Hi, a little of topic, but I have had some good advice from you guys before.
I am after fitting a 750 triumph piston into a b31 engine 400cc it is fitted in my m20 .

I have seen a few triumph pistons for sale one with a compression ratio 8.6:1 then fit a 2mm decompression plate I believe.
Also a low compression piston ratio 7.4:1, what I wanted to know was would I need a decompression plate with the low ratio piston, or which is the better option to go with.
Thanks for any advice.

Ian will probably be able to give you the definitive answer but he did supply me with a piston which is currently being installed in my B31.

Low compression (7.5:1) with some machining to the crown and a suggested decompression plate of 4-5mm to bring the CR to around 8:1.

I have been messing with this conversion for a while now and hope to get it on the road very soon (need more shed time!)

I tried going the 400 route a few years ago with the higher compression piston and a 3mm plate. Not good, pinked like crazy and got very hot.


T140 piston 7.4:1 Part Nr. 71-7086

Cheers
Pete

The low compression (7.4-1) T140 piston is supplied by L.F.Harris to help the Bonnie with modern fuels...

As Pete says a decompression plate of approx. 4mm will be required...I also machine the head of the piston, first machining a flat on top of the 'dome' to produce a crown thickness of not less than .135" after machining....(150" for example, is fine..)

I also machine a flat section around the edge of the piston to account for the difference in bore diameter when the Bonnie piston is fitted (5mm)...

So, you are looking to produce a 'land' 3mm wide minimum...Blend in the edge where the land meets the dome of the head after machining...

Gap the rings if required to .008"-.010" and apply the standard B31 piston clearance for a non split skirt piston when reboring the barrel...

I have found this set up will run OK on standard timing settings..

A pair of Gold Star scrambles cams (65-2446 in. and ex.), a B33 inlet valve and a 30mm inlet port with matching carb would be the logical next step....Ian

email (option): ian@wright52.plus.com

Wrist pin diameter will be smaller in the Triumph piston - easy enough to solve with a new S/end bearing - i use aluminium rather than bronze.

Whoever makes the s/end - MAY be able to machine it eccentric for you - this is what i do when fitting triumph pistons in 350 ES2's - you can lift the piston ~1-2mm using thos method, without compression plates.

The 90 deg hemi-heads used on many pre-war engines have a terrible combustion chamber shape with high comp. pistons (hence twin plugs etc.)

Unless you buy an unmachined piston (+£180 from Omega) and machine a very close 'squish' band - std piston/heads have virtually no usable squish - so theres little point worrying how close the piston is, to the head.

Running a high comp piston, further down the bore - replicates what pre-war tuners used to do - (HARTLEY and such) - can give much better gas flow around TDC/Overlap.

When i raced methonal burning triumphs with the VMCC - i started with 12-13:1 CR and after three years, ended up with a MUCH faster engine (~80bhp at back wheel) with a flat top piston and ~10:1 CR.

My and my cousins CCM 500/550 are routinely the fastest Bitish MX bikes - and notably the most reliable - both burn petrol and run 9:5/9:1 CR's. Unlike everyone else buring Methanol and god knows what CR's.

Anyway less about me - yourself - run about 9:1 CR - and if you have a 12:1 piston, run it down the bore.
Whatever you do - ACTUALLY MEASURE THE CR - 90% of people who talk about CR's never actually measure what they actauly have!!!

As for P C's - 400 running hot ?
Either he had more CR than he thought - or its simple ignition/cab adjustment - no other reason.
I know a guy who runs my old 400 in the CRMC 350 class - for the few years without any problems at all (he's not very fast so no ones ever measured it!)

if you want a 750 piston let me know and i'll send u a used one pay PP only.

B31 and T140 gudgeon pin diameters are the same at .750" (and the B Series small end bush is too thin to be machined eccentrically)....Ian

email (option): ian@wright52.plus.com

Yes my first attempt did result in way too high a CR, that's why it got so hot. I can also confirm that the gudgeon pins are the same size.

here's a Picture of the Piston Ian supplied me with (is that OK Ian?)and what I believe to be an original low comp Version


and that is not a dent on the crown it's muck

I always look at what japanese bike pistons are available - they tend to be much lower expansion, with thinner rings and much, much lighter - often because the wrist pin is a smaller diameter.
They also tend to be cheap in std sizes as no one ever wants standard size!

MITAKA.co.uk website give good details about piston dimensions
otherwise its Wiseco, etc.

I am currently fitting (if i ever get it back!) a 75mm bore (1mm o/s)XL250 honda piston into my B25 - this is basically flat top with a 2mm raised section in the middle - gives a very nice combustion chamber shape and unshrouds the inlet valve around.
compress height is 15.5mm - but it does have a 19mm gudgeon pin (i will have to make some 'top-hats' to fit the original wrist pin).

Always measure Compression Ratios - is always LESS than the manufacturer says in standard bikes and usually MORE than you expected when modifying them!

I had seen a few articles on the net about doing this conversion, but none covered what piston was used.
Thanks for you advice.