From: Dave Baker
Newsgroups: rec.autos.tech
Subject: Re: Camshaft fit in bearings
Date: 11 Jun 2000 02:17:07 GMT

>Subject: Camshaft fit in bearings
>From: Don bjmcpher@usit.net
>Date: 10/06/00 23:26 GMT Daylight Time
>Message-id: <MPG.13ac7cae112618d59896f6@news.usit.net>
>
>How tightly should a cam fit in new bearings. Should it be real easy to
>turn by hand or is some drag acceptable. These were way too tight, so I
>worked them with some 400 emery, then 600 and the cam fits in and turns
>now, but its sort of snug. I dont want to take off too much, but also
>dont want to finish the rebuild with the cam to tight.

A bit more info on exactly what happened here would not go amiss. Cam bearings
are usually manufactured to finished size (i.e. you don't need to fit them and
then line bore) and to similar standards as crank bearings. The process is very
accurate and it is "very" unlikely that they would be faulty. However,
depending on who fitted them and how this was done, one can end up with tiny
burrs at the ends from where they were pressed in. This can be enough to make a
cam feel tight.

Also, there is no guarantee that a cam is straight or even that the journals
are the correct size - is this a old std cam, a new std cam or a new
performance cam?

So before you wreck the bearings with emery paper it might be wise to check the
cam for size and straightness. If that is ok then fit the cam and if it is
tight then turn it a few times and see where the contact marks on the bearings
are. It might turn out to be just burrs on the bearings. If this is an unknown
block then it might also be something wrong with the straightness of the block.

As a general rule, if you can get the cam into the bearings and it turns at
all, even if a bit tight, then it will sort itself out in short order as soon
as you run the engine. Cam bearing material is very soft and any burrs or tight
spots will wear away in seconds. Enlarging a bearing thinking it is tight when
actually it is just a slight error in how straight the cam is and you might end
up with a much larger problem than you originally had.


Dave Baker at Puma Race Engines (London - England). Web page at
http://members.aol.com/pumaracing/index.htm contains technical & tuning
articles on various engines.