Sprint head skim/block deck and CR

[GrahamFountain]

Assuming that even if the TR7 pistons work normally, the indents for the inlet valves are significant for some safety issue, like valve float or chain break, etc.; does any know what it might cost to get done and what the measurements would be?

Graham

[soe8m]

For chainbreak?

Jeroen.

[GrahamFountain]

I've seen the chain wheel come off the cam. Same effect - the cam stops turning but the engine continues.

Graham

[gmsclassics]

I used TR7 pistons in my race car for six seasons from 2006. First photo shows one in place. I needed a new set as pistons 2 and 3 were damaged in the area of the rings on the exhaust side. I was offered these locally for less than a fifth of the cost of new Sprint ones which were not available in NZ. The dished area on the piston face was not that pronounced.

One of the deciding factors was that the ported and polished head that I was using had already been heavily skimmed and I was having to use an ‘extra thick’ gasket (2.5mm) to achieve a desired compression ratio. Comparing a new and unused head (120.8mm) with this one (118.1mm) indicated a total of 2.7mm had been skimmed off. Don’t know for sure but suspect the block had also had a small amount of material removed (it was well used at 40 thou oversized bore and crank).
It was necessary to cut new pockets for the valves and due to using a STR139 cam, these were all checked by trial fitting, based on a standard Payen head gasket. Positioning was based on where the cutouts were on the 'old' pistons that had been removed.

Worked well and the pistons were never an issue, photo you see is after engine was removed at the end of six seasons of racing. It was ‘retired’ due to the head going soft following too many extreme heat cycles as well as the wear on the crank requiring new mains and big end bearings after each race meeting.

The next/current engine was based on an 1850 auto block, still with std bore and crank, that was then bored out to match forged custom made CP-Carillo pistons. The best std head was heat treated to increase hardness and then ported and polished using the previous one as a template.

So, in conclusion I would happily use these TR7 pistons again in a road car provided the compression ratio could be managed appropriately, or indeed to compensate for a heavily skinned head, provided additional cutouts for the Sprint valves were cut.

Hope this is of some help.

Geoff

[GrahamFountain]

Geoff, How did you measure where to add the new cutouts? I assume they're just over the Sprint inlet valve diameter and at 45 degrees.

Graham

[gmsclassics]

There are a few experts on building race engines in Auckland so I left that to them and gave them the TR7 pistons along with the old Sprint pistons to copy from. They made the cutouts in the TR7 pistons in the identical places and angles for the inlet valves but very slightly deeper and then recommended what the cold clearances between valves and piston needed to be in order to ensure when the engine was hot and everything expanded there was still sufficient clearance.

It was very time consuming to do the checking as we had to assemble the engine completely with a thin covering of plasticine over the cutouts, turn the crank over twice (plugs out) before removing the head and then measuring the thickness of the squashed plasticine to ensure there was the required clearance between valve and piston cutout.

Better safe than sorry though and only have to do it once.

Geoff

[GrahamFountain]

Quick check says there's only 35 degrees between the valves, not 45. Anybody know if they are equal about the centreline?

Graham

[GrahamFountain]

Going off one of the company drawings, not a head, the inlet is 19 degrees to the centerline, so I assume the exhaust is 16. Is anybody in a position to check those angles?

Graham