Griffon 250- Piston edge/transfer port position

[Kim275]

Hi All, (especially Chris kenny or Druid !! - or anybody who knows about 2 stroke porting)
Am having some challenges tuning my 250 Griffon - which had to have a new cylinder liner - reworked head and new piston ( a new forged one from Simon but to original griffon spec). I am aiming to end up with a 12:1 compression ratio but optimise squish band clearance.

My basic problem is that to get the compression any where near spec - I will need to take off some of the top of the liner. In addition the piston edge is slightly proud (above the bottom of the transfer port) when at BDC - so adjusting base gasket isn't an option as this would raise the piston edge even further at BDC.

How critical and accurate does the piston edge need to be to the bottom of the transfer port at BDC (assume it MUST not be below it)?

Do we have any factory drawings of barrel /port dimensions so I can check against? - e.g At BDC - what should the effective height of the transfer ports - an the height form port to cylinder top edge?

So my assumption is i need to do the following.

1. Adjust base gaskets till the piston edge is flush with the bottom of the transfer port @BDC
2. Check compression ratio
3. machine off top of cylinder liner - to reduce piston edge to top of cylinder gap to raise compression ratio

In all the above - assume I will maintain a minimum squish of 1.5mm (does anybody have any data on optimum squish?)

Many thanks for any assistance
Kim

[dave higgins]

Hi kim, another possibilty is to reduce your base gasket & machine out your barrel transfer ports to match your piston port height at bdc, depends how much you are looking to shave off, this would increase your piston position at tdc. dave.

[druid]

Griffon 250

Kim,
How has the head been reworked?
If your liner has been replaced with an exact replica of the original and Simon's piston is the same the original then there shouldn't be too much of a problem.
There are liner drawings about as there are for pistons (I supplied the 250 drg to Simon).
If there are changes to the new liner and piston as well as the head you're in for a busy winter.

If you can't get a barrel drawing from the club drop me a pm and I'll see if I can find one.

Contrary to popular belief, the factory did know about porting. You can clean them up but I wouldn't alter the timings.
Druid

[Kim275]

Dave Druid, thanks for your good suggestions.

Agree that Greeves were masters of piston porting - The barrel and piston were done by simon so should be to spec - but I think my starting point is to validate that my piston and liner were done exactly to original first.

The head was worked by Stan Stevens - to align squish band to piston and improve the shape. He tuned barrel and piston togther - but did not alter port timing.

Once i have worked out any differences vs factory blueprints settings this should give me a clue which way to go to get the compression correct. And hopefully when I can achieve that I shoudl get some good performance.


Cheers Kim

[Rob]

Kim - I have a piston drawing. Just looking to see if there is anything else. I have various 250 barrels kicking about if you need any dimensions.

[Kim275]

Hi Rob - Key dimensions off Barrel I need are vertical height of transfer port - and bottom of transfer port to top edge of cylinder. Also a piston diagram would be great just to cross check - although very unlikely simon made a mistake there.
Cheers Kim
p.s front axles/spacers WILL be measured tomorrow !!

[Rob]

I'll see what I can dig out. Bit of a panic at the moment as I am putting the Anglian together (only been a box of smashed up bits for 30 years) for the Perce Simon next Saturday. Got half an engine together and a sort of rolling chassis. MOT is booked for Wednesday morning! Argghhhhh!!!!!

[Kim275]

A quick update on 250 Griffon progress. Druid kindly lent me the Piston drawing from 1969 (and fascinating detail it is !!) - that confirms my piston is as per specification.

The port heights for the transfers and exhaust give reasonable timings (thanks Kenny for your info) - with standard base gasket - which gives me a squish clearance of 1.3mm - and compression ration of 11.5 to 1 . I have put engine back with standard exhaust and standard inlet (piston without cutaway) - and engine runs pretty sweet.
When I get a chance I will machine the head/liner top to reduce squish to 1mm- and increase CR to 12:1 - but apart from that I think I am there .....just need some decent weather for a test ride .
Cheers all for help.
Kim