Griffon 380cc engine rebuild - barrel & piston

[Martin H]

Hi All

Some time ago (years) I bought a Griffon 380cc that was displayed at Kidlington. Following several letters to DVLA and HMR&C I managed
to get it road registered...... A big THANK YOU to Colin.

I have not been on the dark side (two stoke) for decades. About 40 years ago I had a Bultaco, but have never looked inside a 2 stroke.

As the Griffon has not been started for many years, and I do not know the detailed history, I am reticent to start it without looking inside.

Even with a bit of oil in the bore it was easy to turn over by hand and seemed to lack compression - although I do not know how easy a
Griffon is to kick over ?

Last week I took off the head and barrel. The studs and nuts were rusted externally and looked to be original "stiffnuts"

With my Mitutoyo digital calliper I measured (averages) the bore (after hone) and piston:

Top of bore = 81.96 mm
Bottom of bore = 82.03 mm
Top of piston = 81.74 mm

I took the barrel and piston to a neighbour who has owned a motorcycle repair/restoration business for 40 + years. We agreed that the
machine marks in the bore were probably original indicating little and he gave it a light hone. The piston has some "pick up" marks
probably from the engine being turned over with oil in the bore - not me. The piston has dark petrol/oil stains showing the engine has
been "used" in the past.

Putting the old piston rings in the top of the bore gave a gap between the sharp pointed ends of 0.050". Seems a bit big ?

Questions:

From website data a standard piston seems to be 82mm so a + 0.020" piston would be 82.508 mm.
My piston with little/no wear (?) is 81.74mm
Do I have a standard piston ?

My piston ring gap is 0.050".
Do I need new rings ?
Are standard rings available ?

I will be at Kidlington next weekend and may well bring my piston and barrel for people to fondle and pass judgement

Thank you
Martin

[AndyB]

Hi Martin,
I've approved your post - it was probably down to that 'f' word that you used in the last sentence that upset the censor-bot!

PS Ian is quite busy with work so I've been delegated to do moderation on this forum.

[Martin H]

Hi Andy

Thank you, much appreciated.

I'm a bit surprised at my gentle f-word

Thanks
Martin

[Jeff Morgan]

I've never built a Griffon engine but there some general rules of thumb that may help. Piston ring gap for an air cooled motor is usually around 4 thou per inch of bore, yours is roughly 3.2 inches so you'd want a ring gap of slightly over 12 thou, so 50 thou is excessive. not from a manual but gives you some idea.

Dark brown staining on the piston indicates combustion gases 'blowing by' the ring, not desirable, and another indication of worn rings.

Piston skirts taper out from top to bottom, and from the measurement at the top it looks like a standard piston, but try measuring at the bottom of the skirt as well, it may be bigger.

A quick and easy way to determine whether piston to bore clearance is 'acceptable', without measuring, put the barrel upside down on a flat surface, without studs in so you get a seal between the surface and the top of the cylinder liner. put the piston, without rings, in its correct orientation relative to the bore, and let it drop under its own weight. when it gets below the level of the ports the air trapped beneath it will slow its descent, unless the clearance is excessive, in which case it will not slow down. wouldn't pass muster to an engine builder but gives some idea if you don't have the correct measuring instruments to hand.

my thoughts only, happy to be proven wrong..........

[Martin H]

Hi Jeff

Thank you very much for your informative reply, most useful.

Measurement of the bottom of the piston gave an average of 81.79mm. Perhaps a small taper ?

With a thin smear of 3in1 oil the piston speed dropped marginally when allowed to free fall down the barrel as you described.
Slightly slower when resting on a plastic bag rather than newspaper.

My thinking at present is to try some new rings to check the gap, but maybe buy a new standard piston due to the marks on the current one.

Thank you
Martin

[John Wakefield]

Gary Bamford has some new std 380 pistons @ GBP 85 each, https://www.britishbikes1970.com/par...t-number-g1430 Looks like you current piston is within tolerance and should be OK with a new set of rings. Gary also has in stock both top (dykes) and bottom rings for standard piston at GBP 12.50 each

[Jeff Morgan]

Hi, if you were building this engine to race I would say rebore it, (assuming oversize pistons are available) but if its just for a bit of green laning etc I would do as John suggests and put a new piston in it, those prices seem very reasonable to me. It can be difficult to determine the amount of wear on the skirt, (as opposed to damage) without spec from a new one, but I would expect a greater degree of taper than you have. If the rings have worn to that extent it follows that the piston will have significant wear on the skirt as well.

Having said all that, it would be good to know if the bore is worn as well (pistons generally wear quicker than bores), you can get some idea using a vernier gauge 10 - 15mm from the top and bottom across the thrust/anti thrust axis but they tend to wear more in the middle section, where the ports are (less material), and a Mercer gauge is required to reach that bit. if the bore looks to be in good condition, uniform colour with no irregular blow-by staining and no damage, a new piston could bring it back to a usable level but rebores aren't that expensive.............depends on your budget I guess!

[IanCordes]

Martin. Why don't you ask Gary to bring a standard and 1st oversize piston to Kidlington, just in case? That is assuming he has oversize as well as standard. Don't leave it 'til the last minute to ask him, though.....

