mk 2 square four- piston crown height

[bob cottam]

Hi All, me again with tonights tech. question:

My engine has seen some action, (like most Sqares!) and i have had to have the crankcase mouth skimmed, and also the barrel top face to take out marks/distortion etc. It may not be the first time this has been done to this motor, who knows?

Anyway, with barrel on (no Gasket) and a new IMD piston in, at TDC the piston top protrudes from the barrel top face by very nearly 1mm (not the central raised boss, the outer area of the piston face).
I assume that this should be flush/slightly below the barrel face? .

So it seems my engine has lost quite a bit of metal over the years, I have no base gasket at the moment, but i assume they are roughly about 0.020"/half a MM thick , which would still leave 0.5 mm of piston at least protruding.
I am thinking of making a thicker base gasket from 1mm material, and possibly turning , say 0.010-0.020" off the piston tops for good measure.Alternatively, I could use 1.5mm gasket material, and then possibly leave the pistons as supplied?
The head gasket should thereafter provide plenty further safe clearence.

Anyone have any thoughts/ alternative solutions on this ? Cheers, Bob C.

[john.bebb]

Hi Bob; - I have a similar situation with my Mk1... and will observe this post with interest!

As far as I'm aware; - the (Mk1) Otto head gaskets are >=1.5mm uncompressed and consider they would loose a maximum of 20% thickness when the head is torqued down correctly - and so adequate headroom should remain.

Clearance of valve head to piston is not an issue as the lift (on a new cam) is only 5/16", the pistons are way down the cylinders before full lift and even if the head has been often skimmed - the (Mk1) valve faces rest some 1/2" deep in the combustion chamber (= 3/16 clearance from mating surface at full lift). There should be more than adequate clearance even with the high compression pistons as the crown raise is only 3/16" and again; the pistons are well down from TDC whel the valves are fuilly open.

The fitting of a thicker base gasket seems rational should you still be concerned - I'm a bit suprised Drags do not offer thicker base gaskets (hint, hint!) as there are surely several Squares in similar predicament. Though I'm fairly certain tolerance controls have improved since Squares were produced!

I think to skim the pistons would be over dramatic and (assuming you are a perfectionist...) may then require re-balancing the crank & piston assemblies to Factor = ZERO as the removed mass is from the extreme diameter of rotational throw and so apply maximum variance.

So that's my views on the issues... (please note I'm not acquainted with the Mk11) - I leave it for comment by fellow Squengineers & cognoscenti...

Cheers, John.

[bob cottam]

Hi John, thanks for the reply. I am presently at the point where the crankcase halves are ready to be joined together, but before i do i have been checking all sorts of things like the club rods clearance with the barrel spigots (with no gasket-rods very close, but do just clear) I have the offside case half in a work stand, joint face up, with the cranks sitting in place , and the barrels fastened on to the one crankcase half-so i can see right inside to check everything. I fitted 0.8mm alloy shims under the barrel to simulate a "compressed" 1mm gasket,with the barrels thus shimmed, the pistons are some 0.005-6 proud at TDC, which i am thinking should be OK -as you say the head gasket will provide the clearance. Indeed, there may be no need to skim the pistons.
I measured my barrel casting thickness today using a surface plate and good straight edges- the top and bottom faces are about 112.4mm apart- marginally under 4 and 7/16". It would be interesting to know what the original ex-works dimension was for the barrels (mk2),to see how much has been skimmed off and calculate the thickness of gasket needed to compensate-(ditto the crankcase mouth height from crank centreline)
I wonder if the Club (Bruce?) has any drawings from factory records or elsewhere that could be referred to,it would make life a lot easier....Bob.

[john.bebb]

Hi Bob - ty, & yw... I like your "check" set-up; - your measurements imply maximum piston protrusion (with zero gasketing)to be 0.95mm o/a.
Compressed gaskets (abs. min): Head + Base = 1.1 + 0.3 = 1.4mm (values assume gasket compressions of > 25%)...

therefore worst case min headroom = 1.40 - 0.95 = 0.45mm = 0.0177" - plenty of room!... so no need to bastardize the pistons!!!

Am intrigued to know why it was nescessary to skim the crankcase mouth... damage 'cos a rod let go? crack repairs? or had some previous
Luddite used hammer and chisel to split the barrels off...?

Assuming you've fitted new mains (& you're UK based)... plz - who did your line reaming?... (cost & time?).

Will also be interested should anyone supply original dimensions... for Mk 1 & 11... Crankcase; - crank centre to mouth? Block; - head mate to base mate? & Head; - rocker spindle centre to head mate. Such dimensions would save much cold assembly & strips...

Btw - are you aware of the excellent suggestion (elsewhere in forum) for fitting block to crankcase with pistons & rings already in the bores...
then fitted to conrods via the gudgeon pins?
I've never tried such method... but thinking on it it seems a marvellous method!
REMEMBER; - 1/to fit all innermost gudgeon pin circlips first... 2/ ALL piston "bias arrows" to point to camshaft (as front crank contra-rotates!)...
& 3/ when you fit the outer circlips; - do try not to drop them in the cranckcase...

Cheers, John

[simon.holyfield]

John,

Some years ago, Bernard Ashpole in Bishops Stortford did mine:

http://www.autoinsider.co.uk/local/engi ... i_8147.php

Good people, they know older engines.

