Sprint head skim/block deck and CR

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cliftyhanger
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Re: Sprint head skim/block deck and CR

#16 Post by cliftyhanger »

GrahamFountain wrote: Thu Oct 29, 2020 5:31 pm 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
No, UK spec. I had the TR7 block with the OEM pistons that were dished. I think the LC type had a much deeper dish? (these are circa 1mm, not much at all. I wonder f it meant the TR7 and 1850 could use the sameheads? )
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Re: Sprint head skim/block deck and CR

#17 Post by SprintV8 »

TR7 pistons on EBay 370426792009
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1980 Dolomite Sprint with a touch of BLTS
Balanced Lightened and Tweaked 13B Rotary and SuperCharged.
Back in my possession 22 September 2019.
Rebuilding the Sprint time taken so far, 111Hrs@15/12/2020
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352 @ 28/11/2022
455Hrs @ 20/10/2023
555Hrs @ 28/10/2024
This is time taken at the Sprint not necessary time worked.

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Re: Sprint head skim/block deck and CR

#18 Post by cleverusername »

Those are UK TR7 pistons. I know this because I have a brand new set of OEM ones in the garage I bought off ebay a whiled back, when I discovered I had damaged a couple of my Sprint pistons removing them from the block.

There is some debate about whether they will work, I have seen TR7 pistons with extra cutouts but some claim they are not needed. I am certain that they would lower the compression and you would lose some of the benefits of the Sprint head.

I managed to source some usable Sprint pistons on here for reasonable money and I think that it would be worth trying that before using the TR7 ones.
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Re: Sprint head skim/block deck and CR

#19 Post by cliftyhanger »

cleverusername wrote: Thu Oct 29, 2020 11:36 pm Those are UK TR7 pistons. I know this because I have a brand new set of OEM ones in the garage I bought off ebay a whiled back, when I discovered I had damaged a couple of my Sprint pistons removing them from the block.

There is some debate about whether they will work, I have seen TR7 pistons with extra cutouts but some claim they are not needed. I am certain that they would lower the compression and you would lose some of the benefits of the Sprint head.

I managed to source some usable Sprint pistons on here for reasonable money and I think that it would be worth trying that before using the TR7 ones.
The engineering company supplied the pistons. And teh block was rebored to suit them. Being a proper company, they rebore to the measured piston size, no assumptions etc. This time all pistons measured identical, but it is not always the case, and clearance at teh skirt only wants to be a couple of thou? not much room for manoeuvre.
There has been debate, but Russ had them in a running engine, Robsport use them, and I expect others too. Nobody else has picked up on the CR issue, but as stated, got a sprint head that has been skimmed 40 thou? these pistons would be perfect, but may need machining for the valve cutouts.
As my head is unskimmed (or max 5 thou) I am tempted to just get the block decked enough and lathe off the piston edges to get a CR of 9.5 You are correct, reducing compression is poor news, especially if using a high lift cam where more compression is required. (I am using a std cam)

As always, with building an engine, the devil is in the detail.
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Re: Sprint head skim/block deck and CR

#20 Post by cleverusername »

Oh I see, you have reduced the depth of the head. Could work but the recess in a TR7 piston is way more than 40 thou, so there would still be so loss of compression compared to using flat Sprint pistons.
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Re: Sprint head skim/block deck and CR

#21 Post by GrahamFountain »

There's an old thread I can't find about this where I wrote about talking with Steve at S&S. He said they tested this issue fairly thoroughly, I think that may have been when they owned an engine re-conditioning business and ran that for a while.

He said that there's no problem using TR7 pistons under a Sprint head, as they are, without extra cutouts, and IIRC they told Robsport it would work.

But if you've any questions on that, it may be worth talking to Steve on 01706 874874.

As to the cutouts in the sprint pistons, I do wonder if they are there for the unusual cases when the piston reach's the top and one or more valve inlet valve is fairly well open. I know I had a Sprint engine that got turned over quite a bit (S&S doing compression tests on all 4 cylinders) when the cam wasn't turning (screws holding drive wheel had fell out or broke) without causing any damage to valves or pistons. I assume at least one valve would have been fairly well open in that case, wherever the cam had stopped. And I wonder if that would have been safe without the cutouts.

