.0025".
Sorry about that.
Cheers
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Mosin On the bright side it runs OK on one cylinder....some bikes this week don't even cough.
Oilflow is easiest check. Try swapping the banjo bolts in case there is some hidden difference. Next, with the box off, check the rocker shaft is clear, warm the box to easily remove the shaft.
If all in order, you need to have a look at the valve. Check for bending and/or bow in the stem, and witness marks of higher loading along the length. Piston crown....has it touched? Maybe worth swapping the inlet valves over, it may just be a case of on the limit tolerances for guide and stem. If you can not find anything simple amiss then bite the bullet and poke a bit more from the guide, like musky says, it will only need a light internal polish, honing will be fine.
Swarfy.
Thanks for the positive view. It actually runs really well on one cylinder and was running beautifully on two up until the valve seizes. There is loads of oil getting into the rockerbox though and the rockers seem to be rocking nice and freely when I push them with my finger. Sadly the inlet valve is seized open in the guide so it looks like the head is going to have to come off again anyway in order to free it. (luckily it doesn't appear to have touched the top of the piston though). Unless I suppose it might have freed itself when it cooled down, but I suspect that is just wishful thinking...
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Mosin... At least you know what the problem is. Give me a mechanical problem any day! I mentioned the oilsupply, the rockers may be free, but oil, drops or mist must be able to reach the valve stem.
The surface finish of the valve stem and its corresponding coefficient of friction against the guide internal bore also need to be considered. You say the valves are new, do they have a similar surface finish to the originals? You could swap an old one back if you still have them.
Swarfy.
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swarfy its still a mystery to me how my shooting star engine did well over 60 miles with no big end oil pressure as such and no feed to the rockers when I put the oil pipes on the wrong way round. even though I know it was dragging a bit from the return pipe in the tank which I had over filled and on a slight left hand lean it was enough to get some down the pipe , the colisbro guides are still in the head and it didn't effect them at all. it got very hot threw a circlip and ground the left hand cylinder with wrist pin-- haha wrist pin love it--- pulled out a flat topped camshaft and slightly concave followers, rebuild with timing bush, big end shells different barrels and pistons ,cam and followers and re designed oil pipes *whistle*
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swarfy its still a mystery to me how my shooting star engine did well over 60 miles with no big end oil pressure as such and no feed to the rockers when I put the oil pipes on the wrong way round. even though I know it was dragging a bit from the return pipe in the tank which I had over filled and on a slight left hand lean it was enough to get some down the pipe , the colisbro guides are still in the head and it didn't effect them at all. it got very hot threw a circlip and ground the left hand cylinder with wrist pin-- haha wrist pin love it--- pulled out a flat topped camshaft and slightly concave followers, rebuild with timing bush, big end shells different barrels and pistons ,cam and followers and re designed oil pipes *whistle*
I feel positively lucky in comparison!
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Hi Mosin,
The guy who did the work said that he did not want to over-ream the guides
That sounds like the person does not have much experience with old Brit iron (or alloy) *????*
Work to the clearances in the service sheets or manual and "All will be OK" when using standard Bronze guides
The valves stems are usually the correct clearance undersize from the nominal 0.3125 (5/16in.)
so (usually) the guides are reamed or honed to 0.3125
John
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Sounds like there is something more fundamentally wrong with this one. Ordinarily if you have a sticky valve, you strip it and ease it - once.
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I've been thinking that as well cb.
Hay Mosin, ask the mechanic to measure the valve stem on both. May be an off size valve. As John said the clearance is built into the valve stem.
Cheers
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Thanks for the thoughts guys. Whatever the case it is looking like the whole thing is going to have to come apart again. The strange thing is that the engineer who did the work on the head specialises in classic/vintage/veteran vehicles and runs a few classic bikes including a Vincent himself. That is why he was recommended.
It will be interesting to see if it is the same inlet valve which has seized this time or the other one. I will also have a good look at the banjo bolts. I know that there is good pressure getting to the rocker box, but I guess that if one of the banjo bolts is partially blocked this would not necessarily guarantee that the oil was actually getting to the rocker in sufficient quantity to keep things moving. If this turns out to be the case then that would explain why it was not picked up on when the head was off and in the shop.
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Mosin The hollow rocker shafts have small metering holes which feed into an oilway running lengthwise along the outside of the rocker shaft. Well worth taking apart to check for a blockage. For the oil to evenly lubricate the top and bottom of the rocker forgings, the rocker shafts should be fitted with this oilway uppermost.
Swarfy.
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OK. I have just removed the rocker box (for the fourth time in as many days) and two things stand out immediately:
- The inlet valve rocker on which the pushrod had become disengaged this time (OS) was not the same one as disengaged last time (NS).
- Both inlet valves were now fully closed
To my mind this suggests not so much a valve sticking through heat or lack of clearance, but rather a lubrication problem. I have removed the rocker gear and checked this over. There were no obvious blockages, but I am not convinced that the oilway on the outside of the spindle was facing upwards. Is this crucial? It certainly doesn't get mentioned in Haynes.
Mosin
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Mosin... I never bothered which way it went, so one day in discussion with fellow alcoholic backyard mechanics, the general opinion was that oilway on top was best.
This is what we reasoned.
Oilway to the bottom means very little oil to the top surface of the shaft and also less bearing surface at the underside, where you need it, but possibly more oilflow to the loaded area.
Oilway to the top gives a wide oil film which can enter the low load side of the shaft/rocker, and the remaining oil in the oilway stays there by gravity to flow to the radial groove at each end.
You could argue that the reduced shaft diameter where the oil holes are forms a more than adequate oilway, and as the radial groove at each end coincides with the rocker oil hole feeding oil to the underside of the rocker arm and hence the valves, it really does not matter anyway.
The pictures in parts books and service sheets illustrate the oilway on the side of the shaft. So, top, side or bottom, I suppose you take your choice, even then you can bet it will move round when you tighten the acorn nut.
Swarfy.