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Big bore 74.7mm project, head gasket question
https://www.ausmini.com/forums/viewtopic.php?f=5&t=61777		 Page 1 of 2
Author:  73GT [ Mon Mar 15, 2010 12:01 pm ]
Post subject:  Big bore 74.7mm project, head gasket question
Hi

I have acquired a block that has been bored out to 74.7mm to take triumph 2500 pistons.
This should give 1425cc with the std stoke.

I took the block to the engine reconditioner & he expressed concern about the head gasket sealing.

What he said was pretty logical in that there is .075” metal between the bores in the block, whereas the head gasket is .150” wide between the bores

Therefore the edges of the gasket will be exposed to the combustion heat/pressure.

I know that this overbore has been used by others before, ie detailed by Vizard in his books, Matt Read has a 1510cc version

So my question is what gasket do you use to get a reliable motor & is the fact that the edge of the gasket exposed not a problem.

Once I know that the engine will be reliable I will look forward to finishing the motor & getting it running.

Hopefully this will be another example of minis don’t die they just get faster.
Author:  TheMiniMan [ Mon Mar 15, 2010 1:12 pm ]
Post subject: 
yes it is an issue & i`ve just used the std BK450 Payen head gasket , but i do keep the compression ratio down to a reasonable level,,, if you try to knock the comp ratio too high then the intruding head gasket will be a hot spot & cause knocking/pinging/detonation

But, at 10.3:1 my 1510 has never had an issue at all

My theory here is that the thin bit of block actually "Bites" into the head gasket very well & seals,,, :-) a bit like a lot of the "old" guys used to use a piece of fuse wire between the cyls to help bite into the head gasket,,, i`ve never bothered to do that myself, but in effect this very thin bit of block between the cyls on such big bores does actually do a similar thing

But yes the problem is simply an overhanging gasket edge & yes it is an issue, but it`s never given me any grief (Yet) :-)

there was a Lager gasket avaliable (once upon a time) but i have never bought one & have no idea where to get one or who makes them (if anyone) anymore

also ,,, just be aware that if you use the T-2500 pistons,,, they`re a very "short" piston & will make a "Very" low compression height,,, either longer con rods or shave the block down quite a bit

where-as T-2000 pistons are quite tall,,,

But with either of these you may need to make a "Knotch" into the bottom front thrust side of #3 piston to clear the dissy drive shaft---(((depending if you stroke the engine or not)))

I just skimmed my dissy drive shaft down very thin to clear the piston tho

:-)

:-)

:-)
Author:  lmbm [ Mon Mar 15, 2010 7:17 pm ]
Post subject:  Big bore 74.7mm project, head gasket question
I've seen in the past some special gaskets made by Cometic in US for big overbores, not sure if they still are available. Expensive stuff they were.

cheers
Luis
Author:  850man [ Mon Mar 15, 2010 9:04 pm ]
Post subject: 
How about a laser cut steel shim, made to suit your block, Holden V8s had them fom new. Or you could get the machine shop to do a nice fine surface on the block & head & run no gasket, like a RR Merlin. Or you could have O ring groves cut into the head surface & use O rings, like some HP turbo motors.
Author:  slacko [ Mon Mar 15, 2010 9:15 pm ]
Post subject: 
could you use a copper gasket , laser cut ? my 2 stroke motor bikes use copper head gaskets , might be a dumb suggestion I dont know , just thougth I would put it out there :?
Author:  drmini in aust [ Mon Mar 15, 2010 9:22 pm ]
Post subject: 
Lots of race cars use solid copper gaskets. Water jet cutting is better than laser for cutting these.
O'Brien Aluminium used to do them, maybe still do.
However I've heard they are not too great on an A series.
Author:  slacko [ Mon Mar 15, 2010 9:32 pm ]
Post subject: 
When I take the head off , if the gasket looks ok , I clean it up , spray it with copper head gasket spray and so far this has been OK , but these are only 2 stroke engines no problems so far , and my boys do give them a flogging
Author:  73GT [ Tue Mar 16, 2010 6:36 am ]
Post subject: 
hi all

It is reasuring that matt has not had head gasket problems using a BK450

is the BK450 a composite gasket not copper?

