New piston +Rebored block = Short

[BOXIE]

Hi

 

Datsun 620 J15 motor compleate overhalled.(assembled my self)

 

Problem:1)The piston top of no1 and no4 is not level with the block when cranck is on TDC.

When measured it was found to be 0.95mm short from the top of the block.

 

Is this normal?

 

 

Futher investigation: Measured compression and found 6bar only (Same on all pistons)and manual says 8.3bar.

gave 3 x squirts of oil on 1piston and compression rose to 8bar.

 

Must the rings wear in first to have the right compression?

 

 

Problem:2)Engine starts with choke on 80% and when pushed back the engine dies imediatly.

carb cleaned, new ht wires , condenser, points,dist cap, rotor.

 

 

Any idea what my be wrong?

 

Frustrated big time.

 

Engine was using 5 liters of oil every four days of 20km/day. that was the reason for overhalling the motor.

 

Please help

 

Boxie

[banzai510(hainz)]

Must the rings wear in first to have the right compression?

 

YES!!!!!!!!!!! Rings gapped????????

 

ck tightenness of carb intake and manifold for vaccum leak.

 

I know nothing about J motors but have to assume the valves are also tight

 

If pistons 2 and 3 were the same height at 180 deg I would assume the pistons are all ok . But would assume you saw this when you took it apart what the piston height was.

[BOXIE]

Hi

 

Pistons:

I have not checked the old pistons hight when i removed it.

 

I have sprayed under arm spray at the intake manifold side and did not nitice any increase in revs.

 

I am going to strip the carb tomorrow and double check the idling curcuit .

 

A fter a long relax and cool down in the house , i have come to that only conclution The idling curcuit.

 

It behaves just like a fuel starvation senario.

 

Thanks for the shap reply, i will keep you posted.

 

I will also ask my engineering guy who did the machining for me and ordered the spares.

 

I am trying to look for the specks on the new pistons (part no:PNS407802X) and the original ones, because i suspect it might be forlift pistons and not ldv j15 pistons.

 

Boxie

[datzenmike]

Were the pistons measured and the cylinders bored to match?

Were the rings put in the cylinders and the end gap measured?

Did you stagger the end gaps so they don't all line up?

[banzai510(hainz)]

I worked on a Datsun 521 once and th acrb was loose and then I adjusted the mixure screw and got it running.

Mixtures screw not adjusted correct can make it look like a idle jet is plugged. Later Datsun had a electric anti desiel selinoid to plug the idle circut to keep it from RUN ON after shuutting the motor down

[BOXIE]

Hi

 

The engineer did the reboring and i am dealing with him from 1999 and not one come back jet.

 

The piston rings was placed as follows when assembling

top: gap facing the rear of the engine

second: facing the fron of the engine and

The oil rings was spaced also 180 degrees appart with +- 20mm of set from the gudion pins.

 

I will let you guys know tommorrow what if find.Will post more or less same time.

 

Thank for the help so far.

 

Boxie

South Africa

Durban

[datzenmike]

Ring gaps should never be above the piston pin nor in the thrust direction (right side)

 

Pick any 45 degree direction for the top, 180 from it for the second ring, top oil control ring 90 degrees to them and 180 for the bottom. This is the ideal positions but won't be the cause of your low compression.

[zed]

The piston top of no1 and no4 is not level with the block when cranck is on TDC.

When measured it was found to be 0.95mm short from the top of the block.

not all motors have pistons level with block at TDC. Some are 'out the hole', some are 'down the hole'. Your manual should have a spec for this - maybe given as 'deck clearance'. It is critical for compression. You may be right that your machinist has given you forklift pistons.

[BOXIE]

not all motors have pistons level with block at TDC. Some are 'out the hole', some are 'down the hole'. Your manual should have a spec for this - maybe given as 'deck clearance'. It is critical for compression. You may be right that your machinist has given you forklift pistons.

 

Hi to alll my helpers in time of need.

 

The carb was striped compleatly (exept for the butterflys) again and i used carb cleaner and a spray nozzel with etended tube to get in all the ports.

 

I dit found some thing strange about the carb according to the manual.

 

Carb no:214282-30 185 02 - The senter part of the carb is a J16 model and the rest of it is according to spec of the 15 model.

I have noted that there is some wear on the butterflys by the shaft side about 0.3mm. not big enough to worrie now about that , but will replace this soon for fuel consumption reasons in the near future.

 

I will install the carbi back to morrow and see how it goes.

I will use minimal locktite gray sealer for the bace sections and the manifold gaskets , just to ensure no air leakage is present.

 

I will post you guys tomorrow again to let you know.

 

One thing i know is that the engine must at least idle and have some driveability.

 

cheers

 

Boxie

[EricJB]

.95mm deck height is only about .037 thousands in inches. Should be fine. Get it running right and drive it a bit to seat the rings in. Then check compression.

[BOXIE]

Hi to all

 

Thanks for all your help (realy).

 

As you all know , being a hobby motor rebuilder for years i have never came accross a problem like this.

 

i have to share this in shame with you , after all your help.

 

Condition: Motor start with choke out and will rev with very difficulty.

 

Todays condition: Motor idle only when excilerator is being tapped on and of and when the little arm on the side of the carburettor (squirts fuel when exellerator is pushed) is pushed in a pulsing manner , the motor idles in some way.

 

Conclution:The idling cuircuit is dead and the first mai jet circuit is dead , SO IT BE THE PRIMARY MAIN JET.

 

Please kick my but for me now, here is the fault - The Main jet hole was still blocked after the jets was twice in my hands for cleaning but never tought to clean the simplest thing on it -

 

The hole on in the center of the main jet.

 

I took the LDV for a spin and it was like the van was asking me "you can be very stupid if you want to" and carry on running as if it came brand new from a car dealer ship floor.

 

I can derust the inside cabin floor and driver side window winder now.

 

Thank to all you guys

 

Cheers from South Africa Durban.

 

This forum is the greatest!!!!!!!!!!!!