Triumphrat

josh

Hi Everyone, I have been reading on here for a while but this is my first post. I owe you all a thankyou for what I've learned from this board already, a great resource for someone new to Triumphs. Anyway, I did a top end rebuild on my 55 6T Thunderbird (650 engine), everything went very smoothly and it started right up when everything was back in the bike (after a few kicks to feed oil through). But, after about 4 or 5 minutes running, the engine would begin to make a clunking sound and lose RPM and sputter until I shut it off. If I let it cool off a little it would run great until it warmed up some and the clunking would begin again. I don't know the frequency of the clunk but it seemed constant with the engine revs (not random). So out came the engine and I began to tear it down. I was really worried the noise was related to the bearings. I did not disturb the bottom end at all during the rebuild, because there was no slop and my main concern was the lack of compression I was getting that caused the engine to run on one cylinder, plus it was just more work (and special tools) than I wanted to take on...

Today I was reading this thread:
http://www.triumphrat.net/classic-vi...ml#post1264146

I grew concerned that I had never checked the piston to valve clearance, so I went to check my pistons and this is what I found on the left side exhaust valve relief:

And on the valve:


It looks like the valve was making some contact! It doesn't look like it was extremely heavy, the piston doesn't look cracked, but now I am wondering if this was the source of the clunk, and if I need to proceed further and tear open the bottom end to inspect the bearings, etc, or if I just need to put it all back together, check the valve clearance and then have larger reliefs machined into the pistons? And check out the valve stem/guide? The pushrod passes the roll test. What do you guys think?

During the rebuild, the cylinders were bored to .060 over, and it got new pistons. Also the head was completely worked by the machinist my local britbike supplier/rebuilder uses. The previous owner(s) had ground the valve seats out so much that they had to fit gigantic valves to compensate. The machinist installed hardened seats to fit more common valves from a unit 650. I think those valves are slightly larger than the original pre-unit sized valves.

Sorry for the long post but I wanted to include everything I thought might be important. Has anyone had a similar experience? I found a few posts regarding the importance of checking the piston to valve clearance, but nothing about what has happened to those that neglected to do so...

-Josh

pookybear

Josh,

Are those high compression pistons you got there? Stock
units should not give you that problem, unless the head
has been decked or you are using too thin of a head
gasket.

Take a good look at that valve, pull it out and make sure the
stem is not bent. When you take it out, just roll it on a flat
surface with the tulip part of the valve overhanging the
table. And just roll it on the stem, if it is bent you will see
the tulip part wobble.

Pookybear

tr6tom

I cant see by the photos, but are the pistons installed right? the large valve relief should face the intake valve IRREGARDLESS of any aftermarket stamping..Ive disassembled many engines and found them backwards before, sometimes its an issue sometimes not.

TBS

Have you checked the fitted valve spring length? You can tell by this whether the valve position in the head is correct, may well be too far into the combustion chamber and the seat will have to be adjusted back. Also do the Plasticene test as I wrote about before. If the valve spring length is correct and the valve is hitting the face of the piston, not the lip of the piston cutaway, then go for an 80thou head gasket

josh

TBS - I did my best to measure the spring installed height with calipers, it is close to 1.320" for the outer spring. I can't find a spec for this in the workshop manual though, any idea where I can find it?

The left side exhaust valve would not move in the guide after I took the springs off. I had to tap it out. It isn't bent, but it is all scored up. Also - the valve guide started to move with it when I was lightly tapping on the valve. The valve guide on the right side was loose too, I could move it by hand. But that valve moved freely and slid right out, no apparent damage. The intake valves moved freely and their guides were solidly fitted in the head.

I am thinking that when the engine warmed up those guides may have been moving all over the place. When the left exhaust valve got jammed up I wonder if that's when it was making contact with the piston (due to springs not being able to return it fully closed).

The pistons and valve guides I was told were .060 over stock replacements, but I got those from Marshall Hagy, no idea if they are high compression or normal for this engine. And the guides were installed by his recommended machine shop whom I had to take the head back from because they had no idea how to put seat inserts in...that's when I started using a different dealer and machine shop. So it is possible that the first guy didn't check the bores were the proper size for a press fit. Or is it possible the brass guides loosened up after being exposed to high temp from the exhaust? Don't tell me this is normal!

Also, the pistons are installed the right way, although the intake valve relief is only about .015" deeper than the exhaust.

Thanks everyone for the advice thus far!

GrandPaulZ

I just spotted your glaring problem:

You mentioned "giganic oversize valves", but you failed to have them compatibility checked with the previously mentioned plasticene test to make sure they'd clear each other.

They obviously aren't "plug-n-play" compatible...

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pookybear

Paul,

good eye there, I must have been useing my bad eye to read
with last time, went right over the larger valve line.

Pookybear

josh

Well, the valves that were in place when I bought the bike were gigantic compared to what I have now. All four of them were different, and the exhausts were just as big as the intakes. There weren't any clearance issues with those valves though. Currently, I have valves from a unit 650 that are much closer to stock size (not sure the exact size difference), made possible due to the new seats. I will do the plasticene test. Do you have any thoughts on the loose valve guides?

jimmy bush

They make oversized valve guides. Sometimes when an old guide is removed, there is some damage to the inside of the bore that the guide fits in. Before removing a guide, great care should be taken to remove all the carbon buildup around the guide but even then, sometimes some of the bore is removed along with the guide. The bores should be checked for size and the appropriate size guide installed to give an interferance fit. I'm not sure right off hand how much interference is needed between the guide and the bore but it sounds like yours didn't have near enough. The head should be heated and the guide frozen when installing. When I remove a guide, I heat the head and freeze the removal tool to get some extra shrinkage on the guide. If I have to do mutiple guides, I freeze some snug fitting bolts to be placed in the other guides until I can get to them. All I have done have come out easy using this technique. Maybe I should put this in the Tech Tips section.

black&chrome

hi there,
i've been trying to work out the cause of a similar problem so this thread has been very appreciated, however, TBS mentioned "If the valve spring length is correct and the valve is hitting the face of the piston, not the lip of the piston cutaway, then go for an 80thou head gasket" mine, however, is hitting the cutaway on both sides just slightly to the left side of intake pockets. so what do you do in this case that would be different??

my fairly new valve guides are leaking - despite having a 'pro' shop doing the overhaul. i was on a rebore with standard size and compression pistons. a single deepish gouge on my barrel necessitates getting it overbored already as well unfortunately!
it has never had the sludge trap cleaned to my knowledge, and as i don't know what cams are in it for sure, so it's all coming apart.

breaks my heart that it is happening at the start of summer.