Graham
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

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Re: Sprint head skim/block deck and CR

#22 Post by cleverusername »

GrahamFountain wrote: Fri Oct 30, 2020 9:18 am There's an old thread I can't find about this where I wrote about talking with Steve at S&S. He said they tested this issue fairly thoroughly, I think that may have been when they owned an engine re-conditioning business and ran that for a while.

He said that there's no problem using TR7 pistons under a Sprint head, as they are, without extra cutouts, and IIRC they told Robsport it would work.

But if you've any questions on that, it may be worth talking to Steve on 01706 874874.

As to the cutouts in the sprint pistons, I do wonder if they are there for the unusual cases when the piston reach's the top and one or more valve inlet valve is fairly well open. I know I had a Sprint engine that got turned over quite a bit (S&S doing compression tests on all 4 cylinders) when the cam wasn't turning (screws holding drive wheel had fell out or broke) without causing any damage to valves or pistons. I assume at least one valve would have been fairly well open in that case, wherever the cam had stopped. And I wonder if that would have been safe without the cutouts.

Graham
I think we can assume that the original engine designers wouldn't have got to the bother and extra cost of the cutouts if they weren't needed. I have read in other threads that people have had problems at higher revs with valves kissing the pistons.
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Re: Sprint head skim/block deck and CR

#23 Post by Carl »

The following are the minimum head thicknesses I have, measured from the base of a cam journal (located by a ball bearing) to the head face :-

Dolomite/TR7/Stag - 4.427"

Sprint - 4.745"

These are from hand annotations made to the Service Training Notes by a chap I used to know who attended the training courses, and worked on the cars back in the day.

He told me that these are a guide only. The factory believed that if a head needed skimming beyond this it would generally be twisted excessively, manifested by the cam journals being out of line. If they are still in line it's ok to skim further. I've gone beyond 40thou (1mm) in the past, although that was an 1850. My current Sprint head has been skimmed down quite a bit and the block skimmed also. This was intended from the start to be used with a mildly higher lift/longer duration camshaft though.

Head skimming and decking the block is going to increase the compression ratio. The important figure for engine running is not the static compression quoted (9.5 : 1 as standard for a Sprint) but the dynamic compression which is a function of the valve timing and duration relative to piston movement. IIRC the recommendation used to be no more than about 8 : 1 dynamic compression for a road car. With a lightly skimmed head I'd highly recommend a light skim on the block face also. I think this will be fine with a standard Sprint cam (which is actually a very good cam).
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Re: Sprint head skim/block deck and CR

#24 Post by GrahamFountain »

cleverusername wrote: Fri Oct 30, 2020 10:03 am I think we can assume that the original engine designers wouldn't have got to the bother and extra cost of the cutouts if they weren't needed.
The trouble with that assumption is it ignores what drove the designers of the engine at Coventry Climax. It's said they and the BLMC team that got them involved (Spen King, et al) were primarily interested in the 16-valve version of the slant-4 engine for competition, presumably for the Dolomite Sprint as a Group-1 race car, though possibly with an eye on Groups 2 and 4. And the restrictions and allowed modifications for those homologation groups may mean they designed in aspects that were only significant to the engine at the limit of tune the groups allowed.

I'm sure I've read the Group-1 engines ran to something like 175 bhp on a standard spec engine (at least to the tolerances the FIA's CSI allowed in 1973) and SU6s. On better carbs it could come close to what the far more specialized 2L Cosworth BDG engine gave. It's true that Group 2 and 4 engines could have used mods allowed under the 100-off rule from Appendix J to 1975 - that allowed alternative cylinder heads and transmissions just on production of 100 kits of parts to buy. But while that may have allowed alternative cams and other mods to the head they could do 100 fold, it doesn't appear to have allowed alternative pistons. So I assume even the Group 2 and 4 engines were using the pistons visually indistinguishable from the Mahle piston (homologated as evolution 3 in April 1975) in CSI inspections or CSI instructed scrutineering, like happened to DTV [Dealer Team Vauxhall] in Portugal 1978.