as this is mainly a race motor(it came with a 649 cam which i intend to use) i was planning a higher compression ratio of around 12:1

the block had already had the top machined to suit the 2500 pistons so no stroker for me.

i guess the gasket overhang is a problem in a 1380 as well but not to the same degree & these seem to be reliable
Author:  drmini in aust [ Tue Mar 16, 2010 7:13 am ]
Post subject: 
BK450 heat rings round the chambers are much more heat resistant (S/S?) than the ones in a so-called `copper' gasket, which are just the bottom mild steel sheet rolled up and over the top copper sheet.

I reckon you will get away with it, particularly if the head chamber line covers the gasket as usual.
Author:  awdmoke [ Tue Mar 16, 2010 8:54 am ]
Post subject: 
I have a 1430cc stroker running reasonable compression (but not 12:1)
A daily driver, and has been used in our local sprint & hill climb series
I put a BK450 on it a couple of years ago & have just had to replace it two weeks ago due to burning out the heat rings between #2 and #3. Yes they are composite.

Reckon that's a pretty good lifespan, so I put another BK450 back in there.
Author:  TheMiniMan [ Tue Mar 16, 2010 9:56 am ]
Post subject: 
we have run these big bore engines with high comp ratios planty of times,,, however weith comp ratios hugher than 10.5:i,,, anything over that & we will run Av-Gas,,, 100+ octane fuel

no issues with our supercharged 1426 engine

no issues with Stews 1591cc full race engine running same bore size & BK450 head gasket & 13.7:1 comp ratio

no issues with plenty of other race engines of the same size,,, just "NEED" to run the very best fuel avaliable , thats all

Make sure you re-torque them tho
Author:  73GT [ Tue Mar 16, 2010 12:20 pm ]
Post subject: 
thanks guys, it is great to have your reassurance, since i am spending the money & the shop i am paying to do the work(crank & rods that will go with the block) are not positive, it sounds like this should be a good outcome

good to know that a BK450 will do the job & do special/expensive gasket needed, esp if it has already been done with supercharged & full race engines already.

i also agree with the point made that stainless steel should be a better material in this situation of the gasket edge overhanging into the bore, as it has a higher melting temp than copper, so should take longer before it burns away & blows.
Will make sure that the cylinder head has enough surface land to support the gasket.
Author:  GT mowog [ Tue Mar 16, 2010 4:58 pm ]
Post subject: 
I know that there are a few here who use the BK 450's however, I recently fitted one (there was nothing on the gasket to say that it was a BK 450 however, it was packaged that way). The motor was fairly fresh and when rebuilt was actually fitted with a copper composite gasket (which I have been using for some 25+ years without problem), however since these BK 450's are spoken of so highly, when the cylinder head was changed, I used one of these.

When removed again after only about 2000 km, it was almost on the point of failure. There are no sealing rings on the water jacket as is on the composite copper gaskets. It was in an advanced state of failure between cylinders 2 & 3 and evidence of coolant leakage.

My first thought was that either the head and / or block deck faces were not flat, however they were both found perfect.

If this is what a BK 450 is, they are crap. I have heard many reports from the UK with similar problems with the BK 450 and I think asphalt (here on ausmini) also raised similar concerns.

I know that many here say they have had problems with 'copper' head gaskets (brand / part number unknown), however I can only speak from experience and my experience with composite copper head gasket has been nothing short of excellent and I will continue to use these.

Beware however that if the deck faces are anything less than purfect then composite copper is NOT the answer here.

Going back to the original question, I would suggest either a Cometic which are still available or a water cut copper or laser cut steel.
Author:  drmini in aust [ Tue Mar 16, 2010 6:13 pm ]
Post subject: 
If it was not marked BK450, it ain't one. They DO have seal rings round all the holes... except the 3 large ones near the middle of the head.
:wink:

There are various other black composite gaskets about, not all are created equal.
Author:  GT mowog [ Tue Mar 16, 2010 6:27 pm ]
Post subject: 
Then it would appear that it was NOT a BK 450. Are the BK 450's actually stamped with that P/N on the Gasket?
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