The Triumph Dolomite Club - Discussion Forum

I bought a sprint engine.On stripping bores/pistons scuffed badly enough to warrant a rebore.Also jackshaft journal in teh block oval.
My old TR7 block had a good jackshaft journal, so had that bored out, and the company suggested/supplied TR7 pistons on the premise that robsport and S+S use TR7 pistons in their rebuilds. (I phoned robsport who confirmed) However, TR7 pistons are dished so will have a rather low CR? Dish appears to be about 40thou (actually, 40thou/1mm does not sound enough, need to re-check), and over about 90% of the piston top.

Looking at the block, it seems the top surface has some damage from storage. So it will need decking. This damage appears to be about 20 thou deep right on the edge of the bore. So a 20 thou decking should clean it up, pistons in a lathe to remove a little on the edges. That should approx halve the lost CR. (and new cutouts required in the pistons seems likely)

I have found a good head, it may have had a light skim but I can't find any measurements to compare. However, I am thinking of skimming this head to bring the CR up to the std 9.5 (is that correct??) or even a little extra. Are there any figures available for this sort of work?

The engine will be on a std cam etc. Just want it running as a well built std engine.
Any advice in head heights for unskimmed heads appreciated. Also what is the std combustion chamber volume?

Sorry for all the questions, what I hoped would be a simple job when I got the engine has got complex, with twists thrown in along the way. Keeps the brain working though.

Hi Clive, I am pretty sure Robsport don't fit TR7 pistons to Sprint engines! Sprint pistons are available so I would be inclined to do the job properly and fit Sprint pistons.

Trying to deck the block and taking a bit off TR7 pistons is a bit of a bodge, you'll need to ensure that the valves still clear the pistons, which I suspect you will have an issue with after decking at 20 thou plus shaving the piston crown, it will just create a whole world of problems.

I have bought from these guys before and although they do come from Oz, they are cheaper.

https://www.autosurplus.com.au/triumph- ... 20~5799333

I’ll be inclined and stick with Sprint pistons.
Especially if your using a Sprint 16v head.

Decking the block and piston’s is just part of Blueprinting.
So perfectly acceptable in my book.

James467 wrote: ↑Thu Oct 29, 2020 12:46 pm Hi Clive, I am pretty sure Robsport don't fit TR7 pistons to Sprint engines! Sprint pistons are available so I would be inclined to do the job properly and fit Sprint pistons.

Trying to deck the block and taking a bit off TR7 pistons is a bit of a bodge, you'll need to ensure that the valves still clear the pistons, which I suspect you will have an issue with after decking at 20 thou plus shaving the piston crown, it will just create a whole world of problems.

I have bought from these guys before and although they do come from Oz, they are cheaper.

https://www.autosurplus.com.au/triumph- ... 20~5799333

I spoke to robsport, who confirmed they do.... (in retrospect, I didn't ask if they always fitted them, but teh answer was affirmative and clear)
Too far down this route now, the budget won't extend to a near enough £800 set of pistons and possibly another rebore if the pistons are out of spec (likely) Or indeed try to find a decent block to start again.
The block needs decking because of some marks on the face, so that has to be done anyway. So even new sprint pistons would need work. I am happy to use a lathe to machine the pistons down by the corresponding amount. Valve cutouts I have a cunning plan......

Sometimes I wonder if fitting a modern engine would have been the easier option

Anyway, anybody got info on head depths?

Some info in this thread about dimensions

https://forum.triumphdolomite.co.uk/vie ... nt#p311112

Tony Burd wrote: ↑Thu Oct 29, 2020 1:37 pm Some info in this thread about dimensions

https://forum.triumphdolomite.co.uk/vie ... nt#p311112

Thanks Tony
minimum 120.5 from base of cam journal to face. About to pop up the garage...

Clive, remember when you get the block decked to make sure they do it with the timing cover on as well.

For anyone else looking at this, the pistons from OZ cost £450 delivered.

