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Pair of SP250 or saloon cylinder heads

timmartin
Posts: 49
Joined: Wed Feb 17, 2016 1:37 pm
Location: Berkshire

Re: Pair of SP250 or saloon cylinder heads

Post by timmartin » Thu Jul 11, 2019 10:00 am

Thank you Mike.
I had already discussed this (to me) fairly encouraging head with Russ.

My first reservation was that it looked as if part of it had been fettled using a coal chisel. Russ, of course was more professional. He pointed out that:
1. A piston had broken up in the left hand pictured cylinder, broken pieces of ring having been knocked into the head, and
2. The head had already been skimmed too much for my use. This is liable to cause over-thinned parts to overheat and pre-ignition (pinking). (This is the problem with my own heads)

Thanks again,
Last edited by timmartin on Thu Jul 11, 2019 10:21 am, edited 1 time in total.
Tim
Blue 1964 SP250 in Berkshire

timmartin
Posts: 49
Joined: Wed Feb 17, 2016 1:37 pm
Location: Berkshire

Re: Pair of SP250 or saloon cylinder heads

Post by timmartin » Fri Jul 12, 2019 12:39 pm

Colin and Graham. Just in case this changes your opinion about how much your heads have been skimmed already, this is how Russ checks whether there is enough metal left for him to rework a head. (Measuring the overall thickness of a head can sometimes produce either an over-pessimistic or an over optimistic-conclusion!)

1. Straight edge on the machined face, across the middle of each cylinder.
2. Measure depth to centre of combustion chamber, between the valves.
3. The minimum useable depth is one inch. (When new they are up to one inch and 40 thou. (1.040”)

My posting this does not imply that, unlike me, you need to be told how to suck eggs. Just double checking...

Thank you.
Tim
Blue 1964 SP250 in Berkshire

grahamemmett
Posts: 573
Joined: Mon Feb 08, 2016 8:12 pm
Location: Northwich, Cheshire
Contact:

Re: Pair of SP250 or saloon cylinder heads

Post by grahamemmett » Fri Jul 12, 2019 12:56 pm

Good information as I was to measure the depth of the head being 3.000 inches
Graham Emmett
Northwich, Cheshire
Joint DB18 Registrar (with Marcel Renshaw)
DB18 1949 LCV522 (Yes that one with the P100s)
https://www.db18.org

Warsash 2
Posts: 273
Joined: Sun Feb 28, 2016 1:53 pm

Re: Pair of SP250 or saloon cylinder heads

Post by Warsash 2 » Fri Jul 12, 2019 3:19 pm

Hi
I have been in touch with the SP club and they said that the heads when new the thickness was 3 inches, which indicates that Grahams head have not been skimmed. As such they are hen's teeth.
Regards

Colin

timmartin
Posts: 49
Joined: Wed Feb 17, 2016 1:37 pm
Location: Berkshire

Re: Pair of SP250 or saloon cylinder heads

Post by timmartin » Fri Jul 12, 2019 6:12 pm

Thank you Colin.

I too understood that the nominal overall thickness of a new, machined head was three inches.
But I have heard of thicker ones being seen, up to as much as 3.060”.

It does seem unlikely that one or more of these thicker heads will turn up during my search, but it does suggest to me that measuring the combustion chamber depth may be the more certain way of assessing a used head?
Or have I got it wrong somewhere?
Tim
Blue 1964 SP250 in Berkshire

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