Sex Engineering Articles

Articles that deal with the neuroscience and bio-mechanics of Sex. Concrete advice on how to improve your sex to make it more pleasurable and how to play safe.

Mini Private’s Perspective

Mini Private is my better half.
Her articles cover what she see as the most important insights and information on human sexuality from a woman’s and a bottom’s perspective.

The Hardware Store

Where you can buy the best tools for re-engineering your sex life.

Join Our Community

Free access to all R18+ content
Free order tracking
Post your own comments and Questions

Join Now

Recent Articles

Common Anal Sex Problems

Common Anal Sex Problems First published December 10, 2019 © Max Private – All rights reserved Feature image by Nati's SweetPeach🍑 Introduction Most of the problems people have, or experience, [...]

Ready for Anal Sex

Ready for Anal Sex First published April 23, 2015 © Max Private – All rights reserved Feature image by Nati's Sweet Peach🍑 Question: Staying in anal shape when flying [...]

Total Colon Clean Out

Total Colon Clean Out 3rd edition - August 16, 2022 First published June, 2013 © Max Private - All rights reserved Total Colon Clean Out for Serious Anal Sex Activities This [...]

Eating and Enemas for Clean Anal Sex

Eating and Enemas for Clean Anal Sex 3rd edition - August 16, 2022 First published April 11, 2014 © Max Private – All rights reserved Feature image by Clker-Free-Vector-Images The #1 [...]

Got a question? Ask Max

Our in-house sex intelligent expert.

Ask Now
Can’t clean out my colon2023-10-15T12:49:05+11:00

Q. I have tried to worked my way through your Total Colon Clean Out multiple times and I just can’t take more than 1 liter of water at any stage. After all my attempts I feel empty but then later, and most frustratingly, during anal sex/play everything gets messy. Very messy. What is going on?

A. The 1L limit you describe is discussed in the Total Colon Clean Out but there is a lot to consider and this is a typical stumbling block, so let’s see if we can clear this up. Throughout most of the colon the fecal mater has a high fluid content. Something approximating diarrhoea with (maybe) just a few solid bits. It is not until the descending colon (which slides straight into the sigmoid) that things start to solidify. The rectum is the storage compactor (handles both functions). It take about 500ml (0.5L) to fill your rectum. If your rectum is not empty then 500ml will start filling the sigmoid. 1L will fill the rectum and sigmoid and a descent amount of the descending colon.

So you are getting 1L up your ass but it clearly isn’t all coming out when you want it to. When you sit to expel the enema, the colon pinches off the rectosigmoid flexure (the top of your rectum) to prevent anything going back up while rectum contracts to push its contents out. That empties your rectum but not your sigmoid or descending colon. With patience, your might get a second rush as the sigmoid empties into the rectum, but you still have enema fluid retained in your descending colon. The colon responds to the excess fluid (water) in the sigmoid and descending colon by keeping the rectosigmoid junction closed off. It tries to absorb all the excess water. After all, the colon is not suppose to move stuff into the rectum until it is ready to be compacted. Your brain “feels” that your rectum is empty but does not connect to any of the sensory neurons in the colon. You don’t “feel” the descending and sigmoid colon still have stuff in them.

So you will be stuck with the descending and possibly the sigmoid colon still full and an empty rectum. Your brain only senses the empty feeling of your rectum. Eventually (maybe minutes or hours) peristalsis will start to push the contents of your transverse colon into the already full descending colon. Your colon has no option now but to expel the watery sludge from the sigmoid colon into the rectum where you want to play. It doesn’t help either that activity in your rectum stimulates the colon to push things out.

One additional function of the gastrointestinal that can either help or hinder clean anal sex is the Gastrointestinal Reflex (Gastrocolic Reflex);

“The gastrocolic reflex is a physiological reflex that controls the motility of the lower gastrointestinal tract following a meal. As a result of the gastrocolic reflex, the colon has increased motility in response to the stretch of the stomach with the ingestion of food.”

In simple terms, eat to make yourself go empty your bowels before anal sex. Eat before an enema to make the enema fore effective at emptying your bowels. DON’T eat after have an enema. That will refill your bowels. For more on this issue the article Eating and Enemas for Clean Anal Sex.

Inserting an anal dildo 7″ plus2023-12-29T12:49:55+11:00

Q. I’m having trouble getting  a dildo deeper than 7 inches up my ass. I have a smaller one which hits a turn at about 7 inches in, but it’s skinny and flexible enough so I can push the last inch in. The bigger dildo however, hits a hard stop at this point and won’t progress any further. Is this something I can train or is it just the way my insides are?

A. You are hitting the underside (anterior surface) of the sacrum and probably getting stuck at the mid-transverse rectal fold. To get your larger dildo past this fold, you need to make sure the walls of the rectum are very slippery going through the fold. To lubricate the rectal wall up to (and hopefully through the fold) repeated insertions of your smaller dildo with lots of lube (or even the added use of a lube shooter) is where to start. The next step is to manipulate the base of the dildo to tilt the head in the right direction – towards your belly button. There are two techniques for achieving this.

  1. If there is any flex in the toy, reach between your legs and pull the protruding base of the dildo forward. Inside, the head will be pushed against the anterior surface of the sacrum and it will bend into the shape of a banana. Then you gently push the dildo in further. The head of the dildo will follow the curvature of the sacrum, slide out from under the sacrum at the base of the spine and into the abdominal cavity.
  2. If the dildo is too rigid to bend as described above, then you reach around behind and pull the protruding end back. This will cause the shaft to pivot (effectively) on the coccyx tilting the head out from under the sacrum and into the abdominal cavity.

