Seizing engine

[chotus52]

Hi all, my 1955 shooting star was rebuilt about 4000km ago. It's fitted with 8.5:1 pistons & thorspark ignition. It runs lovely except when cruising at 100kph for maybe 10km the engine locks up. By the time I roll to a stop, the engine kicks over & starts normally. It seized twice & I have since done a compression test & have 160psi in both cylinders. Any thoughts?

[orabanda]

What brand pistons are fitted?

[JulianS]

Check the oil feed.

When it was rebuilt was the oil pump to crankcase gasket one with holes which were the same size as the holes in the pump body? Some gaskets being offered have small holes which obstruct the oilways and significantly reduce oil flow. See first photo.

Second photo show a new old stock pump gasket you can see the difference in size of the holes.

[muskrat]

G'dah chotus52 
 The reason orabanda asks is some brands need more clearance than others. Forged need more than cast.
1: not enough piston bore clearance.
2: getting hot (see 1:), retarded ignition or lean mixture.
After two partial seizures I'd be lifting the top end to see the damage.
A few years ago my 51 (57SS top end) A7 nipped up. a little scoring on one piston & bore. A light hone and new rings. Bad piston in good bore, good piston in bad bore. Still fine after10 000 miles.
Cheers.

JulianS got in 1st with a good point

Now get over to Pictures, Stories and Intros to give uss your story.

[orabanda]

Musky,
You are onto it!
I have had to increase bore clearance to 0.006"to stop seizures if JP pistons are fitted.
Then; no problems; no more seizures!

[Triton Thrasher]

Your symptoms very much resemble insufficient piston to bore clearance.

Was the rebore done by someone who usually works on modern, or water cooled engines?

[chotus52]

The rebuild was done by the previous owner so not sure of any clearances. SRM 8.5:1 pistons fitted 4000km ago.

[Triton Thrasher]

100 km/h is 64 mph, which is usually not engine labouring rpm in top gear, on a 500.

Unless someone has messed about with the sprocket sizes.

[muskrat]

G'day chotus52.
Where abouts are you in this great land? We have members in most states.
Do you have access to an endoscope camera to have a peek down the bore through the plug hole? If not rip the head off and show us some pictures.
My little A7 will sit on 70mph all day.
Cheers

[chotus52]

Hello Muskrat, here in Cranbourne, 35km southeast of Melbourne. Previous owner rebuilt the engine from the crank up. Sent the crank away to get the required end float exact, SRM 8.5:1 pistons (clearance unknown), new valve guides & valves (told me the guides are a tight fit to Ensure no blow by, as engine used to be a smoker) & geared it up one tooth on the front as it pulled so well after the overhaul. Dynamo rewound, 12 volt system, thorspark ignition etc etc etc. After the 2nd seize, I cleaned out the fuel tank (some small flakes of debris inside), replaced a clogged inline fuel filter, which I suspected was causing it to lean out & ping plus found the needle jet loose in the carby. All fixed & back together but seized again yesterday Disappointing.

[muskrat]

G'day again chotus52.
I think the nearest member to you is 20km NW of Melbourne.
The damage will only get worse the more it happens. Time to get the topend off.
"(told me the guides are a tight fit to Ensure no blow by, as engine used to be a smoker)" The guides are a shrink fit in the head, are you sure he didn't mean the pistons or ring gaps?
Cheers

[JulianS]

Guides a tight fit sounds worrying.

Did he mean less than normal clearance between valve stem and guide bore or oversized guide in head?

If the former that could well be the cause of your seizure, valve and guide nipping up when hot due to lack of clearance.

[muskrat]

That is feasible JulianS. But wouldn't they be "taped" shut by the piston  .
Cheers

[RichardL]

If Julian's observation is the cause, I'm thinking removing the rocker box and looking for bent pushrods might tell you right away. The question is, would this be a seizure or just ceasing to run? Then, would it start again with bent pushrods? Honestly, I don't know the answer. It's possible that the "...guides are a tight fit..." comment is a sideline to the main issue, and may just be an indicator of the PO's thinking as to clearances.

Richad L.

[JulianS]

That is feasible JulianS. But wouldn't they be "taped" shut by the piston  .
Cheers

They could well be! And there would be witness marks on the piston crowns.


I think the only way forward is, as you suggest, to have the top off and check .