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Sprint piston heights
http://forum.triumphdolomite.co.uk/viewtopic.php?t=34843		 Page 1 of 1
Author:  dursley92 [ Mon Apr 01, 2019 2:36 pm ]
Post subject:  Sprint piston heights
Just starting to rebuild a Sprint engine and have come across a surprise, the pistons are above the block face at TDC by about 0.025".

Image

The block needed a liner because one bore had some water damage after the engine was left on the garage floor so the machine shop had to skim the block face afterwards.
Question is, have they taken too much off and will I have to get them to take a bit off the top of the pistons as well?
The cam is supposed to be Newman PH3 but it has no markings at all.

I do have a "thicker" Payen gasket but will that be enough to avoid excessive compression ratio.

I posted this on the Official Triumph Dolomte FB page as well.
Author:  dursley92 [ Tue Apr 02, 2019 12:19 am ]
Post subject:  Re: Sprint piston heights
Discussed with the machine shop today and we decided to machine the top of the pistons to bring them dowm level with the block face. Two will need 0.025" and two need 0.027" taking off.

Image
Author:  Carledo [ Tue Apr 02, 2019 9:25 am ]
Post subject:  Re: Sprint piston heights
Will you not have to deepen the valve cut outs by a corresponding amount too? Especially as you (allegedly) have a hot cam!

Steve
Author:  xvivalve [ Tue Apr 02, 2019 10:22 am ]
Post subject:  Re: Sprint piston heights
Will not that bring the top land above the rings too close to the top? Having seen what can happen to lands from using lead free fuel and detonation, I'd not want to reduce the 'meat' there, particularly if using a hotter cam and altered timing.

Be careful that the machine shop aren't offering you a solution to their problem that compounds your problem later on...
Author:  tony g [ Tue Apr 02, 2019 1:20 pm ]
Post subject:  Re: Sprint piston heights
I'd use a thicker head gasket, although with a std gasket if the piston was 0.8mm from the Head at TDC I'd leave it there for good squish and alter valve cutouts if needed to keep the valves clear of the pistons. Cam timing will be more important of course

Tony
Author:  dursley92 [ Tue Apr 02, 2019 2:37 pm ]
Post subject:  Re: Sprint piston heights
Quote:
Will not that bring the top land above the rings too close to the top? Having seen what can happen to lands from using lead free fuel and detonation, I'd not want to reduce the 'meat' there, particularly if using a hotter cam and altered timing.

Be careful that the machine shop aren't offering you a solution to their problem that compounds your problem later on...
We only plan to take 0.025" off the pistons so I think the top land should still be OK.
I am hoping to keep the engine fairly civilised and nice to drive so trying to be a bit conservative and not too high a CR
Author:  dursley92 [ Tue Apr 02, 2019 2:38 pm ]
Post subject:  Re: Sprint piston heights
Quote:
Will you not have to deepen the valve cut outs by a corresponding amount too? Especially as you (allegedly) have a hot cam!

Steve
Yes I will get them to do that as part of the job.

ps Steve, its the engine that came out of the Red Sprint you had from Mark Maynard to break about 4/5 years ago.
Author:  Carledo [ Tue Apr 02, 2019 3:12 pm ]
Post subject:  Re: Sprint piston heights
Quote:


ps Steve, its the engine that came out of the Red Sprint you had from Mark Maynard to break about 4/5 years ago.
Glad to see it's getting built again, but dream on! I got that car from Mark in February 2012, more than 7 years ago! It's soul still lives on in my garage with the Dolomega project which is still under way, completion projected by June 2020!

Steve
Author:  xvivalve [ Tue Apr 02, 2019 8:57 pm ]
Post subject:  Re: Sprint piston heights
Can you not source another block?
Author:  dursley92 [ Wed Apr 03, 2019 12:06 am ]
Post subject:  Re: Sprint piston heights
Quote:
Can you not source another block?
Yes I have another couple. One needs dipping to clean all the crud out of the waterways, one needs a plug getting out from the water pump position because it had an EWP and my old TR7 one that needs boring.

I just planned to use this one so I will see how it works out before going for Plan B, C or D!
Author:  SprintMWU773V [ Thu Apr 04, 2019 9:51 am ]
Post subject:  Re: Sprint piston heights
This is a similar solution to grinding valve stems to get the right valve clearances rather than sort the seats out i.e. a bit of a bodge. I'd source another block.
Author:  xvivalve [ Thu Apr 04, 2019 11:38 am ]
Post subject:  Re: Sprint piston heights
Quote:
This is a similar solution to grinding valve stems to get the right valve clearances rather than sort the seats out i.e. a bit of a bodge. I'd source another block.
That's kind of my thoughts too, a block has been compromised, so why compromise a set of pistons too, in the hope it will work out OK? If it doesn't work out, you've scrapped a block and a set of pistons...
Author:  GrahamFountain [ Sun Apr 07, 2019 10:26 am ]
Post subject:  Re: Sprint piston heights
On the issue of replacement blocks, has anybody replaced the TR7 dipstick insert with one from a Sprint (I don't know if the 1850 is like the Sprint or not).

As far I understand, apart from early Sprint blocks being a slightly different composition of the cast iron alloy, the blocks are otherwise the same.

Graham
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