STD TD13 marking on Pistons???

I have pulled the head and barrows to check for a broken ring. The rings are not broken but the engine did a lot of smoking before I took it apart. The markings on the piston are "STD TD13". The pistons are dome shaped with the cut outs for the valves. Does this mean the pistons are standard? If so what brand of rings would you recommend to use as replacements?

Second question.... if the oil pump isn't adequatly returning the oil to the oil tank would this cause a lot of smoking of the engine? I am asking this because the rings and bore appear to be fine. There is a constant spitting of oil in the crankcase breather line instead of a fine mist as I would expect and I am now wondering if the oil pump is my real problem. I should have removed the return oil line to check for returning oil before everything was taken apart but it's way to late for that now.

Thanks in advance for any information.

Cheers

What bike do you have? Jim

Dirt2Oil said:

The pistons are dome shaped with the cut outs for the valves. Does this mean the pistons are standard?

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They may be standard bore size, but unlikely to be standard Commando pistons if they're "dome shaped".

How did you measure the rings? Bore? Visual inspection is not enough. The scavenge oil spitting is normal observation.

It is a 72 750 Combat Commando. It was a quick visual at this point as I only got it all apart last night. I tried measuring the pistons but the calibers I have are not great so I don't have much faith in them.
I was hoping the markings would make an easy identification.

Dirt2Oil said:

It is a 72 750 Combat Commando. It was a quick visual at this point as I only got it all apart last night. I tried measuring the pistons but the calibers I have are not great so I don't have much faith in them.
I was hoping the markings would make an easy identification.

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"STD" means "STANDARD" bore size. Any oversize would be marked ".020" or such. Industry standard nomenclature.
Calipers would show an overbore, but too course for measuring wear. Bring it to a machinist/engine builder familiar with and equipped to do this type of work, ask him to measure (all of) your parts.

A quick check is to put each piston ring in the bore, measure end gap with a feeler gage, report back with your findings.

Since it is a 72 motor it probably has a damaged oil pump or a plugged scavenge passage in the bottom of the cases -or both. It will need to have the scavenge pump pickup modification done.

Pistons with a domed top are probably Powermax pistons. If you are running them along with a Combat head then you will have a extremely high compression ratio. Jim

Dirt2Oil said:

STD TD13

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Could "TD" be ID = ID13 ?

L.A.B I think you may be correct, I looked at the markings again and it is more likely to be "ID13". Does it mean anything now?

The ring end gaps are as follows
Left cylinder
Top compression ring 0.024 in
Second compression ring 0.024 in
Right cylinder
Top compression ring 0.022 in
Second compression ring 0.024 in

I tried getting a reading on the pistons with my calibers but was getting readings of 2.862 and 2.864 which is less then what the manual states for standard(2.8750 to 2.8758). It appears as if the calibers could not get around the piston enough to get an accurate reading.

With regards to the oil scavenging in the cases, it has been verified by folks on here the cases have been replaced at some point in the bikes history. The breather at the lower back side of the left case is there and there is a plugged hole on the back of the timing case. Also the cases have no numbers which I have been told may indicate them being replaced under warranty.

The ring gaps in the manual I have states
Top ring fitted gap 0.010-0.012
Middle ring (taper) fitted gap 0.008-0.012

I plan on getting new calibers to accurately measure the pistons and then order a new set of rings and
hope the smoking stops. The crankcase modification for oil scavenging may be beyond my abilities at the moment. :roll:

I can tell ya that when ring gap gets to .022"+ oil smoke and fouling usually shut it down. I had this from air boot leaks but found .020+ pistons in spec and bores .002" still under size so got away with just new rings leaving bores alone sealed smokeless. Got to have good measures to make good decision. The pushrods are a tad long in Combats so to both lower CR for new age octane and better valve geometry, could put in base plate or thicker head gasket.

Dirt2Oil said:

L.A.B I think you may be correct, I looked at the markings again and it is more likely to be "ID13". Does it mean anything now?

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Well, 'ID' (+number) is a common AE/Hepolite marking, so they could be Powermax, as Jim says.

comnoz said:

Since it is a 72 motor it probably has a damaged oil pump or a plugged scavenge passage in the bottom of the cases -or both. It will need to have the scavenge pump pickup modification done.

Pistons with a domed top are probably Powermax pistons. If you are running them along with a Combat head then you will have a extremely high compression ratio. Jim

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Jim
I was going to try these with a combat head just to see ,do you think it is at all viable?


Somebody has worked them over with a driver at some point.
J

not if you want to run it on the street with pump gas.

auldblue said:

Jim
I was going to try these with a combat head just to see ,do you think it is at all viable?

Somebody has worked them over with a driver at some point.
J

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auldblue said:

comnoz said:

Since it is a 72 motor it probably has a damaged oil pump or a plugged scavenge passage in the bottom of the cases -or both. It will need to have the scavenge pump pickup modification done.

Pistons with a domed top are probably Powermax pistons. If you are running them along with a Combat head then you will have a extremely high compression ratio. Jim

Click to expand...

Jim
I was going to try these with a combat head just to see ,do you think it is at all viable?

Somebody has worked them over with a driver at some point.
J

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The pistons are probably fine but as Bill says, they will be race fuel only, when combined with a combat head. Jim

Thanks for the replies gentlemen ,the pistons came out of my old bike two years ago and I would like to use them again.
J

That valve pocket peckering sure looks like a piece of carb slide or such got sucked in >> bounced a bit >> spit out. If head don't show peckering signs then may be a new head but no mechanic even me would use screw drive corner pounding in to de coke pistons, instead of chiseling its flat edge along properly. :roll:

If those dome piston must use race level gasoline today then what fuel was used in its hey day?

Steve



This is what the piston were like when the head was pulled and the drive side of the head. They have since been on a bike but it was fouling the plugs and after pulling the head a second time I just stuck on the barrels and pistons from the combat and other things and that cured it, but I would like to try them again as it's not much trouble swapping engines,heads and the like. Just a bit of fun.

Thanks for taking time to photo document the crusty sewer surfaces, but head photo is so dark I can't make out anything but light though plug holes. I think it was the extra hi CR of my P!! that allowed it to idle and ride in 4th w/o throttle and smoke tire from just off idle throttle to avoid if in a hurry. I suspect one could grind off some valve pocket surround to bring CR down into pump gas range &/or cam with more over lap-later intake closing to make it behave like a low rpm low CR lugger on hi test. i look up to your sense of mechanics ease to switch out most an engine, as I've been putting off most a year to re-seal Trixie. My Ole SuVee650 feels so UFO smooth modern until I spend 20 min or it for valve train sense annoyance or over 80 then miss the no down shift pull of plain ole factory Combat waking up. Btw Steam turns carbon cake into a soft grey power that just blows away or wipes off though your thick stuff may need some careful scrapping too.