Yes, I know about the arguments that BLMC was short of cash in the early 70's. But that involved going so far bust by 1975 that the Ryder Plan reckoned it needed nearly £3 billion to dig BL out of the wreckage BLMC had become - something like £20-25 billion at today's values. And the leeway in that amount of misspending would make any, possibly all, of us rich.

And at only 5000 units a year maximum, the production of the Sprint engine would have been so far down the balance sheet, the production cost chasers wouldn't have noticed it. If you don't know about the work of production cost chasers in large Co.s like BLMC, you've probably never died of boredom.

Also, there was the supposedly significantly expensive cost reduction programme in the early-mid 70s (apparently ended in 1975 with the Ryder Plan), intended to make the 16-valve slant-4 suitable for mass production so SD2 could break into the US market (and separate from the the programme to meet emissions legislation from 1976). I think that must show that the engine really wasn't initially designed with large scale mass production in mind. You might feel qualified to say that's down to bad design. But I don't feel able to criticise Lewis Dawtrey and Harry Webster's, Harry Mundy's, and Spen King's design teams in that way.

Graham
Last edited by GrahamFountain on Fri Oct 30, 2020 3:31 pm, edited 2 times in total.
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).
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Re: Sprint head skim/block deck and CR

#25 Post by GrahamFountain »

On the issue of Sprint piston tops while I was researching the prev. post, I came across these pics of the pistons homologated for the Sprints in 1974 and 1975 respectively:


Image


Image

Graham

EDIT: It may be that the first picture is just a wrong one, as the second is in an errata not evo. My apologies. GF
Last edited by GrahamFountain on Fri Oct 30, 2020 5:22 pm, edited 1 time in total.
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).
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Re: Sprint head skim/block deck and CR

#26 Post by GrahamFountain »

Carl wrote: Fri Oct 30, 2020 12:00 pm
Head skimming and decking the block is going to increase the compression ratio. The important figure for engine running is not the static compression quoted (9.5 : 1 as standard for a Sprint) but the dynamic compression which is a function of the valve timing and duration relative to piston movement. IIRC the recommendation used to be no more than about 8 : 1 dynamic compression for a road car.
Can that equate to a compression test result? If so, what pressure would that be?

Graham
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).
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Re: Sprint head skim/block deck and CR

#27 Post by soe8m »

There is no relation or ratio between compression ratio and compression test figures. A dynamic compression ratio is non existent so advising about dynamic figures isn't possible.

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Re: Sprint head skim/block deck and CR

#28 Post by dursley92 »

Found this picture of some TR7 pistons modified with extra pockets for use in a Sprint.

Image

Also in the BL Slant Four training booklet it shows early Sprint pistons with a recessed crown.


Image
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Re: Sprint head skim/block deck and CR

#29 Post by GrahamFountain »

dursley92 wrote: Sat Oct 31, 2020 4:03 pm Also in the BL Slant Four training booklet it shows early Sprint pistons with a recessed crown.
Interesting. Does that coincide with the switch to Mahle pistons and is either about 75?

Graham
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).
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Re: Sprint head skim/block deck and CR

#30 Post by GrahamFountain »

dursley92 wrote: Sat Oct 31, 2020 4:03 pm Also in the BL Slant Four training booklet it shows early Sprint pistons with a recessed crown.
That means amendment 8/3E to its recognition (#5542) isn't really an erratum, but a change/variant/evo.

Graham
The 16v Slant 4 engine is more fun than the 3.5 V8, because you mostly drive it on the upslope of the torque curve.

Factory 1977 TR7 Sprint FHC VVC 697S (Now all of, but still needs putting together)
B&Y 73 Dolomite Sprint UVB 274M (kids!)
1970 Maroon 13/60 Herald Convertable (wife's fun car).
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