If a guide is loose enough that it is sliding in and out of the head (happened on my friends 280 4.5). You will notice one is loose because by trying tp pull the seal off the guide, instead the whole guide will come up! We used the standard size and carefully tapped it in. Use the old guide to hit against as to not damage the new one. You may have to go to the next oversize, but I can't emphasize how easily they are damaged from hitting them in. I've even heard to put the guide in the freezer overnight before hand so it will slide in easier. I guess it contracts somewhat microscopically? I'm not sure, I'm definitely no scientist.
Thanks
David

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2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

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