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Combustion chamber tolerance

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I would appreciate a confidence check please on my rebuild of a 1971 750 Combat import before I bolt the head down.  The pistons over extend from the barrel finished face by 1.7mm and a new copper head gasket is 1mm uncompressed leaving the pistons to occupy the recess of the head which is 1.6mm by about 0.7mm uncompressed.

There has clearly been some historical skimming and I have compensated by using a metal base gasket to lower compression / reduce stroke.

As it is I am heavily reliant on the valve clearances being able to avoid contact with the pistons

Have I a reason to be concerned ?

 

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On an 850 you are looking for a minimum gap piston to head in the squish band area of 40 thou or 1mm to avoid piston to head contact at high revs, so you need an extra 0.3mm to meet this requirement. 

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Thank you John

So before I compress the head gasket I would have a squish of 1.6 - 7 = 0.9mm relative to the combustion chamber recess leaving only the piston indents to take the valve movement ?

As I have increased by adding a metal base gasket , is there an option of a thicker copper head gasket that you know of ?

Hopefully I have now compressed the photos

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With your figures above you would be gambling, a composite gasket compresses about 7%, not sure about a copper gasket.

You mention the head has been skimmed further, so check the depth of squish band around the chamber in the head. If the depth varies forward to aft and across the head, then you are not going to gain anything, so it makes sense to not worry about the squish band gap so increase it to sensible gap. Poorly skimmed heads which leave the recess with a variable depth defeat the idea of reducing squish.

Unless you have a shelf of spare engine parts, check the squish clearance by sacrificing a gasket or then anneal it, and check the valve to piston clearances. 

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I will assemble and tighten and check for valve to head  clearance

I note that on US and Aus sites there are 2mm copper head gaskets which would give me a comfort zone 

I also note on forums that 2 copper head gaskets on bike engines are not unknown ?

What is the opinion of this - in extreme measures  ?

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On my trawling on't interweb

www.accessnorton.com/NortonCommando/750-cylinder-barrell-very-flat-top.18888

concerns a guy with a flat machined barrel face with much the same questions ! 

The answers extend to page 3 with an indication that racing practices have much finer tolerances 

I would just like to ensure I can kick it over to start and not trash my top end !

Anybody in the UK with thicker copper gaskets ??

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Hi Robert,

I have just had a similar problem building an 850 Mk 3 where the head had been previously heavily skimmed.

I contacted Lani Visconti at Copper Gaskets Unlimited. He was very helpful.

He supplied me with a 2mm thick solid copper head gasket which overcame the issue I had. He can supply head gaskets in a range of thicknesses.

email details: coppergasketsunlimited@yahoo.com

 

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Don't forget to keep an eye on the bigger picture with regard to all these gasket additions. A chunky gasket under the barrel and another on top is going to make life for the pushrods very uncomfortable. They may end up too short to cope with all this vertical stretching and then you run out of adjustment at the rockers. This is not a complete disaster as early 650 and Atlas rods are longer and a set may be able to make up the shortfall.

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If the length of the pushrods have not been changed, wouldn't they be OK if all he has done is to restore the total effective height?

Mike

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Combat pushrods were not shortened by the head skim at the factory, so likely too long not too short. Assuming it is a Combat as no Combats are made 71 model year. An early 72 Combat made late 71 may have been titled a 71 on first registration.

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It is in fact a Dec 71 but appears to be 72 spec

Nos identify it in the Combat range

I have tightened down as it is and doesn't appear to foul the valves

I will now final torque and update accordingly

Thank you for all the suggestions ,much appreciated

Bob

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Torqued down and tappets adjusted and appears to be clear of fouling

Lets hope !

I can't register it for the UK until complete according to dating officer so only went on year and date on the vin plate, Thank you John, still learning !

Thank you 

 

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