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Valve guide questions

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After doing a pretty much top to bottom rebuild on my 87Xr250 I struck a slight problem. After starting up from cold (after the bike has been sitting a day or two) I get white smoke for a minute or so till it warms up. About the only thing I didn't do was fit new valve guides (I fitted new kibblewhite valves and springs)

Since the valve guide stem seals were replaced I'm thinking it is either a worn guide or one of the O-rings seals on the EX guides have burnt out letting oil dribble through to the cylinder (as per pic below which I stole from an old thread on here).

CIMG0806.jpg

I am thinking to fix this I need to

1. buy a new head gasket, oring seals, stem seals, valve guides.

2. replace the old guides/seals

3. reassemble

My questions are:

If I buy new valve guides what diameter do I buy?

guides_Layer1.jpg

I see in the kibblewhite catalogue above there are a range of diameters avrying in 0.001 inch increments (dimension G). I am pretty sure the guides in the bike are stock and are not loose as I recall.

I know it is not wise to lap factory honda valves as you can wear off the thin hardened coating on the back of the valves. When the bike was rebuilt it has a 3 angle job but I am guessing it would be wise to give it a quick lap before I reassemble it? Is it OK to lap the kibblewhite black diamond valves?

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You can replace the valve stem seals with out removing the head.

If it was me, that's the route I would go since it's the least invasive and cheapest fix for an older four smoker.

Now if you have the head sitting on your bench then go with new everything.

Valves, guides, seals ect.

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I put new stem seals, (blackdiamond valves, springs, 3 angle job) when I rebuilt the bike (has only been running a few weeks). Hence my guess that it is the O-rings under the guides or the guides themselves.

Bike is not burning oil, and runs fine so I think the break in went OK.

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If you can live with a puff of smoke at start up then leave it be.

Next time in (several years) you have it torn down replace the valve guides, get her ported and polished, new valves and seating and she will be better than new.

Or pick up a used XR2.5 head, get the work done to it and make it a simple swap out.

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It has been my experience that, if there is no measureable loss of oil, and the startup smoke clears up right away, it is not worth pulling the head to replace the guides. I have owned XRs that blew a little smoke on startup the whole time I rode them and never had any problems. Replacing the guides is kind of a PITA anyway. Replacement requires heating the head to about 250 deg. F and using a guide driver (Honda tool 07742-0010100) to remove and replace the guides, then the guides have to be reamed (Honda tool 07984-2000000).

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Yeah, I knew you guys would come back with that, its just that given all the effort I poured into this thing it roots my boot that its not quite perfect :thumbsup: .

I knew this could be an issue and I should have tackled it at the time so I am kicking myself.

Can some one humour me on which size valve guides to buy and tell me if the kibblewhites are OK to lap. I'll get a friend who's a mechanic to replace the guides so I don't goof up, it'll be a rainy day job.

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I would measure the ID of the valve guides and the OD of the valve stems that are in the head now to make sure of the proper size new guide. Do you have a service manual?

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Do you have a service manual?

No unfortunately I do not have the genuine Honda manual, only a haines. The kibble white chart suggests the internal diameters of the guides is constant only the outer diameter is variable, see below.

guides_Layer1.jpg

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I believe that the different ODs available are oversizes in case the guide bore in the head needs to be oversized due to wear/damage, but I cannot say this for a fact. I would contact Kibblewhite www.blackdiamondvalves.com, look under tech support for the contact e-mail. I am sure that they can answer your questions.

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I searched around on the web and looks like as a general rule the press fit of a valve guide into an alloy head to replace a guide is 0.002" assuming no wear in the head.

My problem is that I need to get the guides shipped to New Zealand, so to press the guides out, clean up, measure the holes, then order the guides would take weeks I would rather just go for it!

I will try and contact kibblewhite, but I have tried before and got no response, kind of ironic I thought at the time given that kibblewhite Snr is a also a kiwi

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Here's the reply from kibblewhite which clears things up

Our standard guides are typically designed to offer a .002"-.0025"

interference fit. In fact they are typically made .0003" larger on the OD to

compensate for any "broaching" of the head that may have taken place during

installation and removal of the OEM guide.

The oversizes are offered for the customer who may have a damaged guide bore

in the head, or a bore that is slightly worn from the guides being changed a

few times.

In most cases you should be OK using the standard guide. And, I would not

recommend using a +.002" unless you have measured the hole and determined

that it what it would take to insure a good fit.

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Here's the reply from kibblewhite which clears things up

Our standard guides are typically designed to offer a .002"-.0025"

interference fit. In fact they are typically made .0003" larger on the OD to

compensate for any "broaching" of the head that may have taken place during

installation and removal of the OEM guide.

The oversizes are offered for the customer who may have a damaged guide bore

in the head, or a bore that is slightly worn from the guides being changed a

few times.

In most cases you should be OK using the standard guide. And, I would not

recommend using a +.002" unless you have measured the hole and determined

that it what it would take to insure a good fit.

Good info to have on hand, thanks.

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