LCGB Forums

Bit of advice/help required please guys!

TS1 sleeved to suit 70.94mm Wossner piston, Bored out for 0.005" clearance. Avanti head for 225, ie 71mm across bore, Now the head has very little clearance and I wanted it machined out to 72mm to give 0.5mm clearance around the edge of cylinder bore. I have checked the squish and with 2.00mm head packer squish across 4 corners of piston is 0.7mm, too low, want at least 1.2mm for safety.

Now when I take head to get machined do I ask machinist to take out 2.5mm from head and out to 72mm, I'd like to run head without any packer or gaskets? Any advice please as I want to get scoot back on the road asap. Ta.

The current trend (at last!) is to inset the head into the bore which can serve to eliminate the need for a head gasket, but also centralises the head to the piston & therefore the squish band.

Unless you can afford to add base packing because your port timings will allow it, you are going to have to recess the head, as you have suggested. Bear in mind the effect upon your head volume…

However, dependent upon your machinist & your own comprehension of this reply (I tend to be a bit technical. Sorry!) you can still create a small spigot on the top of the barrel & a concentric recess in the head somewhere outboard from the bore.

Otherwise, unfortunately, to ensure your squish is centralised, you should consider truing the head to the bore via the piston crown & then using dowels to locate. I would advise against relying entirely upon the bigger (Ø72 mm) head recess to allow for errors.

In my opinion, always do away with head gaskets if you can.

Think I understand what your saying, I'm running a 60 x 110 crank with 1mm base and 2mm head packers, no gaskets, so if I can get the transfer timings right with a base 3.5 mm packer , get head machined to 72mm dia, then dowel it centrally hopefully I should get the squish right? I did use the 4 cross solder method and having read some of Darrell Taylors methods he reckons that can give a slightly false measurement?

If I remember correctly, the recess on an Avanti head Is 2mm,
I measured a head and the head is 14 mm thick above the combustion area.
I would guess this is plenty to remove the "spigot" and basically start again.
Or drop Ron a line he might be able to sort you an unmachined head.

Scooterlam wrote:If I remember correctly, the recess on an Avanti head Is 2mm,
I measured a head and the head is 14 mm thick above the combustion area.
I would guess this is plenty to remove the "spigot" and basically start again.
Or drop Ron a line he might be able to sort you an unmachined head.

The heads Ron sells for non Avanti use are machined flat, I have one on my TS1.

Am I missing something or can't you just add thick or thin base gaskets to get the squish right? Ron sells them in various thickness too.

I think you will find that Darrel favours Plasticene for checking squish, I used some on his recommendation and it works a treat, 4 blobs and it is very easy to measure with a vernier.....careful you don't let it fall down the transfers on the final check though

Yes I'm going to use plasticene method as what Darrell says makes sense, especially with crap fuel about and fine tolerances required! Coaster who machined your head?

Think I understand what your saying, I'm running a 60 x 110 crank with 1mm base and 2mm head packers, no gaskets, so if I can get the transfer timings right with a base 3.5 mm packer , get head machined to 72mm dia, then dowel it centrally hopefully I should get the squish right?

As I said earlier:

Unless you can afford to add base packing because your port timings will allow it, you are going to have to recess the head, as you have suggested. Bear in mind the effect upon your head volume…

To be able to vary the packing so much between head & base seems a little unscientific to me, but as long as your state of tune will allow for that, then that's alright.

I think with all of the positive replies you've had, you probably just need to make a choice & go for it.

Interestingly, nobody has yet argued that head gaskets are much misjudged.....

Yes all positive stuff, thanks for everyone's input, called AF this morning and they are going to machine the head for me so I can negate head packer and retain current base. Cheers

Hi the avanti head can have dowels added to pin the head to the barrel good if the squish disallows intrusion into the bore.

its all well and good having the head recessed into the bore, until that is a re-bore is required, then there are issues. if heads are properly mated to cylinders you can get away with slight variations in bore sizes etc and make things a a little more simple, without any loss in performance or reliability.

Rally220 wrote:Yes I'm going to use plasticene method as what Darrell says makes sense, especially with crap fuel about and fine tolerances required! Coaster who machined your head?

Sorry for the slow response, as I said, Ron sold me a head that was already machined flat. SLJ and Warton have said, the head needs dowling which I did by drilling down trough the fins after cenralising the head on the piston and inserting some dowels (cut down nails).



This was before Darrell told me about the plastecine method


Not sure if it makes any difference but I also removed the material between the fins to improve airlflow. Took me half an hour or so with an angle grinder.....VERY messy I only did the flywheel side, the other side is not covered by the cowl so there didn't seem to e any point.




Colin

^... ha ha ha, I've been thinking about doing exactly the same to one of mine. Fools seldom differ, I guess (lol)!

Adam_Winstone wrote:^... ha ha ha, I've been thinking about doing exactly the same to one of mine. Fools seldom differ, I guess (lol)!

I prefer 'geniuses think alike'

Quality thanks Coaster, more jobs to the ever growing list