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Exhaust Valve Stem Damage


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#1 MiniLuke

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Posted 23 October 2020 - 10:43 PM

Stripped a small bore head today and found the exhaust valves difficult to slide out of the guides and appear to have some damage. Head has done no more than 1000 miles, but has been unused for many years.

The link below is a photo.

https://photos.app.g...4gFg3LdRrEunoM7

I’m thinking the problem has been the valves in new guides being quite tight and all valves having seals in place meaning a lack of oil getting to the stem? Thoughts?

So, what do I do now? With a light polish up the valves do run nicely in the guides again. I’m thinking the damage has been caught early, but do the stems have a coating and so maybe need replacing? And do I reassemble with seals only on the inlets?

Any advice welcome. Thanks

#2 Dusky

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Posted 24 October 2020 - 07:54 AM

No. The guides will have the same damage. Replace all damaged valves and guides sadly

#3 ACDodd

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Posted 24 October 2020 - 08:27 AM

Have the guides and valves replaced. The guides need to re sized after fitting. It is likely this was not done the last time it was worked on.

Ac

#4 Bobbins

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Posted 24 October 2020 - 08:36 AM

So is it okay to use Mpi type stem seals on exhaust valves as well as the inlets without issues?

#5 ACDodd

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Posted 24 October 2020 - 08:47 AM

It is if th clearances are correct and you use the chrome stemmed valves.

Ac

#6 Bobbins

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Posted 24 October 2020 - 09:00 AM

Basic refresh on an '86 998 head, not been touched previously and will be getting unleaded seats. All the valves are still good in the stems but sprung seals were only fitted to the inlets, nothing on the exhaust valves.

#7 MiniLuke

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Posted 24 October 2020 - 12:13 PM

Thanks. The head was done by MED a long time ago complete with valves, so I’d be surprised they weren’t sized at the same time...

I’ll get it re done.... ?

#8 sledgehammer

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Posted 24 October 2020 - 12:20 PM

sounds like the guides weren't reamed to the correct size when fitted (or not reamed at all)

 

7mm IIRC ? (never understood why they were metric)

 

I now use MPI stem seals , on all valves with good results

 

when getting new guides , get the ones with the ridge at the top to locate the stem seal

 

check the above info - as my memory is getting fuzzy

 

best of luck



#9 MiniLuke

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Posted 24 October 2020 - 12:24 PM

Thanks. These have the ridge.

#10 MiniLuke

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Posted 24 October 2020 - 01:58 PM

Me again!

I’ve been reading previous posts about valve guides, and there’s some seriously heated threads out there, so let’s not get into alll that, however;

1. I’ve got cast iron guides. I’ve read these shouldn’t have needed to be reamed or honed? Cooperman wrote that about 10 years ago.

2.obviously mine need replacing, due to damage, so once replaced do I size them? If so, reaming acceptable? What size? Is it 7mm?

3. I’ve read on Minispares site that cast guides should use chrome stemmed valves. Maybe that’s what went wrong? Don’t think my valves were chrome stemmed.

4. Do seats alway need to be recut after new guides? Again, read on here if carefully selected components are used, lapping can be enough, or is that wishful thinking?

I’m thinking the answer might be to hand it to machine shop and request they just sort...

#11 Turbo Phil

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Posted 24 October 2020 - 03:54 PM

Guides always need sizing after installation regardless of material type. 0.28125” is the standard ID.
The seats should always be recut after the guides are replaced to ensure they’re concentric.

Phil.

#12 Moke Spider

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Posted 24 October 2020 - 06:37 PM

Have the guides and valves replaced. The guides need to re sized after fitting. It is likely this was not done the last time it was worked on.

Ac

 

Yes, I hear all the time that 'Cast Iron Guides don't need Reaming or Honing after fitting', however, nothing could be further from that. As a matter of routine, I always put the Reamer through them and nearly always, some material is removed. I don't understand why so many who fit the guides, don't even seem to check !



#13 sledgehammer

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Posted 25 October 2020 - 10:31 AM

I should imagine some valves will stick without reaming ?

 

the guide will distort when being fitted

 

I am a complete bodger where I can be - but even I , ream the guide & re-cut the seat a bit - just enough to get a full circle cut

 

then lap to double check - also don't lap too much - just enough to seal

 

best of luck






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