[Martin H]

Gentlemen,

Thank you for the replies. Much appreciated.

I am building this Griffon for use on the road. Despite owning a Norton Wasp outfit in my twenties, again for the road, but never achieved.
I was no scrambler then and now I'man old codger as well.

Following Jeff's reply I have tried to measure the bore with my rings at several levels and with the gap at several locations
around the circumference. It is exceedingly difficult using two 0.025" feeler gauges. However, the bore appears to give roughly
the same ring gap of 0.050" at every height and every rotation.

In answer to Jeff's remark "If the rings have worn to that extent it follows that the piston will have significant wear on the skirt as well".
I would have thought so, but the piston appears to have little wear as does the bore. Surprising.

Great minds think alike .... "Why don't you ask Gary to bring a standard and 1st oversize piston to Kidlington, just in case?"
I thought the same !

I have already been in touch with Gary and need a few other bits so will email Gary tonight with a list.

I intend to bring the barrel and piston to Kidlington on Sunday for expert inspection.

Thank you
Martin

[Brian Catt]

380 Griffon on the road......A trace ambitious???? I detuned my 250 OSSA as being a bit too much for greenlaning.

[iansoady]

Gentlemen,


I am building this Griffon for use on the road. Despite owning a Norton Wasp outfit in my twenties, again for the road, but never achieved.
I was no scrambler then and now I'man old codger as well.

Thank you
Martin

I always had a (unsatisfied) yen for a roadgoing Norton Wasp outfit after riding with them in a variety of long distance trials back in the 1990s on my Enfield. They were always a sight to see and hear especially taking off on a rocky climb.

[Martin H]

"A trace ambitious" ....... Maybe true for me, but photo of Gary's road going Griffons 380cc & 250cc pair outside Exeter Hall yesterday morning. Tasty

[Martin H]

Sorry for delay in reporting progress ... being playing with my Nortons ... sorry ... now washing my mouth out with carbolic .....

Gary kindly inspected the bore at Kidlington and put a new piston in it, pronounced bore to be good and the new piston a good fit. Thanks Gary

So I bought a new piston, which agreed with my thinking as the old one had nasty marks from pick-up possibly due to kicking over dry ?

New piston measured up about the same as the old one - see first post, but looks a LOT nicer.

Ring gap of new ring in honed bore is 0.023" top and bottom and at various radial positions. See above - old ring gap was 0.050"

I'm now happy that the bore is standard, does not require a rebore and I have a new piston, rings and circlips. Thanks Gary

So I took the head and barrel to my local blasting and powder coaters. They specialise in bike stuff and are bikers themselves. One
has owned and raced several Greeves.

I was tempted to have the barrel and head ceramic coated, but they said it would be difficult to ceramic coat evenly between the fins
and there would be an unsightly build up on the fin edges as the ceramic spray is a waxed based solution, so I am having a semi matt
powder coating.

The nuts on the barrel/crankcase sits were corroded and totally fixed. When removing the barrel I had to lift the barrel and undo the
some of the studs from the thread in the barrel base, then use mole-grips on the stud to remove the nuts. Destroyed 2 studs.

I am now having the 2 studs (G1248) and a third as spare made, and depending on cost a spare set of 8 made. Gary only had one stud
left. I probably could have made the studs myself, but the guy suggested to me by the powder coaters was extremely interesting with a
huge workshop of restorations underway... Vincent engine, Commando engine, Frazer Nash car, Pioneer bikes, scramblers....Well worth
the cost of studs! Anybody need any G1248 studs ?

I took a Frazer Nash petrol tank form the "stud man" back to the powder coaters, via a chroming company where I dropped off the front
exhaust pipes. I stopped at a couple of other "shops" on the way to do errands for "the management" but felt uncomfortable leaving the
F-N tank in the car in case it got nicked.... what anybody would do with a F-N petrol tank I don't know but a replacement would be like
looking for Unicorn poo !

[druid]

A previous post on end gaps gave a recommendation of .004" per inch of bore say 12-14 thou for a 380. While I like to have a bit more to be on the safe side 23 thou sounds a bit excessive to me.

One other thing, Greeves spent a fair bit of effort to get those big cooling fins in the airstream to keep the temperature down and you're going to wrap them in a plastic overcoat. Why?

Druiid.

[Martin H]

Hi Druid

Thank you for your reply, very interesting.

Upon inspection by myself and others it would appear that my barrel still has the original machining marks
indicating some, but little use/wear.

Measurement found the bore to be pretty much 82mm so standard with perhaps a little wear.

A new ring from Gary gave a gap of 0.023"

So how do I get a gap of 0.012 / 0.014 with a standard bore and a new ring ?

Looking at original brochures I have and Bob Carrick & Mick Walkers book and Colin's book seem to indicate that Griffons
came supplied with a black barrel. However various photos on the web show Griffons with both black and bare alloy barrels.

Nearly all the old clunkers I have owned over the past 50 years have had black barrels from new.

I have l read articles about the effect, or not, on temperature caused by black barrels.

As stated previously I am not going to race this beastie.

And I note that Gary and Tony's road going Griffons both have black barrels.

But I take your advice and may well just have the likely original (?) black paint blasted off, but not recoated black.

Thank you
Martin