No Compression After Poncam Install

[robbo_rb180]

i have put a set of poncams in my rb25 and sent the car away to get it wired up.

when my mechanic went to turn it over it just kept winding over. after a quick compression check No.1 cyclinder has no compression.

we have checked the timing, its all good.

what would cause this?

its a s2 rb25det with tomei "series 2" cams (late r33 with r34 CAS).

my mechanic thinks i have the wrong cams but i checked them and they are the right cams.

any help would be appreciated.

[Deano 1]

are you sure u got all the right clearance's between the cam lob and the lil cup thing that sits on the vavles? it must have the the vavles always open just a lil bit so theres no comp.

my 2 cents

[tridentt150v]

Haven't snapped a cam have you?

[snozzle]

if you were working on the engine while number 1 cyl 1 was at tdc which is wat u should be doing and moved the cam substantially without a timing belt on you may have opened your valves and smacked the piston causing them to bend, quite possible that ur mechanic tried to remove (undo) the cam gear after he took the timing belt off and the cam rotated during this without the crank moving at the same time

Edited July 15, 2010 by snozzle

[snozzle]

are you sure u got all the right clearance's between the cam lob and the lil cup thing that sits on the vavles? it must have the the vavles always open just a lil bit so theres no comp.

my 2 cents

his engine has hydraulic lifters, no clearance adjustment is possible, neither is it needed

[DVS JEZ]

deano must have thought it was an RB26.

do any of the other cylinders have compression?

[robbo_rb180]

if you were working on the engine while number 1 cyl 1 was at tdc which is wat u should be doing and moved the cam substantially without a timing belt on you may have opened your valves and smacked the piston causing them to bend, quite possible that ur mechanic tried to remove (undo) the cam gear after he took the timing belt off and the cam rotated during this without the crank moving at the same time

i did the cams myself when the motor was out. i made sure i didn't move anything after i set no.1 @ TDC. mechanic only just took the timing cover off to check the timing. when i get the car back home i will pull the cam covers off & see whats happened.

i don't think its a broken cam as everything rotates smoothly.

hope it hasn't bent valve(s).

i have a reco'd head & headgasket if it has though.

if anyone has any other ideas what it might be post it up.

cheers

rob

[robbo_rb180]

deano must have thought it was an RB26.

do any of the other cylinders have compression?

i wish it was a 26, solid lifters FTW.

i haven't checked them yet, it will be the 1st thing i do when i get the car home.

[DVS JEZ]

hope u havent killed the valves in cyl 1 and cyl 6

i did it on my 1st rb26 with poncams i bent cyl 6 valves and i did it by the book.

a mate told me after that to get piston 1 on TDC as normal but then turn it back until all the pistons are at an even height, then install cams (no way the valves can hit cause the pistons are down,) get the cams to the right position then turn the crank forward till at TDC then install the belt, has worked every time for me now. annoying way to learn it but

[The Dan]

If you had the piston at TDC and bolted the cam down so it pushed the valves down on cyl 1, say goodbye to those valves.

You should never bolt cams down when pistons are at TDC

[PM-R33]

Hmmm I swear I did my camshafts a few times with the engine at TDC and never bent anything

Doesn't the engine manual say to make sure cylinder one is at TDC?

Edited July 15, 2010 by PM-R33

[boostn0199]

I think the manual says at TDC but cant remember exactly. But yeah if it is at tdc provided the cam is in the exact correct position and tightened it shouldnt bend the valves I wouldnt think.

[boostn0199]

Actually I checked the manual and it doesnt mention what postion the pistons should be at.

[BoostdR]

Bleed the lifters, should be fine then.

[BoostdR]

also if you have a very late 33. check if you don't have solid buckets. R34 are solid and run much less clearance ~.018', but if that was the case I would expect all cylinders would be leaking comp.

[jiffo]

Hmmm I swear I did my camshafts a few times with the engine at TDC and never bent anything

Doesn't the engine manual say to make sure cylinder one is at TDC?

Probably does say TDC. It's a dreadful manual and shows no understanding of what MIGHT happen.

Compression TDC on No1 means No6 inlet and exhaust valves are in the process of changing over, opening/closing.

Only takes a slight cam rotation and the valves will hit the piston.

Anyone who has installed cams knows they like to rotate slightly as the caps are wound down, so it's simply good practice to rotate the crank slightly off TDC so there is no possibility of any valves interfering.

It also gives you the chance to test rotate your cams (without hitting pistons) to ensure everything is happy.

Oddly enough, Nissan chose No1 BDC for its RD28 camshaft install, cambelt timing the works. A much better option as at No1 BDC, the only protruding valves are poking into cylinders with the pistons way down.

[snozzle]

Ive never had a problem working at TDC, on the rb engine you have to move the inlet cam about 30 degrees anti clock wise to even begin to open, thats quite alot of movenent.. i think you just have to be abit more careful when your bolting teh cam shaft down, make sure the lobes on cyl 1 are looking 10 o clock on inlet side and 2 o clock on the exh

everyone has their own methods, i dont see why you HAVE to work at TDC but its just what i have always done

[PM-R33]

Yeah I have lost count the number of times I changed cams troubleshooting my stupid problems (Stock cams, HKS cams, Tomei cams) and I think I worked at TDC every time, but I always made sure the cams didn't rotate much when bolting the cam caps down.

Edited July 16, 2010 by PM-R33

[Adriano]

Ive never had a problem working at TDC, on the rb engine you have to move the inlet cam about 30 degrees anti clock wise to even begin to open, thats quite alot of movenent.. i think you just have to be abit more careful when your bolting teh cam shaft down, make sure the lobes on cyl 1 are looking 10 o clock on inlet side and 2 o clock on the exh

everyone has their own methods, i dont see why you HAVE to work at TDC but its just what i have always done

Even though you have to rotate the cam 30 degrees for the valve to start opening, #6 piston is also at tdc, and both valves are open.

[DVS JEZ]

I built my rb26 then after a couple months and bit by the power bug i put in poncams and had it at tdc cause thats how u line the timing belt up. I bent the valves in cyl 6. Poncams have more lift then stock cams so its even easier to push the valves into the piston