Shaved heads and timing

[jeffro ra28]

Don't want to hijack this thread, but i've been wondering about shaving heads myself, particularly on the 18RG... my biggest question is how much does a shave of something like 1.5mm translate into compression ratio increase? I know it depends on more then just the amount you shave off, but i'm just looking for rough estimates, or past experiences. Would 1.5mm get you a 10-20% increase in compression? or is 1-5% more realistic?

And is there any real difference between shaving the head or the block?

The percentage increase of compression per shaving heads will be different for every different bore size and chamber design.

Since the 18rg has SFA squish the percentage increase of compression for say a 1mm shave from the head will be more then for say a pent roof chamber with a tighter squish and a higher squish - area ratio because the actuall volume decrease of the chamber for that 1mm shave will be more then that of a closed chamber (remember, less chamber volume = more compression)

You will almost always obtain more compression (decreasing volume) by shaving the block because you are removing a full 1mm of the potential area. Not a percentage of the area like with a chamber.

If you think about reducing volumes rather then increasing compressions you might understand it better. Bare in mind, going to the extremes of shaving heads the percentage of volume decrease per 1mm shave will not be linear as the chamber shape changes. You are removing less volume for every 1mm cut you take. But with the block, you are shaving a perfectly straight cylinder (i hope) so that each 1mm of shave will be the same volume decrease.

Unless you want to do some modelling, the best way is to CC the head as you are shaving it. Or, you can do some rough calcs on how much volume will be removed by measuring your chamber diameter at the gasket surface (should be easy since its 18rg and perfectly round). For the sake of discusion, calculate the volume of a cylinder that is 1mm deep and the same diameter of the 18rg chamber. Im pretty sure you would have worked it out by now. But this measurement will only be a geustimate as the 18rg is hemisperical. So for every 1mm that you cut off the volume decrease will be less per 1mm.

Hope that helps, sorry for veering of the topic.

["Z" UTE]

Do you have a new cam belt tensioner installed along with a new belt and idler? Lotsa talk about other important things in the thread, but have you had an answer as to why the engine worked for a day, then smashed valves? Did you check the relationship between actual top dead centre (TDC), and the TDC timing mark? Was the mark accurate? Does your engine rebuild manual give you a good description of setting up the timeing, and a decent picture/diagram showing the relationship between the cam and the markings in the head?. Is the cam sprocket located to the cam with a dowel, and a multiple choice set of holes, or is it a one position only cam and sprocket arrangement?.

I would like to see this one sorted, cheers Chuck.

[Sagluren]

being that it is an interferance engine and you have originally taken 1.8mm of clearance from pistons and valves, im not surprised you have had some collisions. Keeping im mind that it is not a old v8 but a highly enginered modern engine, shaving and removing material in general is probably not the best idea for such an engine.
going by the diag infront of me the tensioner looks to be a simple spring/ pivot type and so there for if locking bolt was to fail ie. not tight enough would not cause a clashing problem. providing the spring was still providing some tensions.
If it has a hydrolic tensions and it was to fail, then yes i can see some damage occuring.

It is my understanding that the engine was noisy from the moment you started it. so idealy you should have stoped and investigated not driven it for a day and a half ( not having a go, just something to think about)

In regards to fixing, make sure the tensionser still holds enough tension on the belt, if that still looks good and secure, it is time to get either adjustable cam gear and hope you can adjust it and still make power without death to valves, or get the cam reground/ remade to suit your new found engine specs,

I hope this has helped.


cheers.