Author Topic: Inlet valve/valve guide seizure (Read 3612 times)

bsaketcase650 · « **on:** 20.06. 2012 20:05 »

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

muskrat · « **Reply #1 on:** 20.06. 2012 21:43 »

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

Goldy · « **Reply #2 on:** 20.06. 2012 22:18 »

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.

baz · « **Reply #3 on:** 21.06. 2012 19:03 »

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

bsaketcase650 · « **Reply #4 on:** 21.06. 2012 20:01 »

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

muskrat · « **Reply #5 on:** 21.06. 2012 21:27 »

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

bsaketcase650 · « **Reply #6 on:** 19.07. 2012 16:37 »

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

Mosin · « **Reply #7 on:** 19.03. 2019 09:20 »

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.

Mosin

muskrat · « **Reply #8 on:** 19.03. 2019 09:35 »

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

Mosin · « **Reply #9 on:** 19.03. 2019 09:44 »

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.

muskrat · « **Reply #10 on:** 19.03. 2019 10:03 »

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

Swarfcut · « **Reply #11 on:** 19.03. 2019 10:08 »

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.

muskrat · « **Reply #12 on:** 19.03. 2019 10:26 »

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

Mosin · « **Reply #13 on:** 19.03. 2019 11:40 »

Quote from: Swarfcut on 19.03. 2019 10:08

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...

Swarfcut · « **Reply #14 on:** 19.03. 2019 12:38 »

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.