GT6 mk2: piston clearance for 0.02 oversize bore

[DanielH]

Hi there,

my machinist is asking which clearance for the block bore he should account for?
He is saying that the clearance depends on the particular make of the piston, which makes sense to me.

I've provided him with the +20thou pistons from County:

https://rimmerbros.com/Item--i-149976020COUNTY

The County parts number is 149976.

I am unable to get a datasheet from either Rimmerbros or from the manufacturer even.

What's written in the books?

1. The handbook says (pp 42):
Pistons are available 0.02 (0.508 mm) oversize. 
Before fitting, the cylinder block should
be rebored to 2.9614/2.9609 (73.685/73.683mm) to suit the oversize pistons,
Piston dia: 2.937+0.02 (74.600+0,508mm) = 2.957 (75.1078mm)

2. Vizard says (pp 11):

In the standard bores the pistons are graded in three different sizes and the bore clearance on a new engine will be between 3.5 thou and 4.2 thou (0.0035-0.0042).

Any thoughts?

thanks
Daniel

Edited March 10, 2022 by DanielH
wrong info from vizard provided! changed the reference to pp 11.

[NonMember]

Vizard appears to be just plain wrong on that, but the figures you quote from the handbook include a typo. The standard 2L bore is 74.7mm = 2.94" according to the WSM, so most of your handbook is right, but the 73.685 value is in error - it should be 75.22mm

[DanielH]

The data from Vizard is for the 4 cyl... I just overlooked that info bc it is given in the previous column in the text... oh my...

[DanielH]

Oh, Rimmerbros just coughed up:

"Unfortunately we do not have a datasheet available, however the county part number is [b]CP800K 020[/b]."

[DanielH]

Hi there,

my assumption is, that the County pistons are rather common in Triumph engine rebuild projects. Am I right?

That manufacturer appears to be really opaque...no data, no address, even not in the parcel.

How do your machinists cope with that? Do they look at the pistons and say: thats cast, so I go with the rule of thumb: clearance is 1% of dia??

Any thoughts?

Thank you 🙂

Daniel

 

[jmh]

When I bought County pistons for my mk 3 spitfire FD engine, it had a note inside the box providing the clearance.

It was less than stated in the workshop manual.

 

[DanielH]

WOW, you've got a copy of that note?

[Martin M]

We took a clearance between 5/100  and 6/100 mm. GT6 and Vitesse are both happy with that. No oil real consumption and no problem reving beyond 6.000. 

Important, pistons , crank, flywheel and rods should be balanced.

Check piston Ring clearance.

Cheers

Martin

 

[Nigel Clark]

By coincidence, exactly the same question came up on the TR Register forum last week:

https://www.tr-register.co.uk/forums/index.php?/topic/79707-county-pistons-cp800k-skirt-clearance/

There should be a note advising clearances in the box with the pistons.

Nigel

[jmh]

On 11/03/2022 at 09:02, DanielH said:

WOW, you've got a copy of that note?

Not sure, will take a look.