I have been told that linking from 1 of your sites to another 1 of your sites is a huge no no - according to search engines - why? Is it considered black hat? I don't understand what is so bad about say ( as example ) you have a site about cars and you also have a site about fishing linking the 2 sites to each other? By linking to each other - I mean a post on each site saying "hey check out this other cool site about xyz"
excessive interlinking will maybe get your links ignored, that's all. lots of sites do it and rank very well.
You can probably get away with a bit of interlinking, but if you do heavy interlinking the search engines could penalize you. Google could consider this as either a link scheme or a trick intended to improve search engine rankings. I'm not sure if it is black hat, but the search engines probably don't like this. If you care about the long term success of your websites, you shouldn't risk doing anything the search engines may not like. If you are only doing this for direct traffic, then you should be able to get away with this by using the rel="nofollow" link attribute.
I don't think it will harm your sites. I do interlinking to have traffic on my network and I don't think it should cost any damage on SE.
It's all about intent and why you link them together. If you have a site about Dog Grooming and a site about Forex Trading does it make sense to link them together? If you do that kind of thing in volume you will probably get filtered eventually. If you a have site about Auto Insurance and another about Life Insurance then it makes sense to link them together as they are somewhat related. The intent of the search engines is not to penalize you, but to weed out those who are intentionally tying to manipulate the search results. But the reality is this. Any linking structure is an attempt to manipulate the search engines. What you need to do if figure out the balance of linking so that you rank and provide quality information relevant to your users.
I would only link site that you own that are related in topic. So a site about cars could be linked to a site about trucks or recreation vehicles. But I wouldn't link a site about fishing to your car site...however the RV site could be linked to the fishing site. Just keep them somewhat related and you'll be safe.
It won do good nor harm your site... Google or any search engine will look for relevant links.. Fishing and car site does not make good match of links
be assured google performs a time based indexing so it knows how fast you are building your links and if it considers you have build links too fast then you will be penalized