[bob cottam]

Hi John, regarding the need to skim case mouth, it was a case of "all of the above!"-my engine came to me "newly rebuilt" from a very prominent Yorkshire dealer who specializes in classic bikes.....never, never, EVER again...... total wreck with odd rods,sandblasting sand inside etc etc-long and sorry tale involving lies and deceit from more than one party...... but i ended up with it , older and wiser...I am determined that it will run again -and to a very high standard too.
The motor had previously thrown at least two rods, damaging the "gantry" above the camshaft- i detected some localized distortion in the crankcase mouth nearby, and yes- also "screwdriver/hammer method of separating the barrels was in evidence..... there is no doubt that some people should never be let loose without adequate supervision!!
I have spent a fortune, but as we know there is no other way, so onward and upward...
I had the new mains line bored on the recommendation of a Club Member, at Hamlins in Bridgewater- took them 5 wks -bit longer than promised but no problem with that timescale, and cost very reasonable considering the labour /set up and use of a very specialized line boring machine-(not just guesswork on a vertical mill as some/most firms would do it!) I am very impressed by their standard of work and i would also recommend them.( Ahem: "other engine reconditioners are available" etc etc.)They have also recently done more work for me on the barrels/head (double quick time too)and all seems very good-worth the trip!
Yes, i would go with the piston in barrel first method- especially on the square- (with engine on its side) I was recommended this method when building a v-twin recently- and its a "no-brainer" for me now- saves piston rings and furrowed brows!!
It might be worth gathering together any info available as to engine dimensions as a sort of reference source-The info must be out there somewhere!All contributions welcome!!....Bob.

[john.bebb]

Hi Bob - thanks for the gory details of the case mouth skim requirement! - and the recommendation of Hamlin's.
Good to know you're aware of the pistons first... I've never tried it myself --- I've done my Sq at least six times over the years by the "standard" method,
both in & out of the frame...& yes... single handed - & I've never broken a ring... but when I read about the "piston first" & got MY head around the logistics of the method - I thought what a super idfea!!! - why didn't I ever consider it?

I note that a (Mk1) New Old Stock head & barrel have been found (in Canada) - the price of the barrel was astronomic yet it is already sold...
now, if the new owner could be coerced into doing the measurements accurately... PLEASE - could he post the dimensions???

Cheers, John

[bob cottam]

Well , as a starting point, i took some time today to measure the distance between the crank centrelines and the crankcase mouth face on my MK2 motor.
I turned up a nylon peg to be a nice close fit in the main bush, and used a HSS lathe tool held against the machined gasket face on the crankcase mouth as a ruler stop.I measured the gap and added half the dia. of the main bush.I did this on both mains to check my accuracy, and reassure myself that the crankcase skim was parallel to the crank axes...( It was bang on).
The eventual figure I came up with was 4.131", which is over 4 &1/8" by 6 thou, so as my casings have been skimmed, I suppose the original spec. must have been either: 4& 9/64"(4.140),-or the next imperial "round figure" is 4&5/32" (4.156), unless anyone knows different :

- can anyone supply the ex-works figure for the distance from crank centreline to crankcase mouth (base gasket face), and also measure the height of a unskimmed barrel (alloy casting) accurately to establish the ex works measurement?
Cheers, Bob

[bob cottam]

As my deliberations progress, I have investigated further into the problem of pistons protruding from the barrels at TDC. I spoke to Bruce Longman re this, explaining that my cases/barrels have been skimmed to rectify damage,and thus metal lost. The protrusion of the IMD pistons on my engine i have now measured accurately with a clock gauge and it is 0.036"- very nearly 1mm.
Bruce confirms there are no drawings/documents available that would show the true, factory dimensions for the block etc, but he concurs that at TDC, the outer edges of the piston top should be flush with the barrels, or very near (for a MK2 anyway- disregarding the central boss which has plenty of room in the combustion chamber).
As i began to figure out what sort of compression plate would be needed to raise the barrels slightly to overcome the problem, a thought occurred... Is the IMD piston top higher than original pistons?-could this be a contributor to my problem?
I have no original pistons, but i do have a set of Hepolites that came with the bike. I compared the two types of piston by sliding a gudgeon pin into both at once and immediately saw a significant difference (photo`s).The IMD crown edge is significantly higher- i measured it at 0.35mm (nearly 0.014").
So, it seems my engine has not lost quite as much meat as thought- the IMD pistons do seem to exacerbate the issue (by 39%!) Of course this assumes that the Hepolites are as they left the factory i.e. not machined down , but they do seem unmolested, and indeed also assuming that the Hepolite crown-gudgeon pin distance is identical to factory original pistons(surely it would be?). i spoke to the guy at IMD Pistons today at the Stafford show, and he confirmed that the IMD has a certain amount of dimensional "compromise" in it so that it will fit earlier engines with a little machine work if desired.
So there is a little snippet to bear in mind if you fit IMD slugs- have a good compare with your old pistons. Bob C

[john.bebb]

Thanks, Bob - EXCEEDINGLY useful observation - we really need whoever bought the NOS (Mk1) Barrel, as was flagged for sale in Canada last month; to do some accurate measurements & post same... would be even nicer if he'd have it scanned for a computer 3D model...
Cheers, John