As the head pushed through the transverse fold and springs out from under the sacrum into the abdominal cavity you may feel a distinct “Pop” sensation. This experience is almost universal and gives rise to the urban myth that there is a “second sphincter” up there.

Just to add a comment on the next “step”, the next potential stopping point is the opening to the sigmoid colon (or rectosigmoid flexure). This is encountered by most people between 9 to 12 inches inside. At this point there is a distinct narrowing of the colon. It is the narrowest point of the colon. This is another point where a “Pop” sensation might be felt as the head of a dildo passes through. Again, it is not a sphincter but reinforces the urban myths.

Anal depth play – How do you get elbow deep?2023-10-05T16:11:22+11:00

Q – I am an experience anal player. My ass can easily take the larges diameter toys and even two fist. But there is a hard stop, a solid wall up there that stops anything going deeper than about 6 to 8 inches [15 cm – 20 cm]. What’s the story? Is the pelvis obstructing things? How do I push through it? How do I get a fist elbow deep or burry an anal toy over a foot long up my/anyone’s ass?

A- The “hard wall” you are hitting is the underside of your sacrum [<- medical link].
It is a “wall” (of sorts) you can’t push through it. This wall is encountered about 2/3 to 3/4 of the way up your rectum, 6 – 8 inches from the outer rim of you anus. Don’t worry about your pelvis. You body will easily take a female (any) hand.

The underside of the sacrum has a gentle curved but is not smooth. So the shape of the head, and generally the design, of the toy you try to push through the bend under the sacrum will have a big impact on your success. The head of the toy needs to be smooth and rounded to follow the curved under-surface of the sacrum and not get caught on any of the surface features. Fisting partners also need to work with this curvature. Bending fingers and further tilting the wrist into the abdominal cavity (towards the belly button) is the path. The fingers and hand will then be above the bladder and in a position to easily produce a lower belly bulge and generally push the belly button out.

Keep in mind also that when you are upright or on your back, the weight of all your internal organs and intestines are pressing down on your rectum and anus. The colon is literally squashed flat and the bends/corners become folds that are harder to follow and navigate. Kneeling on all fours or, better still, ass-up with head and shoulders on the matt will dramatically straighten everything out. The rectum will be drawn away from the back and out from under the sacrum. That does not mean you will be able to ignore the natural curves, it just make navigation easier.

If you haven’t already figured it out, the anal canal is roughly in line with a line you can draw from your anus to your belly button. When “relaxed”, the anorectal junction forms a right-angle turn towards the back, around and behind the coccyx. Over time, the body adjusts/learns to straighten out the anorectal junction to accommodate anal penetration. Similarly, once you do successfully navigate out from under the sacrum and into the abdominal cavity, it gets easier and easier very quickly.

NB – The sacrum is the lower part of the spine. Important nerves pass through this structure. If you start feeling tingling in any of the lower parts of your body (legs) you are squashing nerves. Pushing too hard can also bruise or otherwise damage the colon. If you see any blood you have caused a fissure and you need to give it a rest of at least a couple of days.

There are two toys that are particularly good for this type of depth training:-

Am I too damn tight?2022-08-16T10:25:45+10:00


No matter how much I try to relax, no matter how much lube I use I cannot even fit a small anal vibrator in me so there is no way I am going to accommodate my boyfriend’s cock in me. I am hugely turned on by the idea of giving him my ass but I have no idea how that is going to work for me. What can I do?


Most people think of the anal sphincter as if it was just a single ring muscle when it is actually an amalgamation of rings connected to an amalgamation of other muscles, the function of which is to keep things out or push things out. I have little doubt that you have seen larger solids come out of your ass when you look in the toilet then anything you are currently trying to put in. It is essential to realize this point. It goes a long way to answering the question in the title of your OP – No you’re not too damn tight. At least not in a engineering sense. All will become easy when you understand what you must do at a conscious level and what you need to do to train your nervous system allow stuff to go in as easily as it pushes things out and only when you want it to.

I have published several articles that you should find useful. You should also share this with your partner. In no particular order:
Starting with an Anal Sex Virgin
Anal Training by Yourself
Anal Training (together)
Avoiding Anal Sex Pain
Anal Training after a bad incident

You will quickly see the overlap in the advice and information given in each of these articles, but I still get questions on some of the specifics. So, here is an FAQ on the subject of getting past/avoiding pain or tightness – Anal Sex Pain catch-22

Dealing with the smell of an enema2022-08-15T16:32:54+10:00


I have always accepted that there is a damn awful smell from expelling an enema. And it lingers for ages. It totally stinks out my bathroom and it is terrible if you have to share the bathroom with anyone else. Is there anything I can do that will stop the stink?


First up, if the smell is that bad then you are dislodging and mobilizing the contents at that top of your colon. That is the smelliest stuff. So the first thing to consider is whether you need to clean yourself out to that degree. If all you are worried about it making sure that having anal sex is clean then you are either over doing things or you need to make some simple adjustments to your diet. No, I don’t mean that you have to starve yourself before anal sex. That is also over doing it. The first thing is to understand what you need to do to achieve the level of cleanliness you are after. You want to start with the article – Eating and Enemas for Clean Anal Sex.

If your goal is to clean out more than just your rectum, the solution is soap. The first article references the Total Colon Clean Out. This definitely will release the smell with the first high volume flush. Adding some liquid soap to the mix will make the expulsion of the enema foam. The foam will trap the smell. Lavender scented castile soap does a particularly good job by adding a pleasant scent to the water while trapping the stink. You can get a similar benefit from adding the soap to the toilet bowl water. Your expulsion will foam the toilet water and trap most of the smell. After the first hi-colonic you won’t need the soap. You just want to rinse your colon with plain water.

See complete list of Q&A's

Full FAQ list


Go to Top