Hi Clive,

You might want to reread the thread, this is from Dolomite 135's comment:

Sprint heads are measured between the face and the base of the cam journal I think?
I have roughly measured my NOS (early) head in the same manner, and the measurement came out about 112.6mm.

cliftyhanger wrote: ↑Thu Oct 29, 2020 1:26 pm

James467 wrote: ↑Thu Oct 29, 2020 12:46 pm Hi Clive, I am pretty sure Robsport don't fit TR7 pistons to Sprint engines! Sprint pistons are available so I would be inclined to do the job properly and fit Sprint pistons.

Trying to deck the block and taking a bit off TR7 pistons is a bit of a bodge, you'll need to ensure that the valves still clear the pistons, which I suspect you will have an issue with after decking at 20 thou plus shaving the piston crown, it will just create a whole world of problems.

I have bought from these guys before and although they do come from Oz, they are cheaper.

https://www.autosurplus.com.au/triumph- ... 20~5799333

I spoke to robsport, who confirmed they do.... (in retrospect, I didn't ask if they always fitted them, but teh answer was affirmative and clear)
Too far down this route now, the budget won't extend to a near enough £800 set of pistons and possibly another rebore if the pistons are out of spec (likely) Or indeed try to find a decent block to start again.
The block needs decking because of some marks on the face, so that has to be done anyway. So even new sprint pistons would need work. I am happy to use a lathe to machine the pistons down by the corresponding amount. Valve cutouts I have a cunning plan......

Sometimes I wonder if fitting a modern engine would have been the easier option

Anyway, anybody got info on head depths?

Definitely not the easy option.
Trust Me on that one.

Not sure, but I reckon a zetec would go in quite easily, and I have done it before in my spitfire

Seems my better head is unskimmed. That will need a lick off anyway, so 10thou along with the block being decked will get me about right. Then just throw it together

Reckon this engine will have cost me 1k by the time it is built. I suppose I could have paid that or more for a running engine, but this will be new or fresh bearings etc, and should be good for ages. Well, it had better be.

Bit late to the party but here is a copy of some sort of (unofficial) Technical Bulletin regarding minimum head thickness on the Triumph "slant" cylinder heads.

Minimum for the Sprint is quoted as 4.745" (120.523mm) - as already quoted!





How about fitting a "saver shim" to get some of the "thickness" back after decking and skimming. Might save a bit of work.

Roger

For what its worth I bought a Sprint engine that came out of a kit car which had been fitted with TR7 pistons without any apparent ill effects. Lots of other problems with it, but nothing to do with the pistons. I was certainly a bit surprised at the time though.

marshman wrote: ↑Thu Oct 29, 2020 3:41 pm Bit late to the party but here is a copy of some sort of (unofficial) Technical Bulletin regarding minimum head thickness on the Triumph "slant" cylinder heads.

Minimum for the Sprint is quoted as 4.745" (120.523mm) - as already quoted!





How about fitting a "saver shim" to get some of the "thickness" back after decking and skimming. Might save a bit of work.

Roger

Nice one, the 112.6 measurement quoted in the other thread must be for the 1850 head.

dursley92 wrote: ↑Thu Oct 29, 2020 3:53 pm For what its worth I bought a Sprint engine that came out of a kit car which had been fitted with TR7 pistons without any apparent ill effects. Lots of other problems with it, but nothing to do with the pistons. I was certainly a bit surprised at the time though.

Russ, that must lower the compression though. The dish appears to be a little over 1mm, I make it about 6 to 6.5cc. Or over 10% of the combustion chamber volume.
A 40 thou head skim would do it....

Actually,for a heavily skimmed head it is abetter way forward than a thick gasket. Probably.

Are the dished TR7 pistons not US unleaded low compression ones?

It's a long time since I looked inside an 8-valve, but I though they were flat with cutouts for the valves (which don't match the Sprint ones).

Graham